Gross Weight

Fred Choate wrote:
Here is a topic that was of discussion at work today:

How much is too much over gross weight? For example.....the 172 has a gross
weight of 2300 lbs, but what if you are 2345 at time of takeoff.....is that
too much over, even if you are going to be burning enough fuel before your
first scheduled stop to be under weight for landing?

Just to add a little gasoline to the legal vs. practical argument,
there are a good number of STCs out there that allow for gross weight
increases. Power Flow has one for the C-172N that with relatively small
modifications (limit flap travel, a cowl cooling lip that some N's
already have) makes it legal to carry an extra 100lbs.

http://www.powerflowsystems.com/prod...htincrease.htm

It would seem to me that the real risks come from (1) CG limits and (2)
takeoff performance. If you overload the plane beyond a certain point,
it just isn't going to fly. Somewhere below that line, you'll fly in
ground effect only, or have miserable climb rates. In either case, the
lethal failure is the failure to abort soon enough. This is an
experiment best conducted on a very long paved runway with no 50' trees
at the end. In the back-country, where overloading is often committed,
you're committed to flying soon after the plane starts rolling. Can an
average GA plane take off with a load great enough to cause damage in
the event of hitting some chop? I don't know, but I suspect that should
be the least of one's worries.

CG seems a more pernicious issue to me, as the plane's behavior can
fool you. Chances are you won't realize you're thoroughly screwed until
after you're up in the air without any good options. In addition to
takeoff CG, I'd also compute CG with half fuel and very little fuel,
just in case.

And then there's the conditions at the moment of takeoff. A C-172 at
gross on a hot humid day is in a lot worse situation than the same
plane 50lbs over gross in the middle of winter at -10c. One's legal,
the other's not, but which would you rather be in with a short runway
and tall trees?

As a low-time pilot, I choose to maintain wide safety margins,
including abiding by the book where I am not sure.

-cwk.

Thanks for that link.....

Fred


wrote in message
oups.com...




Just to add a little gasoline to the legal vs. practical argument,
there are a good number of STCs out there that allow for gross weight
increases. Power Flow has one for the C-172N that with relatively small
modifications (limit flap travel, a cowl cooling lip that some N's
already have) makes it legal to carry an extra 100lbs.

http://www.powerflowsystems.com/prod...htincrease.htm

Can an
average GA plane take off with a load great enough to cause damage in
the event of hitting some chop? I don't know, but I suspect that should
be the least of one's worries.

That depends on the airplane. If you're dealing with a normal category
airplane with old and possibly deteriorated wing structure, I sure
would worry about it. Some of the heavier Cessna twins that served a
lifetime in hauling cargo (where, contrary to regulations, gross weight
is often exceeded, as is zero fuel weight on short runs). Consider
these accidents:

http://www.ntsb.gov/ntsb/brief2.asp?...FA123& akey=1

This is a narrative of the NTSB accident investigation that prompted
the original Airworthiness Directive against the 400-series Cessnas.
Note that the blame is placed on a manufacturing defect, but that does
not diminish the role that repetitive misloading may have played in the
failure.

http://www.ntsb.gov/ntsb/brief2.asp?...FA208& akey=1

This is narrative of an NTSB accident investigation of a Cessna 402
engaged in Part 135 cargo operations that crashed into the Caribbean
Sea. Only parts of the aircraft and some cargo were recovered, and no
probable cause is listed. However, known circumstances outlined in
this report point to wing spar failure, the report suggests that
misloading played a factor.

Is this is a factor overloading a C-172? Not just no, but hell no.
But here's the problem - you get away with it on a C-172, and unless
you understand what you did and why you did it, you have no frame of
reference to know you can't do the same thing on a C-402.

CG seems a more pernicious issue to me, as the plane's behavior can
fool you. Chances are you won't realize you're thoroughly screwed until
after you're up in the air without any good options.

Well, that's overstating the case by quite a lot. The cg needs to be
quite a bit aft of limits before normal flight and a normal landing are
a problem. Oh, you'll feel the reduced longitudinal stability, but it
likely won't be bad enough to keep you from landing the plane. Just
don't stall. Also realize that as weight increases, generally the cg
limits narrow.

The real issue occurs in a tailwheel airplane. A tailwheel airplane
loaded aft of cg can be a real bear on the ground - lots of weight aft
of the mains give it very poor lateral stability.

In addition to
takeoff CG, I'd also compute CG with half fuel and very little fuel,
just in case.

It's not a just in case. In the Beech Bonanza, it's a real issue. As
you burn fuel, cg moves aft - and unless you have some real big boys up
front and little or nothing in back, you're never far from the aft
limit anyway. On the other hand, you have to try real hard to get a
Brand C or Brand P aft of limits.

Michael

Michael wrote:
Can an
average GA plane take off with a load great enough to cause damage in
the event of hitting some chop? I don't know, but I suspect that should
be the least of one's worries.

That depends on the airplane. If you're dealing with a normal category
airplane with old and possibly deteriorated wing structure, I sure
would worry about it. Some of the heavier Cessna twins that served a
lifetime in hauling cargo (where, contrary to regulations, gross weight
is often exceeded, as is zero fuel weight on short runs). Consider
these accidents:

Point taken. I guess I would lump this in with the T-34 accidents we've
seen which IIRC have involved exclusively planes used in weekend
fighter pilot games.

In your opinion, if you have a structure which has been fatigued beyond
book limits over many years (find a 402 that hasn't?), how do you judge
what's safe?

CG seems a more pernicious issue to me, as the plane's behavior can
fool you. Chances are you won't realize you're thoroughly screwed until
after you're up in the air without any good options.

Well, that's overstating the case by quite a lot. The cg needs to be
quite a bit aft of limits before normal flight and a normal landing are
a problem. Oh, you'll feel the reduced longitudinal stability, but it
likely won't be bad enough to keep you from landing the plane. Just
don't stall.

That was kind of my point. Seems like a departure stall is how a good
number of these flights end.

In addition to
takeoff CG, I'd also compute CG with half fuel and very little fuel,
just in case.

It's not a just in case. In the Beech Bonanza, it's a real issue. As
you burn fuel, cg moves aft - and unless you have some real big boys up
front and little or nothing in back, you're never far from the aft
limit anyway. On the other hand, you have to try real hard to get a
Brand C or Brand P aft of limits.

As I exceed FAA standard dimensions considerably myself, I've never
lost too much sleep on this point

-cwk.

How much is too much over gross weight?

This issue comes up so many times, sometimes I feel like writing an
overgross FAQ. The short answer is - it depends. Now for the long
answer:

Legal aspects:

Even 1 pound over max gross is not legal, and can subject you to civil
penalty - unless you have been granted some deviation. The deviaitons
come in many flavors. Many light aircraft in Alaska are eligible for
up to 15% increases if operated under Part 135 (obviously they fly
differently under Part 91). Some STC's allow you gross weight
increases with certain (often minor) modifications. Some engine change
STC's come in flavors where one gives you a gross weight increase and
the other does not - with the same engine. The FAA will give you a
ferry permit to operate up to 20% overgross without batting an eye if
you show that you understand what you're getting into.

In other words - understand that gross weights are as arbitrary as
speed limits - sometimes they exist for very good reasons, and may give
you very little margin for error (or even none at all, or less than
none), and other times they are arbitrary. It all depends on the
situation. But rest assured the FAA won't see it that way.

So how will you get caught? Well, you could have an accident. You
could be ramp checked - but in that case the overload would have to be
obvious and egregious - nobody will catch you on a 50lb overload. Put
four big guys into a Cherokee 140, though, and you are advertising an
overgross operation.

Insurance aspects:

Your insurance PRIMARILY covers you for pilot error, since that's
(officially, at least) the cause of most accidents. I've had many
insurance policies, and none have ever excluded coverage when operating
contrary to FAR's.

In other words, the idea that you're uninsured when overgross is a
myth. You're covered if you are drunk, overgross, and fly into an
airport without a clearance. Exemptions are clearly stated. Generally
they require you to have a certificate of a certain grade, certain
hours of experience, and an annual and medical that have not expired.

On the other hand, if you overload and fail to get off the ground, you
may have a hard time getting insurance after about the second or third
time this happens, and you will pay more the first time.

The slippery slope:

If you fly overgross, you're breaking the rules. Where will the
rulebreaking stop? The answer is really nowhere. It's like speeding -
you are substituting your judgment for regulation. Once you've shown a
willingness to do that, you will keep doing it when you feel it's safe
to do so. That's worth thinking about, assuming you never ever break
any regulations whatsoever, not only in your airplane but also in your
car, on your bicycle, etc. Otherwise, that ship has already sailed and
it's not a valid question.

A more on-point slippery slope - if 25 pounds over is OK, what about
50? And next time 100? 200? Where does it stop? That's the valid
question - and it has a valid answer, but not one you're going to like.

Simple light airplanes have a single maximum gross weight - one size
fits all (really one size fits nobody, because it's a compromise). The
bigger and more complex the airplane gets, the less true that is.

Some airplanes have zero fuel weights, because the wing attach points
are a weak point in the design. Some have a maximum takeoff weight
higher than the maximum landing weight, because the landing gear is a
weak point in the design. Some are eligible for gross weight increases
with tip tanks, provided the extra weight is fuel in the tip tanks and
nothing else. And when you get all the way up to the airlines, their
maximum takeoff weight depends on the runway length, the required climb
gradient, and the density altitude - in other words, it's not a single
number, but must be computed for every takeoff.

So how much is too much? It all depends on what sets the limit. On
some Cessnas, the limiting factor is being able to show the required
positive rate of climb with full flaps (40 degrees). Limit flap travel
to 30 degrees, and you get a 100 lb gross weight increase. Suppose you
simply limit travel operationally (and don't install the hardware)? Is
that OK? How about if the selector switch gets stuck in the down
position?

Are you protected if you stay within the rules? You only think so.
The gross weight is the same at 7000 ft density altitude as it is at
sea level - but at that density altitude, the plane will NOT climb with
full flaps at gross. Never forget, these limitations are arbitrary -
staying within them won't necessarily keep you safe, and breaking them
isn't necessarily dangerous.

It is possible (but not legal) for you to do the same thing the
airlines do - come up with your own calculations and procedures.
Sufficient data exist in the average POH to extrapolate performance
curves for various operations above gross weight, so you can estimate
takeoff, landing, and climb performance, compute new operational
speeds, etc. You can study the design, and the available modifications
and authorizations, to determine if it is performance, rather than
structural integrity of some component, that limits gross weight. You
can consider the nature of the flight - perhaps 3.0 positive gee limit
is sufficient on a smooth VFR day. In other words - you can approach
operation outside the established envelope the way a real, modern test
pilot does it.

Can the average private pilot do this? Will he? No, and no - and
that's why the rules are what they are. They are written to the lowest
common denominator.

Can you learn how to do it? Probably. If you have sufficient
technical background, you may do it on your own. Or someone can teach
you - but who? Not this guy:

I know that when I was receiving training, my instructor
once had me bring 2 male adults with me to a lesson.

He then had you project performance and handling characteristics by
extrapolating from the POH and maybe other sources. In flight, he
discussed these issues with you so you would know how to handle the
situation in the future, pointing out quirks of the operation. No? He
just told you to get in and go?

I wish that were the exception, but it's the rule. He was teaching you
how to fly overgross, and he was doing about the worst possible job of
it. Yeah - you did it in that particular instance. You learned a
little - mainly, you learned that operating outside the published
envelope has consequences, and a bit about what they can be. But
without an underlying framework for understanding, all you learned is
what happens in that one specific case. Since that specific case is
unlikely to be repeated exactly, you haven't really learned anything
very useful.

So the answer to your question of how much is too much? When the
safety margin associated with the limiting factor in play under the
circumstances becomes uncomfortably slim, that's too much.

Michael

"Michael" writes:

[...] In other words, the idea that you're uninsured when overgross
is a myth. You're covered if you are drunk, overgross, and fly into
an airport without a clearance. Exemptions are clearly stated.
[...]

As a contrary data point, my (Canadian Marsh/Lloyds) insurance
includes an explicit requirement to stay within W&B limits to retain
coverage.

- FChE

As a contrary data point, my (Canadian Marsh/Lloyds)
insurance includes an explicit requirement to stay within
W&B limits to retain coverage.

Which may or may not be enforcable, depending on state laws. It is
true, thought, that some carrier demand in-envelope operation, and that
others required the airworthiness cert to be in full force and effect,
and then argue that over-weight operation voids said cert. Again,
whether this latter approach would work would depend on the state.
Avemco, though, specifically say that you can be over-weight and
they'll still pay. And no-one has yet produce an example of *any*
company failing to pay as a result of an aircraft being over-weight....

Has anyone identified an accident that was caused by being overweight.
Are they common? Certainly not as common as running out of fuel.

On 8 Jul 2005 17:55:19 -0700, "Doug" wrote:

Has anyone identified an accident that was caused by being overweight.
Are they common? Certainly not as common as running out of fuel.

Plenty of accidents caused by being outside of the CG envelope.

I recall a Bonanza that crashed on takeoff leaving KASH with a load of NH
tax-free liquor.


Ron (EPM) (N5843Q, Mooney M20E) (CP, ASEL, ASES, IA)

Has anyone identified an accident that was caused
by being overweight. Are they common?

I've read of quite a few where over-weight operation combined with high
DA to result in an aircraft either running off the end of the runway on
take-off, or, worse, getting into the air and then spinning in upon
being unable to maintain a climb. I am quite convinced that over-weight
operation will cause accidents; I'm not convinced it results in
accidents as a result of structural failure.