Overweight takeoff / flight

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

On 2003-11-26 05:23:36 -0800, (Koopas Ly) said:

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.

Since your stall speed is invariably higher, you'll eat up more runway
when landing.

In both cases, it's not just the higher speeds involved, but the increased weight
also means that acceleration for takeoff and deceleration on landing will be
lower. F=ma with a (more or less) constant F, and increased m means
decreased a.

--
Larry Fransson
Seattle, WA

In article , Koopas Ly
wrote:


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


Once you have burned sufficient fuel to bring the weight back down into
the max gross/useful load range, why should landing parameters be any
different from published?
Published numbers are for max gross weight.

Koopas Ly wrote:

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.

Seems to me that you have listed most of the effects correctly. One thing you
should consider, however, is the fact that the balance envelope for most (if
not all) planes gets narrower at the top. In other words, the more weight you
put in an aircraft, the closer to the center of lift that weight has to be. At
some point, all of the weight will have to be in the front seat.

I have read of cross-Atlantic ferry flights in which the aircraft was loaded to
weigh about 1.6 times the normal MGW. In one account, a Bonanza loaded that way
took over 6,000' to get airborne.

George Patterson
A man who carries a cat by the tail learns something that can
be learned no other way.

An old pilot once told me, when I was a young pilot, "...sumbitch flies a
hell of a lot better overweight than it does outta gas..."

JG

"Koopas Ly" wrote in message
om...

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.



I take issue with Va. At first thought, it should go up as sqrt( m/m0)
with m the new weight and m0 the maximum gross at which Va is
quoted. This since at a higher Va, we can maintain the same AOA
as we did at m0, so the G forces at stalling AOA never exceed the
design limitations.

BUT, there are 2 things (at least) which contribute to the setting of
Va in the first place. One is the limitation of 'heavy things in the plane',
such as a bag of sand in the baggage compartment. If this is the limiting
factor, then Va should indeed scale as sqrt(m/m0). However, there is
also the 'torque on the wings' (low wings) or 'force on the wings' (struts
on Cessna). If you are pulling 3.5G with a higher gross weight, you'll
be exerting more force than was designed for at certified gross.

So to be safe (hah!, we're talking about overloading dammit), then
unless you know exactly which type of failure limits Va in the first
place, you'd be best off using Va for certified gross and not scaling
it up.

--
Dr. Tony Cox
Citrus Controls Inc.
e-mail:
http://CitrusControls.com/

BUT, there are 2 things (at least) which contribute to the setting
of Va in the first place.

Neither one of the things you mentioned is given in Part 23 as a
requirement for Va. Part 23 uses the speed solely to provide the
design requirements of the elevators, ailerons, and rudder.

Since the forces on these control surfaces will not vary with weight,
you certainly can't scale it up.

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

|
| 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.

In article , Greg Esres
wrote:

BUT, there are 2 things (at least) which contribute to the setting
of Va in the first place.

Neither one of the things you mentioned is given in Part 23 as a
requirement for Va. Part 23 uses the speed solely to provide the
design requirements of the elevators, ailerons, and rudder.

Since the forces on these control surfaces will not vary with weight,
you certainly can't scale it up.

G-forces are directly related to weight. Since the size of the control
surface is directly related to the forces exerted on it, control
surfaces are dependent on weight.