John Carrier wrote:
Glide speed generally provides sufficient windmill RPM on the engine(s) to
provide sufficient hydraulic power so that the controls (they don't have to
be fly-by-wire, any irreversible hyd flight control system is effected) have
sufficient pressure and volume to operate normally with moderate control
inputs.

R / John


The B747 and B737 (not sure about the B737 NG aircraft) uses
windmilling of main engines for providing hydralic power but also has
substantial battery backup to provide electrical power. FBW aircraft
are more dependent on electrical power so tend to use RATS but
B747/B737 have power or power assisted controls but not FBW.

The RAT generates hydraulic pressure for the flight controls and then
derive electrical power from a hydraulic motor driven generator.

I believe the DC10/MD11 used ATG (Air Turbine Generator) and thus
reversed the setup with the turbine driving a generator to power a
electrical bus and then deriving hydraulic power from this.

VC10 had both a RAT for Hydraulic power and ATG for electrical.

APU's generally can't be started reliably in flight but 3+ hour ETOPS
certified aircraft like the B777 have special APU that are certified to
start after a cold soak.