Overweight takeoff / flight

"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

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?

"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

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

"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

|
| 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.

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.

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 . com...
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

(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

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