"Dave S" wrote in message
. net...
I guess what Im getting at is.. if the POH and checklist says one thing,
and a homebrew Vref, Vx, Vy, etc. doesnt match "the book" exactly (but
is scientifically correct).. which would prevail if something went wrong
and my decisionmaking was analyzed after the fact by G-men, insurers,
usenet readers, etc..

I don't think there's actually a disparity here between what the POH says
and what physics says. Va, Vx etc. are defined at max gross weight, and are
_intended_ to be scaled down for other weights.

But suppose there _is_ a disparity. If you're executing a high-performance
takeoff from an obstructed short field, would you rather use a speed that
gives you the best climb angle, or one that produces a shallower angle, but
gives you an excuse for the crash investigators? (That's not to say that
your question about the legal consequences isn't still of interest, though.)

I get the feeling (without having done any of the math yet) that this
truly is an academic exercise in the typical 4 seat or less light
spamcan anyways, something akin to a few knots here or there...

Well, there's not much math to do--if you're 30% below gross (quite possible
in a typical 4-seater), then Va, Vx etc. get reduced by about 15%--not a
trivial difference.

--Gary

Gary Drescher wrote:

"Dave S" wrote in message
. net...

Now... a question about realities.. The POH nazi's will say that the
Word as written is good, praise be to the POH... if I base flight
decisions and speeds on MY calculated numbers rather than the max weight
sea level standard day numbers published in the almighty POH.. am I
going to be asking for trouble here?


It depends on what you mean by 'trouble'. The laws of physics prevail
over
the POH in determining whether your engine mount will break, whether
your
climb angle will clear an obstacle, whether you can stop before the end
of
the runway, whether you can glide to a landing spot, etc. And those
things
are what the V speeds are all about.

In fact, though, I don't think there's any contradiction between the
physics
and the way the POH speeds are supposed to be interpreted. But the
question
is a good illustration of why understanding the basic physics helps
understand how to use the POH numbers safely.

--Gary