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.