mm43:
...(i) ACAS will neither track nor display non-transponder equipped aircraft, nor aircraft not responding to ACAS Mode C interrogations; and
(ii) ACAS will automatically fail if the input from the aircraft's barometric altimeter, radio altimeter or transponder is lost.
Note 1: In some installations, the loss of information from other on-board systems such as an inertial reference system (IRS) or attitude heading reference system (AHRS) may result in an ACAS failure. Individual operators should ensure their flight crews are aware of what types of failures will result in an ACAS failure.
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mm43 statement:
Clause (ii) gives 3 causes of failure, plus the IRS in Note 1.
(i) This will not cause or result in TCAS Fail.
(ii) A sudden change in indicated altitude as experienced by Air Caraibe will not trigger TCAS Fail. There was no ACARS report of radio altimeter nor transponder Fail; hence, these do not account for the TCAS Fail. Moreover, there were no reports of ADR Altitude Fail.
1. IRU is inconsequential to Collins TCAS. IRU Fail might trigger Honeywell TCAS Fail. That, however, requires failure of both #1 and #3 IRU, as #3 is automatic backup. Does AF 330 fleet fly Collins TCAS, or Honeywell TCAS?
I agree with the BEA: TCAS Fail report remains unexplained/unrelated to the other Fail reports.
HN39
...position error of that port. The position error correction (PEC) is usually expressed as a pressure coefficient cp, which must be multiplied by the dynamic pressure measured at the pitot, to obtain the pressure correction. Some system must have 'reasoned' that since airspeed was unreliable, altitude was also unreliable, and hence TCAS would not be able to perform its intended function.
The PEC occurred to me while reading The Air Caraibe story, where it notes a sudden drop in altitude at the time that CAS and Mach drop, and the altitude steps up by about the same amount when airspeeds come back.
If the measured altitude was good enough to display to the pilot, it was good enough to report to the transponder. From what little I've seen of airspeed correction tables in other aircraft, they are significant only at high angles of attack, and not at cruise.
Did Air Caraibe have a TCAS Fail during their event?
GB