Overweight takeoff / flight

"Koopas Ly" wrote in message
om...
Climbout: same angle of climb for obstacle clearance at higher Vx speed.

Almost. It should be the same *angle of attack* (and higher speed) for best
angle of climb. But the angle of climb itself (and the pitch angle) will be
less than when within max gross weight. Consider the limiting case where
the aircraft is so heavy it can barely climb; the best angle of climb will
then be just slightly above zero.

Similarly, pilots who are not accustomed to high-altitude takeoffs can get
in trouble if they try to set up the same pitch attitudes they're used to at
lower altitudes for best-rate or best-angle climb. Due to the lower angle
of climb at higher density altitudes, commanding the usual pitch altitude
implies a higher angle of attack than usual, and hence a danger of stalling.

--Gary

In article , Greg Esres
wrote:

Ah. I'm not sure how they determine Vo. They don't specify how it's
to be calculated, and the Part 23 Flight Test guide doesn't say how to
find it experimentally (unlike things like Vmc).

What is the definition of Vo?
I cannot find a definition of/for it.

I've got an old Flying Magazine (circa 1970 or so) where one of the editors
makes the comment that it is better to take off overloaded (with fuel) than
it is to try a launch with marginal fuel in order to stay under gross. The
comment was the same... It'll fly better over gross than outta gas.

I bet the magazine's lawyers wouldn't let them print that now...

Don't tell anyone... but I did that almost routinely training for my
PPL. We were flying an old, tired C-150 in a Georgia summer (which
automatically means density altitudes are incredibly high). I weighed
about 180 at the time; my instructor was about 240. We were usually
over gross by about 40-50 lbs, and when flying dual we were lucky to
get 250fpm out of it.

My examiner was even bigger... I had to check the fuel levels and set
it up so that I'd burn enough on the way over to his airport that we'd
be right at gross for the checkride... which meant coming back home
afterwards was cutting it close (but then, it's only an 11 mile
flight).

Gary,

Thank you for the correction. Everything you wrote makes sense.

Could you please take a peak at my most recent thread entitled "Angle
of climb at Vx and glide angle when "overweight": five questions". I
added a clarification post in response to Mr. Gerry Caron's reply,
within that thread, so I'd appreciate if you could tackle some of my
questions.

Thanks,
Alex


"Gary L. Drescher" wrote in message
news:YBnxb.238322$ao4.855590@attbi_s51...
"Koopas Ly" wrote in message
om...
Climbout: same angle of climb for obstacle clearance at higher Vx speed.

Almost. It should be the same *angle of attack* (and higher speed) for best
angle of climb. But the angle of climb itself (and the pitch angle) will be
less than when within max gross weight. Consider the limiting case where
the aircraft is so heavy it can barely climb; the best angle of climb will
then be just slightly above zero.

Similarly, pilots who are not accustomed to high-altitude takeoffs can get
in trouble if they try to set up the same pitch attitudes they're used to at
lower altitudes for best-rate or best-angle climb. Due to the lower angle
of climb at higher density altitudes, commanding the usual pitch altitude
implies a higher angle of attack than usual, and hence a danger of stalling.

--Gary

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.

"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

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

On Wed, 26 Nov 2003 10:18:14 -0500, "G.R. Patterson III"
wrote:


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.


Not really. You can put 50 pounds 3 feet in front of the zero cg
datum and 50 pounds 3 feet behind the datum and it is the same as
adding 100 pounds at the datum (front seats I guess).

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.

I let students take off at 2000 rpm in a 172. You roll a long way
(very sensitive to temperature) and the climb performance is down
right scary.

George Patterson

Mike Weller

Mike Weller wrote:

Not really. You can put 50 pounds 3 feet in front of the zero cg
datum and 50 pounds 3 feet behind the datum and it is the same as
adding 100 pounds at the datum (front seats I guess).

That would be true enough, except that you can't put 50 pounds 3 feet in front
of the zero cg point in most light singles. That would be where the engine is
in my plane.

George Patterson
Some people think they hear a call to the priesthood when what they really
hear is a tiny voice whispering "It's indoor work with no heavy lifting".

(Rick Durden) wrote
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?

You bet. 14CFR91.323

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.

Really modern airplanes are not eligible for this, but a lot of stuff
a whole lot newer than Trimotors is.

If you've got a reference, I'd appreciate it.

Here you go. A great web site for this stuff is
http://www.access.gpo.gov/nara/cfr/c.../14tab_00.html

14CFR91.323 Increased maximum certificated weights for certain
airplanes operated in Alaska.

(a) Notwithstanding any other provision of the Federal Aviation
Regulations, the
Administrator will approve, as provided in this section, an increase
in the
maximum certificated weight of an airplane type certificated under
Aeronautics
Bulletin No. 7-A of the U.S. Department of Commerce dated January 1,
1931, as
amended, or under the normal category of part 4a of the former Civil
Air
Regulations (14 CFR part 4a, 1964 ed.) if that airplane is operated in
the State
of Alaska by --
(1) A certificate holder conducting operations under part 121 or part
135 of
this chapter; or
(2) The U.S. Department of Interior in conducting its game and fish
law
enforcement activities or its management, fire detection, and fire
suppression
activities concerning public lands.
(b) The maximum certificated weight approved under this section may
not exceed
--
(1) 12,500 pounds;
(2) 115 percent of the maximum weight listed in the FAA aircraft
specifications;

(3) The weight at which the airplane meets the positive maneuvering
load factor
requirement for the normal category specified in §23.337 of this
chapter; or
(4) The weight at which the airplane meets the climb performance
requirements
under which it was type certificated.
(c) In determining the maximum certificated weight, the Administrator
considers
the structural soundness of the airplane and the terrain to be
traversed.
(d) The maximum certificated weight determined under this section is
added to
the airplane's operation limitations and is identified as the maximum
weight
authorized for operations within the State of Alaska.
[Doc. No. 18334, 54 FR 34308, Aug. 18, 1989; Amdt. 91-211, 54 FR
41211, Oct. 5,
1989, as amended by Amdt. 91-253, 62 FR 13253, Mar. 19, 1997]