The danger of assumption

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"

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.

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

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

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

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

VE

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"

"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" 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" 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.