Benjamin Gawert wrote:
Charles Talleyrand schrieb:

Suppose a smaller airplane with it's fly-by-wire controls goes runs out
of fuel. All the engines quit. On a larger jet a ram air turbine
would drop into the airstream and power the controls. How does it work
on smaller planes like the Dassault Falcon or the F-16?

The Panavia 200 Tornado has a EPS battery (one shot battery) that in
case of double engine out situations supplies a few instruments and an
electric pump to have some hydraulic pressure. This allows maintaining
control over the a/c for ~5-7 (max 10) minutes.

These EPS (Emergency Power System) Batteries are usually known as
"Thermal Batteries". They have an eutectic electrolyte of salts that
are melted by a pyrotechnical charge. Because the electrolyte is inert
and sold untill melted shelf lives of 20+ years are possible. Because
of the high opperating temperature very high power densities are
possible.

20 years would not be full life for an airliner and a ram air turbine I
suggest would require less maintenance. (ie it doesn't ever need
replacement)

Thermal batteries appear to be the battery of choice for missiles. I
did read though that the early MANPAD Stingers handed over to the
Mujahidine had thermal batteries that would now be failing and thus
(thankfully) rendering the missiles inopperational.

The Panavia Tornado also has Nickel Cadmium Secondary rechargeable
batteries (perhaps upgraded to NiMH now?) for APU starting and other
power. I suspect that these could keep the system up for a few minutes
on their own if in good condition.


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?

I a lot of airplanes the APUs (resp. their intakes and exhausts) are
mounted in a way that they can't be used inflight.

This would hardly apply to the Tornado. Tornado however uses its
entire slab sided botton fueselage for weapons: there is little room
for ram air turbine.

Besides that, if
there still is fuel in the tanks it's better served for keeping the
engine alive than just the APU...

Benjamin