Overweight takeoff / flight

Michael,

Comments below,

Regards,
Alex

(Michael) wrote in message om...
(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

(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