Gross Weight

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

"Michael" wrote in message
oups.com...
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


Very nice post, and I enjoyed reading it. I especially like your view on
what my instructor did, and you make a very good point. I have admitted to
myself, that he did some things in my training that, although we got away
with, were wrong, and I remember those each time I fly. You make a very
good point with regard to teaching without the proper framework. I will
also remember that. Thank you for you comments.

Fred

If your limit is 45 over gross, how far over your limit is ok?

How about as far as airlines go over-weight on a daily basis as a
result of the FAA standard person not being equal to the actual load
they're dealing with? Not everyone weights 170lbs or whatever the
latest number is..........

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

Excellent post!

I have two comments...

If you fly overgross, you're breaking the rules. Where
will the rulebreaking stop? The answer is really nowhere.

I agree. But as you pointed out earlier, keeping within max gross won't
keep you safe, either. In other words, every flight includes making
decisions about performance that are arbitary, or more precisely, that
are based on experience. If experience didn't matter, why would we keep
log books? If the rules were enough, why would we need anything else?

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.

If it is structural issues -- which I would suggest it very rarely is
-- you'll still have a huge safety margin when 5% or 10% over-weight.
You are not going to be getting that close to the 'g' envelope, and
your landings are hopefully not going to be hard enough to be given 10%
of collapsing the gear!

Further, if you look at accident reports where over-weight operation
was a factor, I doubt you'll find many where structural issues came
into play. In fact, I can't recall reading a single one, and like most
pilots, I eat 'em up to try and learn from others' mistakes. As I said,
the failure mode that matters most is failing to fly, or failing to get
out of ground effect.

If your limit is 45 over gross, how far over your limit is ok?

It depends on the aircraft.

Well then, what does the word "limit" mean to you?

As a test pilot, I agree there is no simple pat answer, and a previous
post went into many of the factors that go into aircraft handling at
high weights. But I'm not a test pilot, and have no intention of being
one. And I've found in life that most of the time I've gotten into
trouble with the laws of physics has been when I've been "over the
limit" in one form or another. I'll stick with the book.

That said, it would be nice if the book addressed limits a bit more
completely (such as a graph showing the maximum takeoff weight vs
density altitude). It doesn't, so I won't go there.

Jose
--
Nothing takes longer than a shortcut.
for Email, make the obvious change in the address.

Exactly what we were discussing at work.....

I guess maybe it might relate to how many drinks can I have in a specified
period before driving home.....

Actually, I've found alcohol to be quite useful in aviation. In
situations where I'd be over gross, I have a few six-packs of beer. If
the pilot is already flying, then his weight can be subtracted from the
manifest, allowing more cargo, or more fuel. By getting the passengers
suitably high also, I can reclaim almost all the payload that way. So
then, after the passengers have boarded the airplane, I calculate how
much extra fuel each one represents, and have the FBO fill up one of
those big fifty gallon drums full and we load it into the cargo
compartment... you know, the one in the back that says "200 pounds max".
Well, ok 50 gallons is 300 pounds, but if we average that over the
whole airframe we're only 25 pounds over per passenger (and the bigger
the passenger, the less that 25 pounds is, percent wise). And more fuel
makes you safer anyway.

So then I get on the runway, open the throttle, and climb as fast as I
can. Really pull back on that yoke - the plane should climb like a bat
out of hell, since it's virtually empty.

Remember - eight bottles, then throttle.

Jose
(*) kids - don't try this at home!
--
Get high on gasoline. Fly an airplane.
for Email, make the obvious change in the address.

LOL!

Fred


"Jose" wrote in message
...
Exactly what we were discussing at work.....

I guess maybe it might relate to how many drinks can I have in a
specified period before driving home.....

Actually, I've found alcohol to be quite useful in aviation. In
situations where I'd be over gross, I have a few six-packs of beer. If
the pilot is already flying, then his weight can be subtracted from the
manifest, allowing more cargo, or more fuel. By getting the passengers
suitably high also, I can reclaim almost all the payload that way. So
then, after the passengers have boarded the airplane, I calculate how much
extra fuel each one represents, and have the FBO fill up one of those big
fifty gallon drums full and we load it into the cargo compartment... you
know, the one in the back that says "200 pounds max". Well, ok 50 gallons
is 300 pounds, but if we average that over the whole airframe we're only
25 pounds over per passenger (and the bigger the passenger, the less that
25 pounds is, percent wise). And more fuel makes you safer anyway.

So then I get on the runway, open the throttle, and climb as fast as I
can. Really pull back on that yoke - the plane should climb like a bat
out of hell, since it's virtually empty.

Remember - eight bottles, then throttle.

Jose
(*) kids - don't try this at home!
--
Get high on gasoline. Fly an airplane.
for Email, make the obvious change in the address.

But I'm not a test pilot, and
have no intention of being one.

If being 1lb over-weight makes you a test pilot, then so does having an
engine which is operating at 1hp less that its rated outputs. Or a wing
that is contaminated with bugs and is producing less than the CoL that
was present during POH testing. Or brakes that are operating at 1% less
than new condition. Or, well, you get the idea...........

Fred Choate wrote:
How much is too much over gross weight?

I will not fly 1 pound over gross, I will not delibrately break an FAR, I
will always do a thorough pre-flight on the aircraft, I will only cancel IFR
once I'm more than 500' below or 1000' above the clouds, etc. As a CFI, I
have canceled lessons because the compass card was missing or not legible -
perhaps the cancelation proves to be a better lesson for the student in the
long run than the instructional flight. Either way, I'm not prepared to be
tempted by the slippery slope.

NAFI Code Of Ethics: http://www.nafinet.org/who/code.html

Hilton