Does the A320 autopilot work in flight with both engines off?

Question

For a series of simulator sessions to determine the speed polar with students we plan to cutoff both engines in our A320 simulator, but we still want to use the autopilot to control speed (in theory, the autopilot would adjust pitch to keep a desired speed). Obviously, some modes, like altitude hold, positive vertical speed and autothrottle modes will not make sense in such a scenario.

In a real A320, can the autopilot (or selected functions only?) be used (activated) when both engines are cut off? (for example, after a double bird strike)? Assume APU running, so electrical power is guaranteed.

Answer 1

You want to assume the APU is running. The FCOM says:

APU start is not available for 45 s after the loss of both engine generators. This 45 s delay prevents any interference with emergency generator coupling.

So the APU can be started following dual engine failure after waiting 45 seconds.

With the APU running, the green (G) and yellow (Y) hydraulic systems will also be powered, so AP1+2 should work. (The Y hyd will run via the elec pump on AC2, and with Y pressurized, so will the G via the power transfer unit.)

Descending by maintaining a speed using pitch control is called OPEN DES in an Airbus (FLCH for Boeings). OPEN DES doesn't require the A/THR to be working:

The engine-out descent strategy requires disconnection of the autothrust, and descent in OPEN DES mode.

Disconnecting the autothrust prevents an automatic setting of THR IDLE; therefore, the autopilot will fly the target speed in OP DES mode, with a thrust manually selected by the crew.

When reaching the FCU-selected altitude, or whenever normal descent is resumed to a lower altitude, reengage the autothrust.

The flight law reconfiguration table in the FCOM says the AP is lost when any of the items in the dashed box below fail:

I tried to disprove it, but it seems like with the APU started, an OPEN DES should work. (Report back please.)

Answer 2

From the A320 Flight Crew Training Manual:

Following an all engine flame out, the flight deck indications change dramatically as the generators drop off line. The RAT is deployed to supply the emergency generator and pressurize the blue hydraulic circuit.

Significant remaining systems in ALL ENGINES FLAME OUT
FLY PFD1, Alternate law
NAVIGATE RMP1, VOR1
COMMUNICATE VHF1/HF1/ATC1

As you can see, alternate law is available, and only one hydraulic circuit (blue) is powered by the RAT (Ram Air Turbine), not the APU.

Alternate law does not control speed with pitch, and particularly

At the flight envelope limit, the aircraft is not protected, i.e.:

• In high speed, natural aircraft static stability is restored with an overspeed warning
• In low speed (at a speed threshold that is below VLS), the automatic pitch trim stops and natural longitudinal static stability is restored, with a stall warning at 1.03 VS1G.