Charles Talleyrand wrote:
Alternatively, I've always wondered if one could not keep a special
reserve tank for the APU. When the engines quit, the APU
automatically starts to power the controls. Would such a system be
safe and would it be certifiable? Would it weigh less than the
existing system?


In a lot of airplanes the APUs (resp. their intakes and exhausts) are
mounted in a way that they can't be used inflight. Besides that, if
there still is fuel in the tanks it's better served for keeping the
engine alive than just the APU...


I know that most Boeing and Airbus planes can run the APU in the air.

I'm just asking if 30 minutes of fuel for the APU might not weigh less
than
the ram air turbine. The APU would clearly produce more power than the
ram air turbine.

If the choice was between flight controls powered for 30 minutes or the
main
engines powered for another one minite, I would take the former.

I must assume that there are some very smart aircraft designers who
have already
considered and rejected my idea. But WHY was this? Is it a
fundementally
bad idea, is it a good idea that's against the rules of certification,
or what?

-Charles Talleyrand


For one thing you'd be hard pressed to get 30 minutes of glide time
from any airliner.

Dan, U.S. Air Force, retired