The engine mount on most light aircraft is designed to withstand 9
G's minimum.


Where is that specified? Does Cessna provide max. g limits for items
on the aircraft or are max. g limits for these items explicitly
specified in FAR 23?


And as I said earlier, the 3.8 figure is based on gross
weight. Reducing gross would allow them to take a higher G figure but
the same net force.


Agreed. This may sound like a stupid question, but how do you define
"gross weight"?


Cessna also states in the 172 POH that it's designed to 150% of
the G figures given, or 5.7 G's. I think the 3.8 figure would be the
yield point, where things begin to bend, and the 150% figure would
break them entirely. Or something like that.


You're correct. An airplane has to sustain ultimate loads, typically
50% beyond limit load (highest in-service load to be experienced) and
show no failure (fracture). An airplane has to sustain limit loads
without permanent deformation (yielding). At least, that's for FAR 25
aircraft.



Dan