Overweight takeoff / flight

[Koopas Ly]

Howdy again,

After reading NTSB reports that attribute the cause of the accident to
exceeding the airplane's maximum takeoff weight, I began wondering
about the effects of an overweight takeoff within C.G. limits.
Specifically, what would I have to do differently when flying an
airplane that's heavier than what the POH specifies. I am not
supporting the practice, of course, so let it be purely educational.

Corrections, additions, and comments welcome.

I would start by considering the increase in weight as comparable to
an increase in load factor. Hence, all your aoa-related speeds would
increase by the square root of the load factor. Vs, Vx, Vy, Vglide,
etc. would all increase. Va would also go up.

Now, by virtue of rotation speed being a function of stall speed, I
conjecture you'd have to liftoff at a faster airspeed which would
equate to a longer takeoff roll.

Then, after pitching for your faster Vy airspeed, you'd notice a
decrease in climb rate at full power due to the increased power
requirement. During cruise, you'd notice a reduced cruise speed and
an increase in stall speed. At approach to landing, should you bump
up your approach speed, you'll find yourself sinking faster when
chopping off the power even though your glideslope will remain the
same.

Since your stall speed is invariably higher, you'll eat up more runway
when landing.

So to sum up:

Takeoff: higher takeoff distance, higher rotation speed.

Climbout: lower climb rate at higher Vy speed, same angle of climb for
obstacle clearance at higher Vx speed. Should Vx not be flown faster,
a poorer angle of climb would result, making obstable clearance
doubtful. *I may be wrong here* I am not sure if the max. angle of
climb is constant regardless of weight...my calculations don't show
so...could someone clarify?

Cruise/Maneuvering: lower cruise speed, higher maneuvering speed,
higher clean stall speed.

Approach to maintain glideslope & descent profile: higher approach
speed, higher sink rate for a given power setting. Higher dirty stall
speed.

Landing: higher landing distance

Question (1 of 2): Seems to me that flying "overweight" is possible if
you're aware of the performance reductions. So why do you read so
many NTSB reports with probable causes listed as "overweight takeoff,
exceeded performance limitations"? As you slowly pull the yoke to
rotate, wouldn't a pilot *realize* through control forces, feel, gut
feeling that something is wrong?

Question (2 of 2): When considering accidents due to exceeding maximum
takeoff weight, do the majority occur during takeoff? If so, is it
typically due to not reaching proper liftoff airspeed for that
increased weight, stalling, and spinning to the ground? Would this
scenario be consistent with failure to set the flaps/slats to their
takeoff value?

Alex

[Larry Fransson]

On 2003-11-26 05:23:36 -0800, (Koopas Ly) said:

> Now, by virtue of rotation speed being a function of stall speed, I
> conjecture you'd have to liftoff at a faster airspeed which would
> equate to a longer takeoff roll.

> Since your stall speed is invariably higher, you'll eat up more runway
> when landing.

In both cases, it's not just the higher speeds involved, but the increased weight
also means that acceleration for takeoff and deceleration on landing will be
lower. F=ma with a (more or less) constant F, and increased m means
decreased a.

--
Larry Fransson
Seattle, WA

[EDR]

In article >, Koopas Ly
> wrote:


> Approach to maintain glideslope & descent profile: higher approach
> speed, higher sink rate for a given power setting. Higher dirty stall
> speed.
>
> Landing: higher landing distance


Once you have burned sufficient fuel to bring the weight back down into
the max gross/useful load range, why should landing parameters be any
different from published?
Published numbers are for max gross weight.

[G.R. Patterson III]

Koopas Ly wrote:
>
> Howdy again,
>
> After reading NTSB reports that attribute the cause of the accident to
> exceeding the airplane's maximum takeoff weight, I began wondering
> about the effects of an overweight takeoff within C.G. limits.

Seems to me that you have listed most of the effects correctly. One thing you
should consider, however, is the fact that the balance envelope for most (if
not all) planes gets narrower at the top. In other words, the more weight you
put in an aircraft, the closer to the center of lift that weight has to be. At
some point, all of the weight will have to be in the front seat.

I have read of cross-Atlantic ferry flights in which the aircraft was loaded to
weigh about 1.6 times the normal MGW. In one account, a Bonanza loaded that way
took over 6,000' to get airborne.

George Patterson
A man who carries a cat by the tail learns something that can
be learned no other way.

[John Gaquin]

An old pilot once told me, when I was a young pilot, "...sumbitch flies a
hell of a lot better overweight than it does outta gas..."

JG

[Tony Cox]

"Koopas Ly" > wrote in message
om...
>
> I would start by considering the increase in weight as comparable to
> an increase in load factor. Hence, all your aoa-related speeds would
> increase by the square root of the load factor. Vs, Vx, Vy, Vglide,
> etc. would all increase. Va would also go up.
>


I take issue with Va. At first thought, it should go up as sqrt( m/m0)
with m the new weight and m0 the maximum gross at which Va is
quoted. This since at a higher Va, we can maintain the same AOA
as we did at m0, so the G forces at stalling AOA never exceed the
design limitations.

BUT, there are 2 things (at least) which contribute to the setting of
Va in the first place. One is the limitation of 'heavy things in the plane',
such as a bag of sand in the baggage compartment. If this is the limiting
factor, then Va should indeed scale as sqrt(m/m0). However, there is
also the 'torque on the wings' (low wings) or 'force on the wings' (struts
on Cessna). If you are pulling 3.5G with a higher gross weight, you'll
be exerting more force than was designed for at certified gross.

So to be safe (hah!, we're talking about overloading dammit), then
unless you know exactly which type of failure limits Va in the first
place, you'd be best off using Va for certified gross and not scaling
it up.

--
Dr. Tony Cox
Citrus Controls Inc.
e-mail:
http://CitrusControls.com/

[Greg Esres]

<<BUT, there are 2 things (at least) which contribute to the setting
of Va in the first place. >>

Neither one of the things you mentioned is given in Part 23 as a
requirement for Va. Part 23 uses the speed solely to provide the
design requirements of the elevators, ailerons, and rudder.

Since the forces on these control surfaces will not vary with weight,
you certainly can't scale it up.

[Bob Gardner]

When considering takeoff parameters, don't forget the increased rolling
resistance of overloaded tires...this will affect acceleration and takeoff
distance.

Bob Gardner

"Koopas Ly" > wrote in message
om...
> Howdy again,
>
> After reading NTSB reports that attribute the cause of the accident to
> exceeding the airplane's maximum takeoff weight, I began wondering
> about the effects of an overweight takeoff within C.G. limits.
> Specifically, what would I have to do differently when flying an
> airplane that's heavier than what the POH specifies. I am not
> supporting the practice, of course, so let it be purely educational.
>
> Corrections, additions, and comments welcome.
>
> I would start by considering the increase in weight as comparable to
> an increase in load factor. Hence, all your aoa-related speeds would
> increase by the square root of the load factor. Vs, Vx, Vy, Vglide,
> etc. would all increase. Va would also go up.
>
> Now, by virtue of rotation speed being a function of stall speed, I
> conjecture you'd have to liftoff at a faster airspeed which would
> equate to a longer takeoff roll.
>
> Then, after pitching for your faster Vy airspeed, you'd notice a
> decrease in climb rate at full power due to the increased power
> requirement. During cruise, you'd notice a reduced cruise speed and
> an increase in stall speed. At approach to landing, should you bump
> up your approach speed, you'll find yourself sinking faster when
> chopping off the power even though your glideslope will remain the
> same.
>
> Since your stall speed is invariably higher, you'll eat up more runway
> when landing.
>
> So to sum up:
>
> Takeoff: higher takeoff distance, higher rotation speed.
>
> Climbout: lower climb rate at higher Vy speed, same angle of climb for
> obstacle clearance at higher Vx speed. Should Vx not be flown faster,
> a poorer angle of climb would result, making obstable clearance
> doubtful. *I may be wrong here* I am not sure if the max. angle of
> climb is constant regardless of weight...my calculations don't show
> so...could someone clarify?
>
> Cruise/Maneuvering: lower cruise speed, higher maneuvering speed,
> higher clean stall speed.
>
> Approach to maintain glideslope & descent profile: higher approach
> speed, higher sink rate for a given power setting. Higher dirty stall
> speed.
>
> Landing: higher landing distance
>
> Question (1 of 2): Seems to me that flying "overweight" is possible if
> you're aware of the performance reductions. So why do you read so
> many NTSB reports with probable causes listed as "overweight takeoff,
> exceeded performance limitations"? As you slowly pull the yoke to
> rotate, wouldn't a pilot *realize* through control forces, feel, gut
> feeling that something is wrong?
>
> Question (2 of 2): When considering accidents due to exceeding maximum
> takeoff weight, do the majority occur during takeoff? If so, is it
> typically due to not reaching proper liftoff airspeed for that
> increased weight, stalling, and spinning to the ground? Would this
> scenario be consistent with failure to set the flaps/slats to their
> takeoff value?
>
> Alex

[C J Campbell]

|
| Question (1 of 2): Seems to me that flying "overweight" is possible if
| you're aware of the performance reductions. So why do you read so
| many NTSB reports with probable causes listed as "overweight takeoff,
| exceeded performance limitations"? As you slowly pull the yoke to
| rotate, wouldn't a pilot *realize* through control forces, feel, gut
| feeling that something is wrong?

You would not necessarily feel heavier control forces if the airplane was
trimmed properly. Heavier control forces as you rotate would indicate a
forward cg, not over weight. You could be grossly over weight and have very
light control forces if the weight was mostly in the back. Most noticeable
is that the airplane does not accelerate as quickly as usual. If you are in
the habit of flying overvweight, you might not notice anything wrong at all.
Add in a hot day, short runway, and high altitude and suddenly you are going
to find yourself bitten by bad habits.

|
| Question (2 of 2): When considering accidents due to exceeding maximum
| takeoff weight, do the majority occur during takeoff? If so, is it
| typically due to not reaching proper liftoff airspeed for that
| increased weight, stalling, and spinning to the ground? Would this
| scenario be consistent with failure to set the flaps/slats to their
| takeoff value?

Many airplanes take off from normal runways without flaps. A pilot can
easily forget to set flaps for short or soft field takeoffs. A lot of pilots
are also taught just 'plane' wrong. Consider the Cessna 172M, for example.
Most pilots are taught to set the flaps at 10 degrees for a short field
takeoff. Most aftermarket checklists tell you to do this, even the ones
designed for older Cessnas. Surecheck sells checklists that are supposedly
designed specifically for the 172M but they contain this error.

But read the manual. It tells you that if you set the flaps at 10 degrees
you will lift off the runway more quickly, but that you will climb more
slowly and you might not clear an obstacle at the end of the runway. The
manual says to use 10 degrees of flaps only when the runway is soft or is
short but there are no obstacles on climbout. But the idea that you use 10
degrees of flaps to do a short field takeoff is so pervasive that I have had
train my students in how to educate examiners on this issue.

Newer Cessna 172s use 10 degrees of flaps for all short field takeoffs, so
when transitioning from one model of Cessna 172 to another, be sure to read
the manual thoroughly.

[EDR]

In article >, Greg Esres
> wrote:

> <<BUT, there are 2 things (at least) which contribute to the setting
> of Va in the first place. >>
>
> Neither one of the things you mentioned is given in Part 23 as a
> requirement for Va. Part 23 uses the speed solely to provide the
> design requirements of the elevators, ailerons, and rudder.
>
> Since the forces on these control surfaces will not vary with weight,
> you certainly can't scale it up.

G-forces are directly related to weight. Since the size of the control
surface is directly related to the forces exerted on it, control
surfaces are dependent on weight.

[Greg Esres]

<<G-forces are directly related to weight. Since the size of the
control surface is directly related to the forces exerted on it,
control surfaces are dependent on weight.>>

Sorry, you lost me. The forces exerted on the control surfaces are
going to depend on airspeed and angle of deflection.

The size of control surfaces is irrelevant in this discussion, since
they are fixed at design time.

[Peter Duniho]

"G.R. Patterson III" > wrote in message
...
> Seems to me that you have listed most of the effects correctly. One thing
you
> should consider, however, is the fact that the balance envelope for most
(if
> not all) planes gets narrower at the top.

A true generalization as far as I know, but I'm sure there are a number of
exceptions and in many cases, the shape of the W&B envelope has as much to
do with what test parameters the manufacturer chose to look at, as it does
any real structural or aerodynamic issues.

The main thing is to make sure one is paying attention to the W&B envelope.
When flying overweight (with FAA approval, of course) one can make an
educated guess by extrapolating the existing graph, but the bottom line is
you don't really know what the shape of the W&B envelope is over gross,
unless the manufacturer has been kind enough to publish it (and they usually
aren't).

> In other words, the more weight you
> put in an aircraft, the closer to the center of lift that weight has to
be.

Not really. In some aircraft, the envelope is sloped on the aft portion too
as weight goes up. For rearward CG configurations, additional weight needs
to be put farther from the center of lift, not closer. All you can say
without seeing the actual W&B envelope is that usually you have a narrower
range at higher weights. You can't say which direction that range trends,
and even that generalization has exceptions.

> [...] At some point, all of the weight will have to be in the front seat.

Even if the previous statement were true, not all airplanes have their
center of lift aligned with the front seat.

> I have read of cross-Atlantic ferry flights in which the aircraft was
loaded to
> weigh about 1.6 times the normal MGW. In one account, a Bonanza loaded
that way
> took over 6,000' to get airborne.

How much runway did Voyager take? I'll bet it was a LOT. :)

Pete

[Peter Duniho]

"Greg Esres" > wrote in message
...
> Since the forces on these control surfaces will not vary with weight,
> you certainly can't scale it up.

Huh? You have to scale Va with weight. Even within legal configurations, a
specific Va is valid only at a specific weight, with lower weights resulting
in lower Va and higher weights resulting in higher Va.

Just because you went outside the design/certification envelope, that
doesn't change the nature of Va.

Pete

[Tony Cox]

"Greg Esres" > wrote in message
...
> <<BUT, there are 2 things (at least) which contribute to the setting
> of Va in the first place. >>
>
> Neither one of the things you mentioned is given in Part 23 as a
> requirement for Va. Part 23 uses the speed solely to provide the
> design requirements of the elevators, ailerons, and rudder.
>

Well, right conclusion even if you don't agree with my method.

Va might be used in the design of the control surfaces, but I
was eluding to how Va is established in the first place.

And certainly one of the limitation is ensuring that you can't
exceed 3.5G at Va by yanking the yoke back. Isn't that
what Va is all about?

> Since the forces on these control surfaces will not vary with weight,
> you certainly can't scale it up.

Now you've lost me. If that were the case, Va would be
the same at any aircraft weight, which it certainly isn't.

--
Dr. Tony Cox
Citrus Controls Inc.
e-mail:
http://CitrusControls.com/

[Rick Durden]

Alex,

You make the assumption that the center of gravity envelope edges
(fore and aft c.g. limits) are linear as one goes above published
gross weight. While they may be, it is still an assumption...and test
pilots usually get paid pretty well...and never seem to take
passengers along when doing those tests....

All the best,
Rick

(Koopas Ly) wrote in message >...
> Howdy again,
>
> After reading NTSB reports that attribute the cause of the accident to
> exceeding the airplane's maximum takeoff weight, I began wondering
> about the effects of an overweight takeoff within C.G. limits.
> Specifically, what would I have to do differently when flying an
> airplane that's heavier than what the POH specifies. I am not
> supporting the practice, of course, so let it be purely educational.
>
> Corrections, additions, and comments welcome.
>
> I would start by considering the increase in weight as comparable to
> an increase in load factor. Hence, all your aoa-related speeds would
> increase by the square root of the load factor. Vs, Vx, Vy, Vglide,
> etc. would all increase. Va would also go up.
>
> Now, by virtue of rotation speed being a function of stall speed, I
> conjecture you'd have to liftoff at a faster airspeed which would
> equate to a longer takeoff roll.
>
> Then, after pitching for your faster Vy airspeed, you'd notice a
> decrease in climb rate at full power due to the increased power
> requirement. During cruise, you'd notice a reduced cruise speed and
> an increase in stall speed. At approach to landing, should you bump
> up your approach speed, you'll find yourself sinking faster when
> chopping off the power even though your glideslope will remain the
> same.
>
> Since your stall speed is invariably higher, you'll eat up more runway
> when landing.
>
> So to sum up:
>
> Takeoff: higher takeoff distance, higher rotation speed.
>
> Climbout: lower climb rate at higher Vy speed, same angle of climb for
> obstacle clearance at higher Vx speed. Should Vx not be flown faster,
> a poorer angle of climb would result, making obstable clearance
> doubtful. *I may be wrong here* I am not sure if the max. angle of
> climb is constant regardless of weight...my calculations don't show
> so...could someone clarify?
>
> Cruise/Maneuvering: lower cruise speed, higher maneuvering speed,
> higher clean stall speed.
>
> Approach to maintain glideslope & descent profile: higher approach
> speed, higher sink rate for a given power setting. Higher dirty stall
> speed.
>
> Landing: higher landing distance
>
> Question (1 of 2): Seems to me that flying "overweight" is possible if
> you're aware of the performance reductions. So why do you read so
> many NTSB reports with probable causes listed as "overweight takeoff,
> exceeded performance limitations"? As you slowly pull the yoke to
> rotate, wouldn't a pilot *realize* through control forces, feel, gut
> feeling that something is wrong?
>
> Question (2 of 2): When considering accidents due to exceeding maximum
> takeoff weight, do the majority occur during takeoff? If so, is it
> typically due to not reaching proper liftoff airspeed for that
> increased weight, stalling, and spinning to the ground? Would this
> scenario be consistent with failure to set the flaps/slats to their
> takeoff value?
>
> Alex

[Greg Esres]

<<a specific Va is valid only at a specific weight, with>>

Show me a Part 23 requirement that says so.

Todd Pattist has lectured on this a couple of times, and he's right.

[Greg Esres]

<<Isn't that what Va is all about?>>

Conventional wisdom says so, but there is no requirement in Part 23
that says this must be true. Part 23 only uses this speed in its
requirements for control surfaces.

In my view, the most correct definition of Va will be it's the speed
above which you cannot make full or abrupt control movements, due to
control surface integrity.

New airplanes are supposed to come with a new Vo speed, which DOES
require that the airplane stall before exceeding the load factor.

Here's a copy from a draft copy of an AC 23.<something> that I found.
The AC was intended to make this clear to test pilots, but I don't
think the draft was ever finished:

------------<snip>-----------------
VA should not be interpreted as a speed that would permit the pilot
unrestricted flight-control movement without exceeding airplane
structural limits nor should it be interpreted as a gust penetration
speed. Only if VA = Vs sqrt(n) , will the airplane stall in a nose-up
pitching maneuver at, or near, limit load factor. For maneuvers where
VA>VS n , the pilot would have to check the maneuver; otherwise the
airplane would exceed the limit load factor.

Amendment 23-45 added the operating maneuvering speed, VO in §
23.1507. VO is established not greater than VS sqrt(n) , and is a
speed where the airplane will stall in a nose-up pitching maneuver
before exceeding the airplane structural limits.

------------<snip>-----------------


<<Va would be the same at any aircraft weight, which it certainly
isn't.>>

It is in some airplanes. My Piper arrow doesn't scale it with weight.

Moreover, Part 23 says that Va is *only* defined at max gross. Some
manufacturers do publish Va's at lower weight, but that appears to be
at their option. As written, it doesn't match Part 23 definition.

[Greg Esres]

Pete:

Let me elaborate on my terse response (and see my response to Tony).
I agree that maneuvering speed, as defined in the aerodynamics books,
must be scaled with weight. However, Va, which is called DESIGN
maneuvering speed by the FAA, doesn't really match the definition of
plain ole "maneuvering speed". They really should have called it
something else, IMO.

However, it appears that most manufacturers are shooting for a
maneuvering speed, even though the regulations don't require it. If,
however, they chose to make the speed higher for some reason, it won't
protect you from overstressing the airplane, and neither will the
speed when you scale it for weight. GIGO. ;-)

Still, all this is of only academic interest. The one thing that IS
known is that the control surfaces must be protected at VA, and that
won't scale UP from published Va. Agreed?

[Peter Duniho]

"Greg Esres" > wrote in message
...
> <<a specific Va is valid only at a specific weight, with>>
>
> Show me a Part 23 requirement that says so.

Part 23 isn't what makes an airplane fly. Aerodynamics are. And those
aerodynamics clearly show that at a given weight, a slower airspeed is
required in order to limit acceleration to a given number.

Oddly enough, many aircraft manuals bear this out, providing lower Va speeds
for lower weights.

> Todd Pattist has lectured on this a couple of times, and he's right.

I seriously doubt Todd has told you that Va remains the same regardless of
aircraft weight. You obviously misunderstood him.

Pete

[Peter Duniho]

"Greg Esres" > wrote in message
...
> Still, all this is of only academic interest. The one thing that IS
> known is that the control surfaces must be protected at VA, and that
> won't scale UP from published Va. Agreed?

No. Just because Part 23 doesn't stipulate that at a lower weight, a lower
airspeed must be used to ensure not overstressing the airplane in
turbulence, that does not mean that the maximum speed at which you can fly
and be assured of not overstressing the airplane does not go down as weight
is reduced.

Put another way: the minimum airspeed at which a given load factor can be
achieved before stalling the aircraft is positively correlated with weight
(i.e. it goes down as weight goes down, and goes up as weight goes up).
This is *known*. The fact that it's not stated in Part 23 does not make it
any less known.

Even your control surface tangent isn't really relevant to this particular
thread since you are intentionally limiting your comments to a single
weight. Again, just because Part 23 only requires a number to be defined at
a specific weight, that does not automatically mean that the number doesn't
exist at a different weight, nor does it necessarily mean that number is the
same at a different weight.

The definition of Va in Part 23 is clear. It has nothing to do with control
surfaces and everything to do with stall speed and load factor. Just
because Va is only used again within Part 23 for some other use, that does
not change the nature of the calculation. It is commonly understood that,
even though by definition Va exists only for a specific weight, that for the
purposes of flying, one needs to adjust the "operational Va" according to
weight if one expects to remain within the certificated load limits.

Pete

[Michael]

(Koopas Ly) wrote
> After reading NTSB reports that attribute the cause of the accident to
> exceeding the airplane's maximum takeoff weight, I began wondering
> about the effects of an overweight takeoff within C.G. limits.
> Specifically, what would I have to do differently when flying an
> airplane that's heavier than what the POH specifies. I am not
> supporting the practice, of course, so let it be purely educational.

The first thing you should realize is that airplanes are LEGALLY
operated overgross all the time. In Alaska, Part 135 operators can
get the max gross raised by up to 15%, depending on the airplane. For
long overwater ferry flights, the FSDO will give you a ferry permit to
operate 20% overgross without so much as blinking, provided you sound
like you know what you're doing. Just for reference, on a plain
vanilla C-172 or Cherokee, that would be 400+ lbs overgross.

> I would start by considering the increase in weight as comparable to
> an increase in load factor. Hence, all your aoa-related speeds would
> increase by the square root of the load factor. Vs, Vx, Vy, Vglide,
> etc. would all increase.

So far, so good.

> Va would also go up.

Not necessarily. Everything depends on where the weak spot defining
Va happens to be. The usual reason Va goes down on an airplane is
that the weak spot is the engine mount. If the weak spot is the
engine mount, the weight carried by the engine attach point (the
engine) is fixed, and thus maximum gee is fixed. Since at lower
weight you can exceed max gee at a lower speed without stalling, Va
goes down with weight.

If the weak spot is the wing attach point, then Va is constant. This
is because the weight carried by that point is NOT fixed. This is a
pretty common situation in gliders, but pretty rare in airplanes. The
real issue is this - once you exceed max gross, you don't really know
where the weak spot is anymore unless you do an engineering analysis.
Therefore, I would assume Va does not increase.

> Now, by virtue of rotation speed being a function of stall speed, I
> conjecture you'd have to liftoff at a faster airspeed which would
> equate to a longer takeoff roll.

Correct, and this speed may be higher than what the square root of
weight correction would lead you to believe. Typically we rotate well
below best rate of climb speed, and count on being able to accelerate
in ground effect and climb out. However, once you load it up enough,
you may not have that luxury. You may have to wait until almost Vy
speed before you have excess power available to accelerate and climb
out. A too-early rotation may put you in the position of flyng in
ground effect without being able to accelerate enough to climb out.

> Then, after pitching for your faster Vy airspeed, you'd notice a
> decrease in climb rate at full power due to the increased power
> requirement. During cruise, you'd notice a reduced cruise speed and
> an increase in stall speed.

All correct.

> At approach to landing, should you bump
> up your approach speed, you'll find yourself sinking faster when
> chopping off the power even though your glideslope will remain the
> same.

Assuming you are still overgross.

> Since your stall speed is invariably higher, you'll eat up more runway
> when landing.

Maybe. Certainly if you want to minimize use of brakes. However,
your brakes will be more effective with more weight on wheels - you
will be able to use them at higher speed without locking them up.

You are also ignoring another important factor - the cg envelope
shrinks at higher gross weights. Because of this, just because you
are within cg limits for max gross does not mean you are still within
cg limits for the increased weight. Usually being forward is not too
bad - the plane will be nose heavy and will need to be landed at a
higher speed and/or with power to keep the nose up through the
touchdown. But if you are close to the aft limit for gross and are
overgross, beware. You are asking for stability problems in pitch,
and the plane may be uncontrollable.

> So to sum up:
>
> Takeoff: higher takeoff distance, higher rotation speed.
>
> Climbout: lower climb rate at higher Vy speed, same angle of climb for
> obstacle clearance at higher Vx speed. Should Vx not be flown faster,
> a poorer angle of climb would result, making obstable clearance
> doubtful. *I may be wrong here* I am not sure if the max. angle of
> climb is constant regardless of weight...my calculations don't show
> so...could someone clarify?

Max angle of climb will be reduced at higher weight, and Vx will have
to be increased.

> Cruise/Maneuvering: lower cruise speed, higher maneuvering speed,
> higher clean stall speed.

See above with respect to maneuvering speed.

> Approach to maintain glideslope & descent profile: higher approach
> speed, higher sink rate for a given power setting. Higher dirty stall
> speed.
>
> Landing: higher landing distance
>
> Question (1 of 2): Seems to me that flying "overweight" is possible if
> you're aware of the performance reductions.

Absolutely, it's done all the time - legally and illegally. It's a
rare piston freight hauler that doesn't routinely operate overgross.
What you have to realize is that all sorts of safety margins are
reduced. If you are aware of the reductions, it's not deadly. That's
why the FAA will give you a permit to operate overgross if you have a
need - as long as you demonstrate you understand what you're doing.

> So why do you read so
> many NTSB reports with probable causes listed as "overweight takeoff,
> exceeded performance limitations"? As you slowly pull the yoke to
> rotate, wouldn't a pilot *realize* through control forces, feel, gut
> feeling that something is wrong?

Maybe - but if he's not expecting it, he may not realize it in time.

> Question (2 of 2): When considering accidents due to exceeding maximum
> takeoff weight, do the majority occur during takeoff? If so, is it
> typically due to not reaching proper liftoff airspeed for that
> increased weight, stalling, and spinning to the ground? Would this
> scenario be consistent with failure to set the flaps/slats to their
> takeoff value?

In general, the overgross accidents fall into two categories. First
there are the ones where climb speed is never reached and the plane
runs out of runway and hits something. Second is when an overgross
twin loses an engine and can't maintain flight. The first is usually
the result of the "It's always worked before" factor and the second is
betting the engines will both keep running.

Michael

[Greg Esres]

<<The definition of Va in Part 23 is clear. It has nothing to do with
control surfaces and everything to do with stall speed and load
factor. >>

Then you haven't read Part 23.

Let me point out the sections to you:

-------------<snip>------------------
Horizontal Stabilizing and Balancing Surfaces

§ 23.423 Maneuvering loads.
Each horizontal surface...must be designed for the maneuvering loads
imposed by the following conditions:
(a) A sudden movement of the pitching control, at the speed VA...

(b) A sudden aft movement of the pitching control at speeds above
VA...

Vertical Surfaces
§ 23.441 Maneuvering loads.
(a) At speeds up to VA, the vertical surfaces must be designed to
withstand the following conditions....

Ailerons and Special Devices
§ 23.455 Ailerons.
(a) The ailerons must be designed for the loads to which they are
subjected -
....
(i) Sudden maximum displacement of the aileron control at VA. Suitable
allowance may be made for control system deflections.
-------------<snip>------------------

Now, the section that may be misleading you is this
---------<snip>-------------
§ 23.335 Design airspeeds.
(c) Design maneuvering speed VA. For VA, the following applies:
(1) VA may not be less than VS * sqrt(n) where -
---------<snip>-------------


Note that it says MAY NOT BE LESS THAN... In other words, it can be
more.


<<Oddly enough, many aircraft manuals bear this out, providing lower
Va speeds for lower weights.>>

Oddly, you didn't read what I wrote.

The point is that at Part 23 doesn't require this. And not all
aircraft publish such variations.

<<does not mean that the maximum speed at which you can fly
and be assured of not overstressing the airplane does not go down as
weight is reduced.>>

Again, you didn't read what I wrote. I said it doesn't scale UP.
Flying over max gross may increase maneuvering speed, but it doesn't
increase VA, because the increased weight won't protect control
surfaces from failure.


<<Even your control surface tangent isn't really relevant to this
particular thread >>

Tangent? It's the essence of what Va is.


<<I seriously doubt Todd has told you that Va remains the same
regardless of aircraft weight. You obviously misunderstood him.>>


Ok, you read what he wrote and tell me:

----------------<snip>----------------------
Note that this is a minimum Va ("no less than"). Thus the
designer can specify a higher Va and then protect the tail
surfaces by limiting stick throw, or by making the required
force at the stick to produce a damaging load on the
protected structure higher than a standard pilot could
exert. Note also that the regulatory definition of Va
*requires" that it be computed only at a stalling speed "at
the design weight" , i.e. max gross. Thus any lower speed,
or any lower weight cannot be Va as defined.
----------------<snip>----------------------

[Tony Cox]

"Greg Esres" > wrote in message
...
>
> In my view, the most correct definition of Va will be it's the speed
> above which you cannot make full or abrupt control movements, due to
> control surface integrity.

This is way interesting & I've got the FAR's in front of me
now to get to the bottom of this.

First, I can't find a specific definition of "Design maneuvering speed" in
the FAR's, but my personal working definition is almost like yours.
I'd substitute "without risk of structural failure" for talk of control
surface integrity. Since control surface failure is indeed structural
failure, my definition would seem more restrictive than yours.

It looks like Va is mentioned twice in pt 23.

In 23.335 we get Va must be >= Vs sqrt(n), with n the load
factor. We also get "Va need not exceed Vc" which
makes no sense to me, at least as far as a regulation goes.

Then, in 23.423 we see Va used in establishing the characteristics
of the (horizontal) control surfaces. Note that this doesn't
say this is how you calculate Va, it says you must use this speed
in the design of control surfaces to achieve certain rates of
response when they are used and/or to make sure you don't
break anything..............I suppose that manufacturers
do such a poor job of designing control surfaces that
they have to restrict Va just to meet this certification
requirement.....Well, bugger me Greg, looks like you're right!

>
> New airplanes are supposed to come with a new Vo speed, which DOES
> require that the airplane stall before exceeding the load factor.
>

Since control surfaces seem to be the limiting factor, I'd assume
that manufactures would design them for as low a Va as possible,
consistent with 23.335. So they'd choose Va = Vs.sqrt(n).

Vo does differs a little from pt 23 certification requirements, in that
Va isn't exactly Vo, because Va calculations assume that airfoil
lift does scale linearly with AOA and as the square of airspeed
when in fact these are only approximately true.

I'd bet that Vo and Va are pretty close. Allowing for the 1.5 safety
factor, I bet they're indistinguishable.

> Here's a copy from a draft copy of an AC 23.<something> that I found.
> The AC was intended to make this clear to test pilots, but I don't
> think the draft was ever finished:
>
> ------------<snip>-----------------
> VA should not be interpreted as a speed that would permit the pilot
> unrestricted flight-control movement without exceeding airplane
> structural limits nor should it be interpreted as a gust penetration
> speed. Only if VA = Vs sqrt(n) , will the airplane stall in a nose-up
> pitching maneuver at, or near, limit load factor. For maneuvers where
> VA>VS n , the pilot would have to check the maneuver; otherwise the
> airplane would exceed the limit load factor.

Isn't this just a warning that Va "may not be less than Vs.sqrt(n)", and
so could be higher?

> <<Va would be the same at any aircraft weight, which it certainly
> isn't.>>
>
> It is in some airplanes. My Piper arrow doesn't scale it with weight.
>
> Moreover, Part 23 says that Va is *only* defined at max gross. Some
> manufacturers do publish Va's at lower weight, but that appears to be
> at their option. As written, it doesn't match Part 23 definition.

I don't see that in pt 23. I see it being defined as 'may not
be less than' some expression involving gross weight parameters,
but there is nothing to say that this applies only to gross
weight (to be pedantic). Nor does 23.423 - which we both
agree partially defines Va - say anything about the weight
of the plane during the certification maneuver.

I'd remind you how we got here. The suggestion was that
Va, should be scaled upward in an overloaded airplane. We
both claim that it should not. I'd also scale my maneuvering
speed downwards if underweight just to stay within load
factor limits, and I bet you would too. To my mind, the laws
of physics trump the FAR's. (and my Va is indeed pretty close to
Vs.sqrt(3.5)). After all, pt 23 just tells me how to certify a
plane, not how to fly it.

I'd claim that Va shouldn't be increased because it is really
the minimum of a number of different speeds where things
start to fall apart, and without further data we don't know
which one does the limiting.

Interesting discussion.


--
Dr. Tony Cox
Citrus Controls Inc.
e-mail:
http://CitrusControls.com/

[Kyle Boatright]

"John Gaquin" > wrote in message
...
> An old pilot once told me, when I was a young pilot, "...sumbitch flies a
> hell of a lot better overweight than it does outta gas..."
>
> JG

I've got an old Flying Magazine (circa 1970 or so) where one of the editors
makes the comment that it is better to take off overloaded (with fuel) than
it is to try a launch with marginal fuel in order to stay under gross. The
comment was the same... It'll fly better over gross than outta gas.

I bet the magazine's lawyers wouldn't let them print that now...

KB

[Greg Esres]

<<This is way interesting >>

I agree, and I appreciate and admire your open mind.

<<I'd substitute "without risk of structural failure" for talk of
control surface integrity. Since control surface failure is indeed
structural failure, my definition would seem more restrictive than
yours.>>

I can live with your defintion. I only used "control surface
integrity" in order to stress that it wasn't necessarily the main wing
we were talking about.

<<Vo does differs a little from pt 23 certification requirements, in
that Va isn't exactly Vo, because Va calculations assume that airfoil
lift does scale linearly with AOA and as the square of airspeed
when in fact these are only approximately true.>>

The only distinction I see between Va and Vo is that Va says "not less
than" and Vo is "not greater than". Where do you see the distinction
you are drawing?

All the lift slope curves I've seen for straight wings are pretty
linear, at least up until the stall. But that does lead us into the
concept of a dynamic stall. Airfoils rapidly rotated to a high angle
of attack can generate a much higher lift coefficient than when in
steady state. (References available upon request.) The whole concept
of Va, or even Vo, protecting the wing are a bit fraudulent.

<<I'd bet that Vo and Va are pretty close. Allowing for the 1.5 safety
factor, I bet they're indistinguishable.>>

I'd say you're right. A friend of mine, who spoke with the FAA's
Seattle Certification office, said that Va might be a maximum of 5
knots over what sqrt(n)*Vs would be.

<<Isn't this just a warning that Va "may not be less than Vs.sqrt(n)",
and so could be higher?>>

Yes, exactly. Some people need it spelled out. <g>

<<I don't see that in pt 23. I see it being defined as 'may not
be less than' some expression involving gross weight parameters,
but there is nothing to say that this applies only to gross
weight (to be pedantic). >>

If I understand what you're saying, I agree. I guess it depends on
what "defined" means. <g>

<<The suggestion was that Va, should be scaled upward in an overloaded
airplane. We both claim that it should not. >>

Agreed.

<<I'd also scale my maneuvering speed downwards if underweight just to
stay within load factor limits, and I bet you would too. >>

Yes. However, those knowledgeable about aircraft structures maintain
that load factors incurred in turbulence are less stressful on the
aircraft than what are incurred via flight control movements.
Turbulence penetration speeds are calculated allowing higher load
factors.

<<I'd claim that Va shouldn't be increased because it is really
the minimum of a number of different speeds where things
start to fall apart, and without further data we don't know
which one does the limiting.>>

Very well expressed.

[Tony Cox]

"Peter Duniho" > wrote in message
...
>
> The definition of Va in Part 23 is clear. It has nothing to do with
control
> surfaces and everything to do with stall speed and load factor.

Actually, it seems to depend on both. I'm all turned around
on this having scratched my head for a while. Greg is
essentially correct.

23.335 says that Va must be >= Vs.sqrt(load-factor)
If we take the equality, then this is the load-factor
relationship we get assuming "Lift prop. to AOA"
and "Lift prop. airspeed**2".

23.423 (and others I'd missed) say how the control
surfaces must behave at Va and above. Designers
can set anything they want for Va as long as it
passes the control surface tests.

But since they are likely to want to minimize
complexity & weight of the control surface
mechanism, they are likely to choose Va to be
the minimum allowed by 23.335. But they don't
have to.

Greg is right. They really ought to have invented
another term for it. Va isn't the maneuvering
speed at all, and should be renamed to something
completely different.

--
Dr. Tony Cox
Citrus Controls Inc.
e-mail:
http://CitrusControls.com/

[Tony Cox]

"Greg Esres" > wrote in message
...
>
> <<Vo does differs a little from pt 23 certification requirements, in
> that Va isn't exactly Vo, because Va calculations assume that airfoil
> lift does scale linearly with AOA and as the square of airspeed
> when in fact these are only approximately true.>>
>
> The only distinction I see between Va and Vo is that Va says "not less
> than" and Vo is "not greater than". Where do you see the distinction
> you are drawing?
>


I assumed that Vo was an actual speed determined by calculation or
flight test.

Va >= Vs.sqrt(n) assumes (in the equality) lift linearity vs. AOA (which
we know drops off near the stall) and a quadratic relationship between
lift and AOA (which is only true for small AOA & probably off by
10% or more close to the stall).

That's why I called the distinction. Nothing profound.

--
Dr. Tony Cox
Citrus Controls Inc.
e-mail:
http://CitrusControls.com/

[Greg Esres]

<<I assumed that Vo was an actual speed determined by calculation or
flight test.>>

Ah. I'm not sure how they determine Vo. They don't specify how it's
to be calculated, and the Part 23 Flight Test guide doesn't say how to
find it experimentally (unlike things like Vmc).

<<a quadratic relationship between lift and AOA (which is only true
for small AOA & probably off by 10% or more close to the stall).>>

I assume you meant "between airspeed and AOA" ?

[Tony Cox]

"Greg Esres" > wrote in message
...
>
> Ah. I'm not sure how they determine Vo. They don't specify how it's
> to be calculated, and the Part 23 Flight Test guide doesn't say how to
> find it experimentally (unlike things like Vmc).

G-meter? Yank the yoke at different speeds on a calm day?

>
> <<a quadratic relationship between lift and AOA (which is only true
> for small AOA & probably off by 10% or more close to the stall).>>
>
> I assume you meant "between airspeed and AOA" ?
>

Yes. slip of the keyboard.

Happy Thanksgiving!

[G.R. Patterson III]

John Gaquin wrote:
>
> An old pilot once told me, when I was a young pilot, "...sumbitch flies a
> hell of a lot better overweight than it does outta gas..."

That postulates a situation in which I those are my only two choices. I'm
betting that I can live my entire life without putting myself in that situation.

George Patterson
A man who carries a cat by the tail learns something that can be learned
no other way.

[Peter Duniho]

"Greg Esres" > wrote in message
...
> Then you haven't read Part 23.

Yes, I have.

> Let me point out the sections to you:

Those sections stipulate, given an existing Va, how the control surfaces
must be designed. They do not in any way define Va.

> The point is that at Part 23 doesn't require this. And not all
> aircraft publish such variations.

You are still hung up thinking that Part 23 is what makes airplanes fly.

> Again, you didn't read what I wrote. I said it doesn't scale UP.
> Flying over max gross may increase maneuvering speed, but it doesn't
> increase VA, because the increased weight won't protect control
> surfaces from failure.

In this context "maneuvering speed" is synonymous with "Va". Yes, you're
right, there is only ONE Va. But in the world of piloting, we commonly
understand the shorthand "Va" to mean "maneuvering speed at a given aircraft
weight" where the weight is changeable.

> Tangent? It's the essence of what Va is.

Not in this context it's not.

> <<I seriously doubt Todd has told you that Va remains the same
> regardless of aircraft weight. You obviously misunderstood him.>>
>
> Ok, you read what he wrote and tell me:

If you think that he was explaining the aerodynamics of maneuvering speed,
you misunderstood him. If you don't think that he was explaining the
aerodynamics of maneuvering speed, then your comments based on that quote
are irrelevant to this thread.

Pete

[Peter Duniho]

"Tony Cox" > wrote in message
hlink.net...
> Greg is right. They really ought to have invented
> another term for it.

That's not what Greg said.

I don't see why a whole thread that is really about aerodynamics needs to be
co-opted by the terminology police. The original question was clear enough
in its intent. The OP isn't asking about how to meet FAA certification
standards. He's asking about over-gross operations and their effect on
airplane performance.

Pete

[Tony Cox]

"Peter Duniho" > wrote in message
...
> "Tony Cox" > wrote in message
> hlink.net...
> > Greg is right. They really ought to have invented
> > another term for it.
>
> That's not what Greg said.
>
> I don't see why a whole thread that is really about aerodynamics needs to
be
> co-opted by the terminology police. The original question was clear
enough
> in its intent. The OP isn't asking about how to meet FAA certification
> standards. He's asking about over-gross operations and their effect on
> airplane performance.
>
> Pete


Well, I'm certainly not the terminology police, and I'm
awfully reluctant to get confrontational, especially with
the holidays about to start.

But there is an important point here, and one that I'd
not appreciated before this discussion. The fact is that
under pt 23, the often-quoted Va speed isn't in fact
the speed at which you can apply full control deflection
without risk of structural failure. Va is determined by
control constraints, and by the requirement that it must
be >= Vs*sqrt(n). This means that it is quite possible
(although, I'd proffer, unlikely) for the POH value of
Va to be above the value where you'd risk exceeding
the load factor.

I think this is an important safety point, unappreciated
by many (and until just recently, by me too).

Cheers & happy holidays.

--
Dr. Tony Cox
Citrus Controls Inc.
e-mail:
http://CitrusControls.com/

[Greg Esres]

<<we commonly understand the shorthand "Va" to mean "maneuvering speed
at a given aircraft weight" where the weight is changeable.>>

Then what is commonly understood is not correct, because that's not
how Va is *necessarily* determined.

<<Not in this context it's not.>>

PARTICULARLY in this context. The control surfaces were designed with
withstand full movements at PUBLISHED Va, not his new, derived, HIGHER
Va.

[Greg Esres]

TC: Greg is right. They really ought to have invented another term
for it.

PD: That's not what Greg said.

GE: Greg said exactly that. He said "They really should have called
it something else, IMO."

<<co-opted by the terminology police.>>

No communication takes place without an agreed-upon vocabulary, which
is why technical disciplines define terms very precisely.

Equating "Va" to "maneuvering speed" is commonly done, but it's
sloppy. You can scale both speeds down with weight, but you can't
scale Va up.

[Rick Durden]

Michael,

In Alaska, Part 135 operators can
> get the max gross raised by up to 15%, depending on the airplane.

Do you have a reference for this?

I've chased it a couple of times but can't find it (I'm assuming I'm
looking in the wrong places); all I've found is for an increase for a
few very old Dept of Commerce or CAA certified airplanes such as
Stinson Trimotors and so forth, nothing modern. If you've got a
reference, I'd appreciate it.

All the best,
Rick

[Paul Cook]

> I began wondering
> about the effects of an overweight takeoff within C.G. limits.

This is impossible. As soon as you go over gross you are out of CofG by
definition.
Paul

[Peter Duniho]

"Greg Esres" > wrote in message
...
> PARTICULARLY in this context. The control surfaces were designed with
> withstand full movements at PUBLISHED Va, not his new, derived, HIGHER
> Va.

There are any number of structural issues raised by overgross flight. It
was clear to me which one he was asking about, and the control surfaces
ain't it.

[Peter Duniho]

"Paul Cook" > wrote in message
...
> > I began wondering
> > about the effects of an overweight takeoff within C.G. limits.
>
> This is impossible. As soon as you go over gross you are out of CofG by
> definition.

I guess that depends on how you define "out of CofG". I define it as not
within the fore and aft limits for the position of the center of gravity.
Under that definition, you certainly can still be within those limits while
being above the maximum gross weight.

I prefer to use the term "out of weight & balance limits" as the general
term for being overweight or out of the CG limits or both. Though, if only
overweight or out of CG limits, I'd actually prefer to just use the term
that actually describes the condition.

Pete

[Andrew Rowley]

"Paul Cook" > wrote:

>This is impossible. As soon as you go over gross you are out of CofG by
>definition.
>Paul

Actually not by definition. CofG (Centre of Gravity) is the balance
part of weight and balance. It is measured in distance from a datum.
There are no weight units in CG.

[Gary L. Drescher]

"Koopas Ly" > wrote in message
om...
> Climbout: same angle of climb for obstacle clearance at higher Vx speed.

Almost. It should be the same *angle of attack* (and higher speed) for best
angle of climb. But the angle of climb itself (and the pitch angle) will be
less than when within max gross weight. Consider the limiting case where
the aircraft is so heavy it can barely climb; the best angle of climb will
then be just slightly above zero.

Similarly, pilots who are not accustomed to high-altitude takeoffs can get
in trouble if they try to set up the same pitch attitudes they're used to at
lower altitudes for best-rate or best-angle climb. Due to the lower angle
of climb at higher density altitudes, commanding the usual pitch altitude
implies a higher angle of attack than usual, and hence a danger of stalling.

--Gary

[EDR]

In article >, Greg Esres
> wrote:

> Ah. I'm not sure how they determine Vo. They don't specify how it's
> to be calculated, and the Part 23 Flight Test guide doesn't say how to
> find it experimentally (unlike things like Vmc).

What is the definition of Vo?
I cannot find a definition of/for it.

[Bob Martin]

> I've got an old Flying Magazine (circa 1970 or so) where one of the editors
> makes the comment that it is better to take off overloaded (with fuel) than
> it is to try a launch with marginal fuel in order to stay under gross. The
> comment was the same... It'll fly better over gross than outta gas.
>
> I bet the magazine's lawyers wouldn't let them print that now...

Don't tell anyone... but I did that almost routinely training for my
PPL. We were flying an old, tired C-150 in a Georgia summer (which
automatically means density altitudes are incredibly high). I weighed
about 180 at the time; my instructor was about 240. We were usually
over gross by about 40-50 lbs, and when flying dual we were lucky to
get 250fpm out of it.

My examiner was even bigger... I had to check the fuel levels and set
it up so that I'd burn enough on the way over to his airport that we'd
be right at gross for the checkride... which meant coming back home
afterwards was cutting it close (but then, it's only an 11 mile
flight).

[Koopas Ly]

Gary,

Thank you for the correction. Everything you wrote makes sense.

Could you please take a peak at my most recent thread entitled "Angle
of climb at Vx and glide angle when "overweight": five questions". I
added a clarification post in response to Mr. Gerry Caron's reply,
within that thread, so I'd appreciate if you could tackle some of my
questions.

Thanks,
Alex


"Gary L. Drescher" > wrote in message
news:<YBnxb.238322$ao4.855590@attbi_s51>...
> "Koopas Ly" > wrote in message
> om...
> > Climbout: same angle of climb for obstacle clearance at higher Vx speed.
>
> Almost. It should be the same *angle of attack* (and higher speed) for best
> angle of climb. But the angle of climb itself (and the pitch angle) will be
> less than when within max gross weight. Consider the limiting case where
> the aircraft is so heavy it can barely climb; the best angle of climb will
> then be just slightly above zero.
>
> Similarly, pilots who are not accustomed to high-altitude takeoffs can get
> in trouble if they try to set up the same pitch attitudes they're used to at
> lower altitudes for best-rate or best-angle climb. Due to the lower angle
> of climb at higher density altitudes, commanding the usual pitch altitude
> implies a higher angle of attack than usual, and hence a danger of stalling.
>
> --Gary

[Koopas Ly]

CJ,

My C172SP POH states that for short/soft field takeoffs with a 50-ft
obstacle, flaps 10 should be used, as well as a climb speed of 56 kts
until obstacle is cleared. Flaps should be retracted after obstacles
are cleared after a safe flap retraction speed (what is that?) of 60
kts. is reached. Pitch for best angle of climb of 62 kts. after that
during the enroute climb, should obstructions again need to be
cleared.

I am guessing that the 56 kts speed is best angle of climb speed for
the flaps 10 configuration, even though that angle is probably less
than the normal 62-knot best angle of climb clean, due to the
parasitic drag induced by the flaps.

From my interpretation of the POH (and the latter doesn't make it
completely clear), if the runway was neither short nor soft, but with
obstacles at the end, I wouldn't use any flaps, lift-off at normal
speed, and pitch for the 62 kts. best angle of climb speed right away.

Thanks for replying,
Alex


"C J Campbell" > wrote in message >...
> |
> | Question (1 of 2): Seems to me that flying "overweight" is possible if
> | you're aware of the performance reductions. So why do you read so
> | many NTSB reports with probable causes listed as "overweight takeoff,
> | exceeded performance limitations"? As you slowly pull the yoke to
> | rotate, wouldn't a pilot *realize* through control forces, feel, gut
> | feeling that something is wrong?
>
> You would not necessarily feel heavier control forces if the airplane was
> trimmed properly. Heavier control forces as you rotate would indicate a
> forward cg, not over weight. You could be grossly over weight and have very
> light control forces if the weight was mostly in the back. Most noticeable
> is that the airplane does not accelerate as quickly as usual. If you are in
> the habit of flying overvweight, you might not notice anything wrong at all.
> Add in a hot day, short runway, and high altitude and suddenly you are going
> to find yourself bitten by bad habits.
>
> |
> | Question (2 of 2): When considering accidents due to exceeding maximum
> | takeoff weight, do the majority occur during takeoff? If so, is it
> | typically due to not reaching proper liftoff airspeed for that
> | increased weight, stalling, and spinning to the ground? Would this
> | scenario be consistent with failure to set the flaps/slats to their
> | takeoff value?
>
> Many airplanes take off from normal runways without flaps. A pilot can
> easily forget to set flaps for short or soft field takeoffs. A lot of pilots
> are also taught just 'plane' wrong. Consider the Cessna 172M, for example.
> Most pilots are taught to set the flaps at 10 degrees for a short field
> takeoff. Most aftermarket checklists tell you to do this, even the ones
> designed for older Cessnas. Surecheck sells checklists that are supposedly
> designed specifically for the 172M but they contain this error.
>
> But read the manual. It tells you that if you set the flaps at 10 degrees
> you will lift off the runway more quickly, but that you will climb more
> slowly and you might not clear an obstacle at the end of the runway. The
> manual says to use 10 degrees of flaps only when the runway is soft or is
> short but there are no obstacles on climbout. But the idea that you use 10
> degrees of flaps to do a short field takeoff is so pervasive that I have had
> train my students in how to educate examiners on this issue.
>
> Newer Cessna 172s use 10 degrees of flaps for all short field takeoffs, so
> when transitioning from one model of Cessna 172 to another, be sure to read
> the manual thoroughly.

[Koopas Ly]

"Bob Gardner" > wrote in message news:<0w5xb.234388$9E1.1274436@attbi_s52>...
> When considering takeoff parameters, don't forget the increased rolling
> resistance of overloaded tires...this will affect acceleration and takeoff
> distance.
>
> Bob Gardner
>


Bob,

Thanks for pointing that out. Never thought about the higher friction
force between the tires and pavement due to the higher weight.

Gracias for your continued contributions,
Alex

[Koopas Ly]

Michael,

Comments below,

Regards,
Alex

(Michael) wrote in message >...
> (Koopas Ly) wrote
> > After reading NTSB reports that attribute the cause of the accident to
> > exceeding the airplane's maximum takeoff weight, I began wondering
> > about the effects of an overweight takeoff within C.G. limits.
> > Specifically, what would I have to do differently when flying an
> > airplane that's heavier than what the POH specifies. I am not
> > supporting the practice, of course, so let it be purely educational.
>
> The first thing you should realize is that airplanes are LEGALLY
> operated overgross all the time. In Alaska, Part 135 operators can
> get the max gross raised by up to 15%, depending on the airplane. For
> long overwater ferry flights, the FSDO will give you a ferry permit to
> operate 20% overgross without so much as blinking, provided you sound
> like you know what you're doing. Just for reference, on a plain
> vanilla C-172 or Cherokee, that would be 400+ lbs overgross.

Thanks for pointing that out.


>
> > I would start by considering the increase in weight as comparable to
> > an increase in load factor. Hence, all your aoa-related speeds would
> > increase by the square root of the load factor. Vs, Vx, Vy, Vglide,
> > etc. would all increase.
>
> So far, so good.
>
> > Va would also go up.
>
> Not necessarily. Everything depends on where the weak spot defining
> Va happens to be. The usual reason Va goes down on an airplane is
> that the weak spot is the engine mount. If the weak spot is the
> engine mount, the weight carried by the engine attach point (the
> engine) is fixed, and thus maximum gee is fixed. Since at lower
> weight you can exceed max gee at a lower speed without stalling, Va
> goes down with weight.
>
> If the weak spot is the wing attach point, then Va is constant. This
> is because the weight carried by that point is NOT fixed. This is a
> pretty common situation in gliders, but pretty rare in airplanes. The
> real issue is this - once you exceed max gross, you don't really know
> where the weak spot is anymore unless you do an engineering analysis.
> Therefore, I would assume Va does not increase.


Whoa! At first, I didn't understand what you meant so I posted
another thread yesterday called "Va: maneuvering speed ad nauseam".
After someone else gave an explantion similar to yours, I re-read your
reply and it's now perfectly clear. Thanks. Why isn't this taught to
pilots when they learn about Va?

>
> > Now, by virtue of rotation speed being a function of stall speed, I
> > conjecture you'd have to liftoff at a faster airspeed which would
> > equate to a longer takeoff roll.
>
> Correct, and this speed may be higher than what the square root of
> weight correction would lead you to believe. Typically we rotate well
> below best rate of climb speed, and count on being able to accelerate
> in ground effect and climb out. However, once you load it up enough,
> you may not have that luxury. You may have to wait until almost Vy
> speed before you have excess power available to accelerate and climb
> out. A too-early rotation may put you in the position of flyng in
> ground effect without being able to accelerate enough to climb out.

In the 172SP, I rotate at 55 kts clean, with a stall speed clean of 48
kts at max. t/o weight. So rotation speed is approx. 1.15*Vs. My
best angle of climb speed at sea level is 74 kts. If I recall
correctly, after I lift the nose up, I instinctively hold the nose
down for a little bit to get some airspeed, after which I pitch up to
my usual attitude to get the 74 kts.

You make it sound like you wouldn't be able to climb before reaching
Vy when overloaded. Assuming that you upped your weight by 25%, you
should still be able to accelerate to your new Vy speed of
sqrt(1.25)*74 = 1.12*74 = 83 kts in ground effect. Granted, you'd
take a while longer to get there. Then you ought to be able to climb
right *after* reaching your stall speed of sqrt(1.25)*48 = 54 kts.
Granted your climb rate will be quite low since 1/ you'd be climbing
at an airspeed much slower than best rate of climb speed (which has
also increased due to the higher weight) and 2/ you'd experience
reduced excess power at that airpseed compared to the normally loaded
airplane flying at that airspeed, but you should still be able to
climb. Indeed, if you're talking about obstacle clearance at the end
of a short runway, you're SOL.


>
> > Then, after pitching for your faster Vy airspeed, you'd notice a
> > decrease in climb rate at full power due to the increased power
> > requirement. During cruise, you'd notice a reduced cruise speed and
> > an increase in stall speed.
>
> All correct.
>
> > At approach to landing, should you bump
> > up your approach speed, you'll find yourself sinking faster when
> > chopping off the power even though your glideslope will remain the
> > same.
>
> Assuming you are still overgross.
>
> > Since your stall speed is invariably higher, you'll eat up more runway
> > when landing.
>
> Maybe. Certainly if you want to minimize use of brakes. However,
> your brakes will be more effective with more weight on wheels - you
> will be able to use them at higher speed without locking them up.
>
> You are also ignoring another important factor - the cg envelope
> shrinks at higher gross weights. Because of this, just because you
> are within cg limits for max gross does not mean you are still within
> cg limits for the increased weight. Usually being forward is not too
> bad - the plane will be nose heavy and will need to be landed at a
> higher speed and/or with power to keep the nose up through the
> touchdown. But if you are close to the aft limit for gross and are
> overgross, beware. You are asking for stability problems in pitch,
> and the plane may be uncontrollable.
>
> > So to sum up:
> >
> > Takeoff: higher takeoff distance, higher rotation speed.
> >
> > Climbout: lower climb rate at higher Vy speed, same angle of climb for
> > obstacle clearance at higher Vx speed. Should Vx not be flown faster,
> > a poorer angle of climb would result, making obstable clearance
> > doubtful. *I may be wrong here* I am not sure if the max. angle of
> > climb is constant regardless of weight...my calculations don't show
> > so...could someone clarify?
>
> Max angle of climb will be reduced at higher weight, and Vx will have
> to be increased.



Could you take a look at my Nov. 26 post entitled "Angle of climb at
Vx and glide angle when "overweight": five questions"?



>
> > Cruise/Maneuvering: lower cruise speed, higher maneuvering speed,
> > higher clean stall speed.
>
> See above with respect to maneuvering speed.
>
> > Approach to maintain glideslope & descent profile: higher approach
> > speed, higher sink rate for a given power setting. Higher dirty stall
> > speed.
> >
> > Landing: higher landing distance
> >
> > Question (1 of 2): Seems to me that flying "overweight" is possible if
> > you're aware of the performance reductions.
>
> Absolutely, it's done all the time - legally and illegally. It's a
> rare piston freight hauler that doesn't routinely operate overgross.
> What you have to realize is that all sorts of safety margins are
> reduced. If you are aware of the reductions, it's not deadly. That's
> why the FAA will give you a permit to operate overgross if you have a
> need - as long as you demonstrate you understand what you're doing.
>
> > So why do you read so
> > many NTSB reports with probable causes listed as "overweight takeoff,
> > exceeded performance limitations"? As you slowly pull the yoke to
> > rotate, wouldn't a pilot *realize* through control forces, feel, gut
> > feeling that something is wrong?
>
> Maybe - but if he's not expecting it, he may not realize it in time.
>
> > Question (2 of 2): When considering accidents due to exceeding maximum
> > takeoff weight, do the majority occur during takeoff? If so, is it
> > typically due to not reaching proper liftoff airspeed for that
> > increased weight, stalling, and spinning to the ground? Would this
> > scenario be consistent with failure to set the flaps/slats to their
> > takeoff value?
>
> In general, the overgross accidents fall into two categories. First
> there are the ones where climb speed is never reached and the plane
> runs out of runway and hits something. Second is when an overgross
> twin loses an engine and can't maintain flight. The first is usually
> the result of the "It's always worked before" factor and the second is
> betting the engines will both keep running.
>
> Michael

[Mike Weller]

On Wed, 26 Nov 2003 10:18:14 -0500, "G.R. Patterson III"
> wrote:


>Seems to me that you have listed most of the effects correctly. One thing you
>should consider, however, is the fact that the balance envelope for most (if
>not all) planes gets narrower at the top. In other words, the more weight you
>put in an aircraft, the closer to the center of lift that weight has to be. At
>some point, all of the weight will have to be in the front seat.
>

Not really. You can put 50 pounds 3 feet in front of the zero cg
datum and 50 pounds 3 feet behind the datum and it is the same as
adding 100 pounds at the datum (front seats I guess).

>I have read of cross-Atlantic ferry flights in which the aircraft was loaded to
>weigh about 1.6 times the normal MGW. In one account, a Bonanza loaded that way
>took over 6,000' to get airborne.

I let students take off at 2000 rpm in a 172. You roll a long way
(very sensitive to temperature) and the climb performance is down
right scary.

>George Patterson

Mike Weller

[G.R. Patterson III]

Mike Weller wrote:
>
> Not really. You can put 50 pounds 3 feet in front of the zero cg
> datum and 50 pounds 3 feet behind the datum and it is the same as
> adding 100 pounds at the datum (front seats I guess).

That would be true enough, except that you can't put 50 pounds 3 feet in front
of the zero cg point in most light singles. That would be where the engine is
in my plane.

George Patterson
Some people think they hear a call to the priesthood when what they really
hear is a tiny voice whispering "It's indoor work with no heavy lifting".

[Michael]

(Rick Durden) wrote
> > In Alaska, Part 135 operators can
> > get the max gross raised by up to 15%, depending on the airplane.
>
> Do you have a reference for this?

You bet. 14CFR91.323

> I've chased it a couple of times but can't find it (I'm assuming I'm
> looking in the wrong places); all I've found is for an increase for a
> few very old Dept of Commerce or CAA certified airplanes such as
> Stinson Trimotors and so forth, nothing modern.

Really modern airplanes are not eligible for this, but a lot of stuff
a whole lot newer than Trimotors is.

> If you've got a reference, I'd appreciate it.

Here you go. A great web site for this stuff is
http://www.access.gpo.gov/nara/cfr/cfrhtml_00/Title_14/14tab_00.html

14CFR91.323 Increased maximum certificated weights for certain
airplanes operated in Alaska.

(a) Notwithstanding any other provision of the Federal Aviation
Regulations, the
Administrator will approve, as provided in this section, an increase
in the
maximum certificated weight of an airplane type certificated under
Aeronautics
Bulletin No. 7-A of the U.S. Department of Commerce dated January 1,
1931, as
amended, or under the normal category of part 4a of the former Civil
Air
Regulations (14 CFR part 4a, 1964 ed.) if that airplane is operated in
the State
of Alaska by --
(1) A certificate holder conducting operations under part 121 or part
135 of
this chapter; or
(2) The U.S. Department of Interior in conducting its game and fish
law
enforcement activities or its management, fire detection, and fire
suppression
activities concerning public lands.
(b) The maximum certificated weight approved under this section may
not exceed
--
(1) 12,500 pounds;
(2) 115 percent of the maximum weight listed in the FAA aircraft
specifications;

(3) The weight at which the airplane meets the positive maneuvering
load factor
requirement for the normal category specified in §23.337 of this
chapter; or
(4) The weight at which the airplane meets the climb performance
requirements
under which it was type certificated.
(c) In determining the maximum certificated weight, the Administrator
considers
the structural soundness of the airplane and the terrain to be
traversed.
(d) The maximum certificated weight determined under this section is
added to
the airplane's operation limitations and is identified as the maximum
weight
authorized for operations within the State of Alaska.
[Doc. No. 18334, 54 FR 34308, Aug. 18, 1989; Amdt. 91-211, 54 FR
41211, Oct. 5,
1989, as amended by Amdt. 91-253, 62 FR 13253, Mar. 19, 1997]

[Michael]

(Koopas Ly) wrote
> Whoa! At first, I didn't understand what you meant so I posted
> another thread yesterday called "Va: maneuvering speed ad nauseam".
> After someone else gave an explantion similar to yours, I re-read your
> reply and it's now perfectly clear. Thanks. Why isn't this taught to
> pilots when they learn about Va?

First, because the average flight instructor can barely handle the
easy parts, never mind this level of complexity. One of my CFI orals
adressed the issue of Va. It went something like this:

"What is Va?"
"It is the maximum airspeed where full deflection of the flight
controls will not overstress the airframe."
"Specifically, which control?"
"The elevator. Va is the maximum speed at which you will stall before
you overstress the airplane. It's the clean stall speed multiplied by
the square root of the maximum positive gee rating for the category
under which the airplane is certified."

That was as far as the examiner wanted to go. Note that neither of
the answers I gave are technically correct. For example, at Va you
can do a full rudder deflection to one side, starting with zero yaw
rate. If you then deflect the rudder to the other stop, you are not
necessarily protected. Therefore, full rudder deflection is not
necessarily permitted. And, of course, if you are rolling as you pull
gees, all bets are off. Therefor, if you are rolling at Va and pull
the elevator all the way back, you risk pulling a wing off. However,
this level of complexity is really beyond the scope of the CFI ride.
Maybe it shouldn't be, but it is. It's certainly well beyond the
complexity of the private ride.

Second, what would be the point? To allow the pilot to competently
asess the consequences of overgross operation? Just because it's done
all the time doesn't mean the FAA will acknowledge the fact and make
it any easier on the pilots. Denial ain't just a river in Egypt.

> You make it sound like you wouldn't be able to climb before reaching
> Vy when overloaded.

Overload enough, and that's exactly what happens. Just because you
are not stalled is not a guarantee you will be able to climb. At some
maximum weight and density altitude, you will only maintain altitude
at one speed, and will descend at any other speed. To be honest, that
would probably require over 100% overload on most airplanes at
reasonable density altitudes, but you certainly can get there.

> Assuming that you upped your weight by 25%, you
> should still be able to accelerate to your new Vy speed of
> sqrt(1.25)*74 = 1.12*74 = 83 kts in ground effect.

Not necessarily.

> Granted, you'd
> take a while longer to get there. Then you ought to be able to climb
> right *after* reaching your stall speed of sqrt(1.25)*48 = 54 kts.

I repeat - just because your airspeed is over stall for your increased
weight does not mean that you have excess power available.

> Granted your climb rate will be quite low since 1/ you'd be climbing
> at an airspeed much slower than best rate of climb speed (which has
> also increased due to the higher weight) and 2/ you'd experience
> reduced excess power at that airpseed compared to the normally loaded
> airplane flying at that airspeed, but you should still be able to
> climb.

I repeat - not necessarily. For the purposes of this exercise, assume
the plane has a constant speed prop (we will deal with the
complexities introduced by a fixed pitch prop later). For all
practical purposes, that means the power available over a reasonable
airspeed range is constant. At best rate airspeed (Vy), climb rate
will be a maximum. Going to either side of best rate will reduce the
climb rate. Your boundaries are Vs and Vne.

Most airplanes are not so overpowered that they have sufficient power
to climb at Vne (though I know of at least one exception, a highly
modified King Air used to haul jumpers). Most are also not so
underpowered that they have insufficient power to climb at Vs, but I
know of some exceptions there too. However, if you overload enough,
you WILL eventually reach a point where sufficient power to climb at
Vs (or 1.05 Vs, or 1.1 Vs) will simply not be available.

Is it possible to become airborne in that situation? Yes it is. If
you are rolling on a smooth surface, on good tires, with the elevator
neutral and the wings at zero angle of attack or close to it, the drag
forces on a grossly overloaded airplane will not be much greater than
those on a properly loaded one. Most of the drag at typical rotation
speeds will come from parasitic drag (since the wings are at zero
angle of attack, and thus not producing lift), with some contribution
from the weight on the wheels. Parasitic drag is quite low close to
stall speed, and normally rotation speed is close to stall speed. The
increased weight on the wheels hurts some, but not much. The
increased weight will mean greater inertia, which will slow the
initial acceleration, but given sufficient runway you will easily
reach rotation speed. When you rotate, what really happens is that
you lower the tail, immediately increasing the angle of attack on the
wings. This will provide immediate lift, and you will become airborne
in ground effect. Now what?

You have eliminated the weight on the wheels, and thus reduced drag a
little. However, you have added lots of induced drag, because now the
wings are supporting the weight at low airspeed and thus a large lift
coefficient. Your total drag an inch over the runway is MUCH greater
than it is on the runway. At the airspeed you are flying, you have
insufficient power to maintain altitude and airspeed. Either you will
lower the nose (impacting the runway) or you will let the speed bleed
off (settling back onto the runway). Or you may have just enough
power to maintain altitude and airspeed in ground effect, but nothing
extra. Now you keep flying until you hit an obstacle.

A fixed pitch prop actually makes the situation worse. At lower
airspeeds, the prop can't spin fast enough to allow the engine to
generate full power. Thus power available decreases with decreasing
airspeed, and you're even less likely to have excess power available
to climb or accelerate.

> Could you take a look at my Nov. 26 post entitled "Angle of climb at
> Vx and glide angle when "overweight": five questions"?

I have, and really don't have much to add there unless you have a
specific question.

Michael