AF447

[Graybeard]

TCAS, the Canary in the Mine?

Of course we don't really care that the TCAS failed with no other traffic within range. What I am pursuing is what failure triggered its fault report. I found the below list on the Italian page. Can you point me to a better list?

The first four items would follow from iced pitot tubes, where there would be airspeed disagree. Airspeed anomalies should not affect altitude computation enough to cause the digital Altitude word to go Fail Warn out of the ADIRU. And airspeed will not affect pitch and roll outputs.

The TCAS depends on Altitude, not airspeed.

The IR faults are down the list, obviously occurring some time after the TCAS Fail. Moreover, if 447 had Collins TCAS, IR Fail would be meaningless anyhow, as the Collins doesn't use pitch and roll inputs. Honeywell TCAS use those inputs, but might not fail itself if those inputs are failed.

ATA 27.91 F/CTL ALTN LAW
ATA 22.83 FLAG ON CAPT PFD
ATA 22.83 FLAG ON F/O PFD
ATA 22.30 AUTO FLT A/THR OFF

ATA 34.43 NAV TCAS FAULT

ATA 22.83 FLAG ON CAPT PFD
ATA 22.83 FLAG ON F/O PFD
ATA 27.23 F/CTL RUD TRV LIM FAULT (Brutta cosa)
ATA 34.11 EFCS2 EFCS1 AFS FAULT
ATA 27.93 EFCS1, EFCS2X FAULT
ATA 22.83 FLAG ON CAPT PFD
ATA 22.83 FLAG ON F/O PFD
ATA 34.10 NAV ADR DISAGREE (ADIRU DISAGREE)
ATA 34.22 ISIS 1, ISIS 2 FAULT (22FN)
ATA 34.12 IR2, EFCS1X, IR1, IR3 FAULT
ATA 27.90 F/CTL PRIM 1 FAULT
ATA 27.90 F/CTL SEC 1 FAULT
ATA 22.83 AFS 1, FMGKC1 (1CBS2)
ATA 21.31 ADVISORY CABIN V/S
------------ END OF TRANSMISSION------------

I believe we can rule out icing of the pitot tubes as cause for TCAS Fail, and we can rule out attitude, as the TCAS Faults came before the IR Faults.

We are left with a few possibilities, one being a TCAS antenna.

Another possibility is Altitude Fail, but that does not appear so early in the reporting, if at all.

Sure, severe turbulence could trigger a TCAS Fail in an intermittent installation, and I would be looking at the airplane's history for that.

Otherwise, I'd be looking for lightning damage, rare as it may be at that fright level.

GB