Hopefully this is the last part of the QZ8501 FBW series of articles covering the FBW, Flight Controls maintenance, and Pilot procedures.

Previous Parts are:

  1. QZ8501: FBW Part 1 – Understanding A320 Computers
  2. QZ8501: FBW Part 2 – FAC and Maintenance

I am disturbed by the leak from unnamed sources within the investigation that the pilots may have switched off the Flight Augmentation Computers (FAC), and that the media has quoted that “Airbus – the plane’s manufacturer – discourages pilots from shutting off power systems as it can affect other components” and quoting a safety expert saying, “particularly with an Airbus you don’t do that” as the way it is written, is open to gross misinterpretation and could result in incorrectly laying blame on the crew as a result. However, I don’t blame the safety expert as he may not have been asked the right question in the right context and therefore the answer may not be the right one.

On the contrary, if the FACs have a problem, Airbus procedures do allow for the crew to reset the FACs. So, let us start by looking at what faults may have occurred on the aircraft on that fateful flight.

AUTO FLT RUD TRV LIM 1 (2)

This is what was reported by one local media occurring between 12 and 19 December. The aircraft’s Electronic Centralized Aircraft Monitor (ECAM) would show the following:

AUTO_FLT_RTL12

There is no action to be done, and in accordance to the manual on Flight Control Law Reconfiguration, the aircraft would remain in Normal Law and the Autopilot can remain functioning with the loss of one rudder travel limiter.

AUTO FLT RUD TRV LIM SYS

If both Rudder Travel Limiter are faulty, or the FAC detects a fault within the Rudder Travel Limiter system as a whole, then the system would generate this fault warning, and the crew would see this on their ECAM:

AUTO_FLT_RTL_SYS

This fault will render the rudder travel limiter inoperative (hence need to use rudder with care above 160 knots), and the procedure does ask for the FACs to be reset one by one, contrary to what one “expert” has said (but am sure he agrees that in this context, it has to be done).

FAC 1 (2) FAULT

What happens when your FAC gets faulty? Surely you don’t want to fly relying on a faulty FAC, right? This is the procedure for such a situation:

AUTO_FLT_FAC12

Again, it is safer to fly without relying on a faulty computer.

AUTO FLT FAC 1 + 2 FAULT: When both your FACs are screwed!

First, you’ll hear the autopilot disengaging, the aircraft then reconfigure to Alternate Law as per the Flight Control Reconfiguration flow:

320-FBW-4

Double FAC failure will lead to Autopilot lost and reconfiguration to Alternate Law. (source: Airbus)

 Then, do the following procedure:

AUTO_FLT_FAC_BOTH

No surprises here, it asks for the FACs to be individually and sequentially reset by switching off then on. If the reset does not solve the problem, then switch both off. When you switch both FACs off, your flight control laws reconfigure to Alternate Law with no protection, and that your autopilots continue to be inoperative until you fix the whole problem on the ground.

What if they had a yaw damper failure?

The aircraft would also enter Alternate Law as per the reconfiguration flow, and you are required to reset the FACs one by one. I am not putting up the procedure because it is similar to the dual FAC fault procedure above.

So, what do we know about what happened?

  • The NTSC and/or its members have stated that the aircraft stall warning had activated. This means the aircraft was no longer working in Normal Law.
  • What caused the aircraft to enter ALTERNATE LAW, may be any of the following:
    1. Yaw Damper system failure. This is a system dependent on the FAC that reported had a fault. The failure would reconfigure the aircraft into Alternate Law.
    2. FAC 1 AND 2 FAULT. This would reconfigure the aircraft into Alternate Law, and require the FACs to be reset sequentially, and if the reset failed, the FACs are to be switched off for the rest of the flight.
    3. Inadvertent reset of both FACs at the same time by the crew. This would reconfigure the aircraft to Alternate Law.
  • A source from the investigation has stated to the media that both FACs at one stage were switched off, although the details of this are too ambiguous at the moment. There is no information as to when during the mishap where both FACs were switched off.

We must in the meantime, take great care in interpreting leaks and incomplete information and not jump to conclusions, nor should we fall into perception shaping that can inadvertently occur when these things are revealed bit by bit.

I personally believe that the truth will eventually be revealed adequately in a timely manner by the Indonesian National Transportation Safety Commission.