The danger of assumption

[Jay Honeck]

The other night, as we approached rotation speed, we skipped half a dozen
times before breaking cleanly. I made some snide remark about this to Mary,
and assumed that she was holding the nose down too long, causing the skip.

"Poor piloting," I harrumphed to myself... "Danged tires are too expensive
to be mistreated this way," I silently grumbled... (This was the night I
was already in a ****y mood, so everything that had transpired to this point
was fitting into my crappy view of the world at the time...)

Ten minutes later, with me in the left seat, I advanced the throttle
smoothly, reached rotation speed, started to gently pull back -- and skipped
half a dozen times down the runway before breaking cleanly. Dang.

Needless to say, of course, Mary pointed this out to me in blunt and certain
terms... :-)

Here I had assumed that she was holding it down too long before rotation --
but it had now become apparent that she (and I) were in fact rotating
prematurely. Wind conditions were calm, temperatures were in the 80s,
humidity was very high, and a ground fog was developing as we landed.
Otherwise, everything was done according to Hoyle, with 2 notches of flaps
set for take-off.

Usually the plane just "flies itself off" the runway in this
configuration -- but not that night. Conditions of flight were fairly
unusual, for us -- the back seat was empty, no wind, high humidity, fairly
light on fuel -- so I suppose it was just pilot error.

It seems odd, however, that we *both* made the same mistake, which leads me
to wonder if there was something atmospheric going on...

Thoughts?
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"

[Paul kgyy]

Might have been an air density change just above the ground? As ground
fog creeps in, the air right at the surface has a higher relative
humidity, and maybe a temperature change as well.

[Jim Burns]

I think you were simply lighter than you are used to being.

> Conditions of flight were fairly
> unusual, for us -- the back seat was empty, fairly
> light on fuel

The airplane was ready to fly before the pilots were expecting it to be.

Jim

[N93332]

"Jay Honeck" > wrote in message
news:HLAVe.346894$xm3.194258@attbi_s21...
> Ten minutes later, with me in the left seat, I advanced the throttle
> smoothly, reached rotation speed, started to gently pull back -- and
> skipped half a dozen times down the runway before breaking cleanly.
> Dang.
>
> Usually the plane just "flies itself off" the runway in this
> configuration -- but not that night. Conditions of flight were fairly
> unusual, for us -- the back seat was empty, no wind, high humidity, fairly
> light on fuel -- so I suppose it was just pilot error.
>
> It seems odd, however, that we *both* made the same mistake, which leads
> me to wonder if there was something atmospheric going on...
>
> Thoughts?

You too??? I think it usually happens to me when there is less fuel and
weight on board than usual and possibly other factors such as humidity,
temperature, technique, pilot error, etc. When it happens to me, I'll do
some touch and goes (or stop and goes) and try it again to see if it
repeats; sometimes it does and sometime it doesn't so I'm not really sure
why...

-Greg B.

[Jay Honeck]

> I think you were simply lighter than you are used to being.

Possibly, but...

> > Conditions of flight were fairly
> > unusual, for us -- the back seat was empty, fairly
> > light on fuel
>
> The airplane was ready to fly before the pilots were expecting it to be.

Actually, the plane was NOT ready to fly before we were expecting it to
be. In fact, just the opposite -- we both rotated prematurely, BEFORE
the plane was ready to fly, and it just skipped down the runway a few
times before finally achieving a positive rate of climb.

That's what's a bit confusing about this scenario. If this were a
"lighter than normal" thing, we should have lifted off *earlier* than
expected, not later -- right?
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation

[Icebound]

"Jay Honeck" > wrote in message
news:HLAVe.346894$xm3.194258@attbi_s21...
> The other night, as we approached rotation speed, we skipped half a dozen
> times before breaking cleanly.


1. If there has been a night inversion already starting to develop, your
own motion through the air is causing turbulence and breaking down the
inversion, bringing down air of a different temperature and wind structure.
The conditions that you had while standing still on the apron, are not
exactly the conditions that you are experiencing while hurtling down the
centre line. Whether your AI is reacting quickly enough to the changing
conditions is probably open to debate.

2. A small corollary of 1. If the surface winds were calm, did you check a
recent surface weather chart to see the *likely* wind aloft? If the
pressure configuration indicated a tailwind, the combination of 1 and 2
might result in your symptom.

3. Even if 1 and 2 were not the case, calm winds still have to be watched
carefully. They easily switch to a small tailwind with no notice, which
could make the difference for a light (and lightly-loaded) aircraft. I make
a point to rotate a few knots higher in calm winds just to compensate for
the possibility of a sudden tailwind gust.

[RK Henry]

On Tue, 13 Sep 2005 13:47:19 GMT, "Jay Honeck"
> wrote:

>Usually the plane just "flies itself off" the runway in this
>configuration -- but not that night. Conditions of flight were fairly
>unusual, for us -- the back seat was empty, no wind, high humidity, fairly
>light on fuel -- so I suppose it was just pilot error.

It might be worth considering the possibility that something about the
rigging has changed. An extra-careful preflight or even a trip to the
shop may be in order. If someone has backed a truck into it while it
was parked, the damage might not be immediately obvious.

I've had dragging brakes produce interesting effects when breaking
ground.

RK Henry

[Jim Burns]

Oops, sorry, I read that wrong. Too many allergy meds increaseing the
density altitude in my head.

What indicated airspeed do you normally rotate at? (no real answer, just
curious) Take off with the twin requires us to hold it onto the runway
until 80mph, we definately do not want to rotate prematurely in case of an
engine failure.
Jim

[Trent Moorehead]

"Jay Honeck" > wrote in message
news:HLAVe.346894$xm3.194258@attbi_s21...

> It seems odd, however, that we *both* made the same mistake, which leads
me
> to wonder if there was something atmospheric going on...
>
> Thoughts?

My first thought was that perhaps you have less power than you usually do,
causing you to hang at the rotation speed a little longer than usual. Did
you rotate by the same point as you normally do? Other things: Mag check
normal? Normal RPM on takeoff run?

-Trent
PP-ASEL

[Mortimer Schnerd, RN]

Jim Burns wrote:
> What indicated airspeed do you normally rotate at? (no real answer, just
> curious) Take off with the twin requires us to hold it onto the runway
> until 80mph, we definately do not want to rotate prematurely in case of an
> engine failure.


The behavior of aircraft at rotation vary quite a bit. It's been my experience
you can rotate a Cessna at any speed above stall, roll along on the mains with
the nose up for a few moments, and then the aircraaft will lift off when it's
ready. Makes for a pretty airline style departure (until you're about an inch
off the ground).

Cherokees are the exact opposite. If you try to rotate before it's ready to
fly, you just skip along until the aircraft is finally ready. I've always
preferred to hold of rotation until I've acheived flying speed, then pull it off
all at once. It's not as elegant but it does avoid the nosewheel skipping down
the runway.

I haven't flown enough Beechcraft to comment on them. Twins I do know,
however... and I'm with you on that one. I generally hold off rotation until
well above Vmc but before blue line. The closer to blue line the happier I am.



--
Mortimer Schnerd, RN

[[email protected]]

"Jay Honeck" > wrote:
> Here I had assumed that she was holding it down too long before rotation --
> but it had now become apparent that she (and I) were in fact rotating
> prematurely. Wind conditions were calm, temperatures were in the 80s,
> humidity was very high, and a ground fog was developing as we landed.
> Otherwise, everything was done according to Hoyle, with 2 notches of flaps
> set for take-off.
>
> Usually the plane just "flies itself off" the runway in this
> configuration -- but not that night. Conditions of flight were fairly
> unusual, for us -- the back seat was empty, no wind, high humidity, fairly
> light on fuel -- so I suppose it was just pilot error.

Is there a reason why you *shouldn't* rotate at the published rotation
speed? ... more precisely, is there a reason why continuing the takeoff
roll beyond the published rotation speed before lifting off is not safe
(assuming you have enough runway ahead of you)?

In the C152, rotating at the published rotation speed nearly always
produced nice, smooth takeoffs. I recently bought a Shinn/Varga (low
wing, tandem, stick, trike), and in early flights, rotating at the
published rotation speed often produced a couple of seconds of "can't
decide if I'm really ready to take off and start climbing" hesitancy on
the part of the airplane (and yes, everything checked out during run-up).

We then tried letting it continue the takeoff roll beyond published
rotation speed, not really "rotating", but pulling back just enough to
take the weight off the nosewheel and letting it lift off in its own
time. Presto, all the takeoffs have been nice and smooth ever since, and
once it lifts off, there is NO hesitation to begin the climb. How far
beyond published rotation speed it lifts off, using this technique,
varies depending on temp, humidity and weight.

Other than runway length concerns, is there a reason why this technique
would not be preferable to abruptly rotating at the published rotation
speed?

[Jim Burns]

> Other than runway length concerns, is there a reason why this technique
> would not be preferable to abruptly rotating at the published rotation
> speed?

Particularly with low wing airplanes, this can produce a wheelbarrowing
effect where your mains get light, and even lift off, but you are holding
the nosewheel on the ground. Not good for the nose gear and any crosswind
gust could produce some rather interesting and dangerous effects.

Jim

[john smith]

In article >,
"Jim Burns" > wrote:

> > Other than runway length concerns, is there a reason why this technique
> > would not be preferable to abruptly rotating at the published rotation
> > speed?
>
> Particularly with low wing airplanes, this can produce a wheelbarrowing
> effect where your mains get light, and even lift off, but you are holding
> the nosewheel on the ground. Not good for the nose gear and any crosswind
> gust could produce some rather interesting and dangerous effects.

With two notches of flaps and two adults in the front seats, your center
of lift was way aft and your center of gravity was way forward. Think
about it.

[Mortimer Schnerd, RN]

Jim Burns wrote:
>> Other than runway length concerns, is there a reason why this technique
>> would not be preferable to abruptly rotating at the published rotation
>> speed?
>
> Particularly with low wing airplanes, this can produce a wheelbarrowing
> effect where your mains get light, and even lift off, but you are holding
> the nosewheel on the ground. Not good for the nose gear and any crosswind
> gust could produce some rather interesting and dangerous effects.


I think you missread what he wrote: not rotating is a far cry from holding it
down. When I accelerate in a Cherokee, I hold the yoke neutral until I'm ready
to fly, then rotate and fly off immediately. In a Cessna, I ease the yoke back
once I'm at or beyond stall speed and let it fly off when it's ready.... rolling
on the mains only until it is.

I would agree holding it down is a poor practice.



--
Mortimer Schnerd, RN

[Jay Honeck]

> What indicated airspeed do you normally rotate at? (no real answer, just
> curious) Take off with the twin requires us to hold it onto the runway
> until 80mph, we definately do not want to rotate prematurely in case of an
> engine failure.

We usually start to rotate (and that's not really what we're doing -- more
like applying mild back pressure) around 70 mph -- but that's not a hard and
fast figure. It all depends on "feel", in my experience -- which, in turn,
is based on weight, wind, temperature, etc.
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"

[Morgans]

"Jay Honeck" > wrote

> We usually start to rotate (and that's not really what we're doing -- more
> like applying mild back pressure) around 70 mph -- but that's not a hard
and
> fast figure. It all depends on "feel", in my experience -- which, in
turn,
> is based on weight, wind, temperature, etc.

Ground fog forms because the ground is cooling the air near it (due to
radiational cooling) faster than the air further from it. You already knew
that, though.

MY guess what happen, is that after you started to rotate, the warmer air
above started getting pushed down into the cooler air, warmed it up, (or you
got into the warmer air) gave you less lift than the cool ground air, then
you started not lifting so good. Just a guess, though.
--
Jim in NC

[Jay Honeck]

> It might be worth considering the possibility that something about the
> rigging has changed. An extra-careful preflight or even a trip to the
> shop may be in order. If someone has backed a truck into it while it
> was parked, the damage might not be immediately obvious.

Everything checked out normally before flight.

> I've had dragging brakes produce interesting effects when breaking
> ground.

Low tire pressure changes things pretty dramatically in our plane.
Take-off performance suffers a surprising amount, for one thing.
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"

[Jay Honeck]

> My first thought was that perhaps you have less power than you usually do,
> causing you to hang at the rotation speed a little longer than usual. Did
> you rotate by the same point as you normally do? Other things: Mag check
> normal? Normal RPM on takeoff run?

Yep, everything checked out during preflight and run-up. The JPI EDM-700
engine analyzer showed 6 good bars (meaning all six cylinders were firing
normally) manifold pressure was 26-27, and RPM was nailed right at 2650 or
so at full throttle.
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"

[Jay Honeck]

> MY guess what happen, is that after you started to rotate, the warmer air
> above started getting pushed down into the cooler air, warmed it up, (or
> you
> got into the warmer air) gave you less lift than the cool ground air, then
> you started not lifting so good. Just a guess, though.

Man, that would require a layer just a few feet thick -- is that possible?

Actually, I know it is, as I've seen it at altitude. I suppose it can form
close to the ground, too, in a calm wind.

Strange how I can study weather in college, I can observe it carefully for
years, and I can spend so much time in the air -- and yet not understand it
very well at all.
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"

[[email protected]]

Jim Burns wrote:
> > Particularly with low wing airplanes, this can produce a wheelbarrowing
> > effect where your mains get light, and even lift off, but you are holding
> > the nosewheel on the ground. Not good for the nose gear and any crosswind
> > gust could produce some rather interesting and dangerous effects.

"Mortimer Schnerd, RN" wrote:
> I think you missread what he wrote: not rotating
> is a far cry from holding it down. When I accelerate
> in a Cherokee, I hold the yoke neutral until I'm
> ready to fly, then rotate and fly off immediately.
> In a Cessna, I ease the yoke back once I'm at or
> beyond stall speed and let it fly off when it's ready....
> rolling on the mains only until it is.

He did misread what *she* wrote. I was not referring to "holding it
down", I meant pulling back *just enough* to get the weight off the
nosewheel (not enough to pull the nosewheel up), letting the nosewheel
and the airplane lift off the runway when it's ready, as you said,
rolling on the mains until it does. Is there a reason (other than runway
length) NOT to do this vs. abruptly rotating it off the runway at the
published rotation speed? The airplane performs better (no second or two
of hesitation before beginning the climb) with the former than with the
latter technique ... yet some pull the airplane up when they see the ASI
reach the published rotation speed even if the airplane doesn't act
ready to begin climbing. What say you?

[Jim Burns]

> Man, that would require a layer just a few feet thick -- is that possible?
>

Yep, we get it here often. You can stand in it and your head will stick
above it and your feet will be below it. Pretty freaky.

Jim

[Mortimer Schnerd, RN]

wrote:
> He did misread what *she* wrote. I was not referring to "holding it
> down", I meant pulling back *just enough* to get the weight off the
> nosewheel (not enough to pull the nosewheel up), letting the nosewheel
> and the airplane lift off the runway when it's ready, as you said,
> rolling on the mains until it does. Is there a reason (other than runway
> length) NOT to do this vs. abruptly rotating it off the runway at the
> published rotation speed? The airplane performs better (no second or two
> of hesitation before beginning the climb) with the former than with the
> latter technique ... yet some pull the airplane up when they see the ASI
> reach the published rotation speed even if the airplane doesn't act
> ready to begin climbing. What say you?


The airplane probably hasn't read the book. If you can get the weight on the
mains without the nosewheel skipping up and down, go for it irregardless of
published rotation speed. The aircraft will fly when she's ready. I don't
particularly care what speed she lifts off at... but I will attempt to
immediately accelerate to best rate.

I'm as bit pickier about numbers in multiengine aircraft, but that's another
story....



--
Mortimer Schnerd, RN

[[email protected]]

"Mortimer Schnerd, RN" > wrote:
> The airplane probably hasn't read the book. If you can get the weight on the
> mains without the nosewheel skipping up and down, go for it irregardless of
> published rotation speed. The aircraft will fly when she's ready. I don't
> particularly care what speed she lifts off at... but I will attempt to
> immediately accelerate to best rate.

Agreed.
I just wondered if there was some reason I was unaware of why this
technique is not as safe, or if there is a reason why the airplane
should be rotated as close to the published rotation speed as possible.
The technique described above certainly results in takeoffs that *feel*
much more solid and safe than an abrupt rotation at some specific number.

Thanks for the input.

[[email protected]]

>
> With two notches of flaps and two adults in the front seats, your center
> of lift was way aft and your center of gravity was way forward. Think
> about it.

I've never flown a Cherokee 235 before, but in the 140 thru 180 models,
but on a paved runway, leaving the flaps fully retracted during the
takeoff roll and kicking in the first notch at rotation always made for
a very good-feeling solid takeoff. I prefer to begin rotation at Vx to
ensure plenty of flying speed. The Cherokees don't seem to eager to
leave the ground until you've got that much airspeed anyway.

[Jay Honeck]

> I've never flown a Cherokee 235 before, but in the 140 thru 180 models,
> but on a paved runway, leaving the flaps fully retracted during the
> takeoff roll and kicking in the first notch at rotation always made for
> a very good-feeling solid takeoff. I prefer to begin rotation at Vx to
> ensure plenty of flying speed. The Cherokees don't seem to eager to
> leave the ground until you've got that much airspeed anyway.

Well, today we flew to Davenport, IA. A cold front had come through last
night, and all of that thick, hot, juicy air is back down south (where it
can *stay*), leaving behind cool, crisp, clean, lovely Canadian air in its
wake.

Wow, what a difference! It's like having 100 extra horsepower! We
literally jumped off the runway after an incredibly short roll, and were
soon climbing out at 1500 fpm. No skipping down the runway this time, even
though the weight and configuration were exactly the same.

Of course, with everything all stirred up by the frontal passage, the air
was rough as a cob, but visibility was a zillion miles. Once we got to
altitude it was smooth and drop-dead, take-your-breath-away beautiful.
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"

[Mortimer Schnerd, RN]

Jay Honeck wrote:
> Of course, with everything all stirred up by the frontal passage, the air
> was rough as a cob, but visibility was a zillion miles. Once we got to
> altitude it was smooth and drop-dead, take-your-breath-away beautiful.


Don't take this the wrong way, but you suck. <G>




--
Mortimer Schnerd, RN

[Jay Honeck]

> Don't take this the wrong way, but you suck. <G>

Thanks!

:-)
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"