AF 447 Thread no. 4

[DJ77]

There are at least 5 computer systems monitoring airspeed: 2 FMGECs and 3 FCPCs (a weird architecture for me but what do I know?).

The previous 2 or 3 days discussions suggested the following possible sequence of events (of course open to discussion):

Pitot 1 tip freezes. ADR1 output indicates rapidly decreasing airspeed/Mach.

FCPCs and the AFS part of FMGECs detect the discrepancy and launch monitoring functions:

AFSs quickly reject ADR1 (tolerance threshold = 20 kt for 0.45 s) and uses remaining ADRs

FCPCs would reject ADR1 if its error relative to median CAS was still above 16 kt after 10 seconds. In the meantime, having detected the median CAS value decreased more than 30 kt in 1 second they launch the “median CAS monitoring function”:

• Master FCPC (FCPC1) broadcasts it intends to change the flight control law from normal to ALT2.

• Master FMGEC/AFS receives the ALT2 request from FCPC1, disengages AP and A/THR then acknowledges the message.

• Receiving acknowledgment that AP and A/THR are off, FCPC 1 activates ALT2 law and notifies other FCPCs.

• All FCPCs start computing flight control commands according to ALT2 law, using limited pitch rate feedback and gains for an initial period of 10 seconds. “F/CTL ALT LAW” is displayed on the ECAM. At this stage, ALT2 law is temporary. If at the end of the monitoring period the median CAS value is found less than the value registered at the start of the period minus 50 kt, ALT law will be latched.

• At some point before the end of the monitoring process FCPC2 loses connectivity with ADR1. For FCPC2 the median CAS becomes the average of ADR 2 and 3 CAS values, practically unchanged compared to the initial value.

• The 10 seconds period ends. Because their final median CAS value is beyond the 50 kt tolerance, FCPC1 and FCPC3 signal normal law unavailable. They also reject ADR1. FCPC2 finds its median value in tolerance and able to compute normal law.

• Before latching ALT law, the FCPC priority logic must be invoked otherwise the master FCPC would force other FCPCs to change law and would never relinquish mastership.

• Priority logic grants mastership to FCPC2 because it is claiming the highest level of flight control law.

• End of “median CAS monitoring function”.

Now comes perhaps the most arguable part of this scenario. Pitot2 and Pitot3 start icing simultaneously but not as fast as Pitot1. CAS does not decay fast enough to trigger another “CAS monitoring function”. However, it triggers the AoA protection law instead, due to the “phase advanced AoA calculation” similar to the TC-JDN A340 Airprox incident. That this function is now inhibited above M 0.53 is irrelevant in this case since the measured Mach number was soon showing less than that.

The A/C start a zoom climb, trying to maintain about Alpha Prot AoA, apparently confusing the PF beyond any understanding.

Perhaps all of the above is just garbage but:
1/ The claim found in the Airbus FCTM that simultaneous obstruction of two pitots is “highly improbable” is a weak case when there is a common cause. These pieces of hardware are built under tight tolerances, live the same life in the same environment and pitots 2 and 3 occupy exactly symmetric positions on the fuselage (contrary to pitot1). Additionally, ice ingestion is not a slow process at cruising speed when the Ice Water Contents in atmosphere reaches 9 g per cubic meter.

2/ I believe the “phase advanced AoA calculation” uses a time derivative of CAS in order to anticipate the interception of Alpha Prot speed and prevent any slower speed. AoA may not be precise or stable enough and its variation is not linear with speed.