Max gross weight

Chris opined

How do aircraft manufactures determine the maximum gross weight for a
given plane? I know I've come across articles that mention on a cool day
you can go over the max. gross weight by a percentage, I think 20% was
thrown out there but not sure. Then again, on a hot day you might not
get any where near that. Is it determined by a standard temp?

The maximum weight is determined by the one of the following, roughly
speaking;

structural strength, nothing breaks before 6gs

climb rate at sea level in take off configuration

climb rate at sea level in landing configuration

stall speed, landing configuration

others that I am not aware of.


At some point adding more weight will cause an aircraft to fail one or more
of the FAAs minimum performance standards.

Changing parts of an aircraft can change the gross weight. Extra fuel can be
put in tip tanks, reducing flap travel can increase gross weight.


-ash
Cthulhu for President!
Why vote for a lesser evil?

In article ,
"Jim Carter" wrote:

Isn't there a 15% tolerance allowed for certain types of flights in Alaska?


Part 135 only.

--
Dale L. Falk

There is nothing - absolutely nothing - half so much worth doing
as simply messing around with airplanes.

http://home.gci.net/~sncdfalk/flying.html

Chris wrote in message .. .
How do aircraft manufactures determine the maximum gross weight for a
given plane? I know I've come across articles that mention on a cool day
you can go over the max. gross weight by a percentage, I think 20% was
thrown out there but not sure. Then again, on a hot day you might not
get any where near that. Is it determined by a standard temp?

You dont´t know what is the reason for the maximum gross weight
(unless you know the designers). It is dangerous to assume that
take-off performance defines MTOW. MTOW is also the weight that has
been used as the "most critical weight" during the whole design. It
means that MTOW is there when you calculate the structural loads.
There is a reason why it is called _maximum_ take-off weight.

"Dale" wrote in message ...
In article ,
"Jim Carter" wrote:

Isn't there a 15% tolerance allowed for certain types of flights in Alaska?


Part 135 only.

And even then it's not just an "automatic" increase.

"Chris" wrote in message ...
How do aircraft manufactures determine the maximum gross weight for a
given plane?

They pick a number and then verify that it is OK through a number of tests.
There are a lot of requirements for aircraft to do "this or that" at various weights
up to gross.

For example, the Navion can be approved for various gross weights, some of
it is climb horsepower related. If you have the stronger gear and a bigger engine
you get a 100 lb increase, etc... Some of it is structural in cruise. Some of it
involves sturctures for landing. etc...

I know I've come across articles that mention on a cool day
you can go over the max. gross weight by a percentage, I think 20% was
thrown out there but not sure

Burn whatever said that. If it were purely a performance issue, that would be
true. But that's not necessarily the case. Note that in the above example,
the two limiting factors on the Navion are performance AND the landing gear.
A cold day might give you a bit more performance but it isn't going to make
the landing gear any stronger.

"John Harlow" wrote in message ...
Chris wrote:
How do aircraft manufactures determine the maximum gross weight for a
given plane? I know I've come across articles that mention on a cool
day you can go over the max. gross weight by a percentage, I think
20% was thrown out there but not sure. Then again, on a hot day you
might not get any where near that. Is it determined by a standard
temp?

Which articles suggest it is OK to fly a plane over gross?

AC 21-4b special flight permits for operation of overwieght aircraft

for one example.

I do believe there is are waivers for operations in Alaska and for
operations with floats as well but I haven't found the documentation
to prove it.

That being said and having spent 10 years involved in aircraft design
I can easily say that Manufactuers determine Gross weight based on a
number of factors. Primarly the limiting factors are minimum aircraft
performance, Cost and Reasonable aircraft Structures.

Many times the Gross Wieght is determined by the Maximum allowable
stall speed for the aircraft.

Say I were to design a 1000 lb airplane stressed for acrobatics
(6G's). Lets say the wings weigh 200 lbs so the non lifting weight is
800lbs. To redesign this same airplane into a Less than Normal
Category airplane(just for easy numbers) (3G's) all I have to do is
change the manual to limit it to 3g's and I can double the non-lifting
weight of the aircraft so the new Gross wieght becomes 1800lbs. Likely
the only part of the aircraft that has not been designed to handle
1800 lbs at 3G's is the landing Gear which was designed for only
1000lbs on the ground and for landing.
Of course the take off and landing distances will increase with the
new gross weight as will nearly all performance speed except Vne.

Common Myths:

Aircraft are required to be designed to 1 -1/2 times the rated load,
so there is lots of safety factor built in.
Actually: All that is required is that aircraft be designed to
repeatly handle the rated load without damaging the structure.
Additionally the structure must not fail if subject to less then 1-1/2
times the rated load, However the strucure may be damaged if the rated
load is exceeded. Wo if I take my aluminum airplane rated at 6G's and
do an 8G pull out the wings will probably not fall off. However it
will probably bend the wings and leave them permantly bent. I have
seen a C-182 that this happened to. Aluminum makes a great material
for aircraft because it complies almost exactly to the regulation. You
can load it up to the limit load with no permanent damage. And you can
load it up to the ultimate load without it breaking however it will
permanently bend it.

Myth#2. Manufactures overbuild the aircraft.
Actually: a former Cessna Engineer told me that the way they
designed a structure was they would have the engineer design the
structure they would then load test it. If the structure did not fail
on the 1st test they fired the engineer for overbuilding it. After it
failed the reeinfored the area that failed and tested it again. They
would repeat this process until the structure just barely passed.

Every lb the manufacturer could take off of the airplane is 1 more lb
of performance they could sell and 1 lb less of material they had to
buy. Cessna didn't paint their airplanes for years in order to save
the weight of the paint. The only reason they changed is they could no
longer get consistantly unscratched aluminum to work with.

Brian

Brian

AAC/USAF did the same thing. WWII and prior birds were built hell for
stout (over built)

After War they started building light as they could and test flew
until they broke and just fixed that part. They ended up with the best
performing A/C at the lowest gross weight.

Big John



On 29 Sep 2004 11:49:20 -0700, (Brian Case) wrote:

"John Harlow" wrote in message ...
Chris wrote:
How do aircraft manufactures determine the maximum gross weight for a
given plane? I know I've come across articles that mention on a cool
day you can go over the max. gross weight by a percentage, I think
20% was thrown out there but not sure. Then again, on a hot day you
might not get any where near that. Is it determined by a standard
temp?

Which articles suggest it is OK to fly a plane over gross?

AC 21-4b special flight permits for operation of overwieght aircraft

for one example.

I do believe there is are waivers for operations in Alaska and for
operations with floats as well but I haven't found the documentation
to prove it.

That being said and having spent 10 years involved in aircraft design
I can easily say that Manufactuers determine Gross weight based on a
number of factors. Primarly the limiting factors are minimum aircraft
performance, Cost and Reasonable aircraft Structures.

Many times the Gross Wieght is determined by the Maximum allowable
stall speed for the aircraft.

Say I were to design a 1000 lb airplane stressed for acrobatics
(6G's). Lets say the wings weigh 200 lbs so the non lifting weight is
800lbs. To redesign this same airplane into a Less than Normal
Category airplane(just for easy numbers) (3G's) all I have to do is
change the manual to limit it to 3g's and I can double the non-lifting
weight of the aircraft so the new Gross wieght becomes 1800lbs. Likely
the only part of the aircraft that has not been designed to handle
1800 lbs at 3G's is the landing Gear which was designed for only
1000lbs on the ground and for landing.
Of course the take off and landing distances will increase with the
new gross weight as will nearly all performance speed except Vne.

Common Myths:

Aircraft are required to be designed to 1 -1/2 times the rated load,
so there is lots of safety factor built in.
Actually: All that is required is that aircraft be designed to
repeatly handle the rated load without damaging the structure.
Additionally the structure must not fail if subject to less then 1-1/2
times the rated load, However the strucure may be damaged if the rated
load is exceeded. Wo if I take my aluminum airplane rated at 6G's and
do an 8G pull out the wings will probably not fall off. However it
will probably bend the wings and leave them permantly bent. I have
seen a C-182 that this happened to. Aluminum makes a great material
for aircraft because it complies almost exactly to the regulation. You
can load it up to the limit load with no permanent damage. And you can
load it up to the ultimate load without it breaking however it will
permanently bend it.

Myth#2. Manufactures overbuild the aircraft.
Actually: a former Cessna Engineer told me that the way they
designed a structure was they would have the engineer design the
structure they would then load test it. If the structure did not fail
on the 1st test they fired the engineer for overbuilding it. After it
failed the reeinfored the area that failed and tested it again. They
would repeat this process until the structure just barely passed.

Every lb the manufacturer could take off of the airplane is 1 more lb
of performance they could sell and 1 lb less of material they had to
buy. Cessna didn't paint their airplanes for years in order to save
the weight of the paint. The only reason they changed is they could no
longer get consistantly unscratched aluminum to work with.

Brian

"C J Campbell" wrote in message
...

"BTIZ" wrote in message
news:uYq6d.114$mS1.39@fed1read05...

"Chris" wrote in message
...
I know I've come across articles that mention on a cool day
you can go over the max. gross weight by a percentage, I think 20% was
thrown out there but not sure.

You should NEVER go over max GW.. that is a limit, not a buffer zone.

Don't ever tell that to the ferry pilots.


Understood CJ... but don't ferry pilots include the waiver for GW operations
as part of the paperwork for the extra fuel tanks?

BT

"BTIZ" wrote in message but don't ferry pilots include the waiver for GW
operations as part of the paperwork for the extra fuel tanks?

Only if it's needed, and the FAA usually will approve 10% over if they
approve it at all.

D.

Thanks, its been many years since I flew out of Boeing Field and up the
coast, so I wasn't sure that kind of thing was still allowed up there.

--
Jim Carter
"Ron Natalie" wrote in message
m...

"Dale" wrote in message
...
In article ,
"Jim Carter" wrote:

Isn't there a 15% tolerance allowed for certain types of flights in
Alaska?


Part 135 only.

And even then it's not just an "automatic" increase.