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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.

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  • $\begingroup$ Talk to them about mcCready and teach to push the stick forward if in doubt :-) $\endgroup$ – Caterpillaraoz May 3 '18 at 6:59
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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.
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  • $\begingroup$ However what happens if you do start the APU then (or have it running in the first place, since you are doing it on purpose)? I believe the normal law should be restored. $\endgroup$ – Jan Hudec May 3 '18 at 20:54
  • $\begingroup$ @JanHudec "With RAT (loss of GEN 1 & 2) the APU is allowed 3 minutes for a start attempt" I will find more info tomorrow, but as far as I remember, you can't rely on the APU. $\endgroup$ – Federico May 3 '18 at 21:07
  • $\begingroup$ I have tested the scenario in our simulator. Indeed, I can activate the autopilot, and I can also activate HDG mode. As you said in your answer, speed mode cannot be activated! I wasn't sure how realistic our simulator depicts the real A320 autopilot... $\endgroup$ – Stefan May 4 '18 at 7:54
  • $\begingroup$ @JanHudec I checked. the training manual says that the APU provides electrical power, and for the "fly" component, still only PFD1 and alternate law would be available $\endgroup$ – Federico May 4 '18 at 8:00
  • $\begingroup$ @Federico - The FCTM mentions that under 'Emergency Electrical Configuration', which doesn't account for the APU. I recommend checking the FCOM's 'Electrical Supply' pages for each of the required systems, such as for the Auto Flight (22-10-60). $\endgroup$ – ymb1 May 4 '18 at 11:34
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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:

enter image description here

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

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  • $\begingroup$ why would G+Y hyd work if you have the APU running? the engine pumps will still be off, right? $\endgroup$ – Radu094 May 16 '18 at 9:46
  • $\begingroup$ Hi @Radu094 - I clarified it: the Y hyd will run via the elec pump on AC2, and with Y pressurized, so will the G via the power transfer unit. $\endgroup$ – ymb1 May 16 '18 at 12:55

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