BEA, DEC-2009, p. 37:
ISIS (22FN-10FC) SPEED OR MACH FUNCTION (2 h 11)
This message is transmitted by the ISIS, and may be the consequence of:
* an internal failure at the level of the CAS or Mach elaboration function,
* CAS or Mach values that were outside certain limits.
The airspeed measured by the ISIS is based on the pressure measurements
from the probes in the standby system, which also feed ADR 3. The static
pressure is not corrected (notably from Mach).
The only cases of excursion outside the validity envelopes compatible with
the CFR are:
* a CAS higher than 530 kt without the Mach value exceeding 1. This
condition implies that the aircraft was at an altitude comprised between
about 4,000 and 14,000 ft;
* a CAS such as the difference between the total and static pressures being
lower than a given threshold. This case implies notably that the static
pressure is higher than the total pressure.
T
he “HARD” nature of the message indicates that the problem lasted longer
than 2 seconds.
BEA, DEC-2009, p.39:
* unusual attitudes: given the relative position of the satellite with respect
to the aircraft and the aircraft’s tracking capability, the antenna would
have to be masked by the aircraft’s fuselage or wings. Examination of
the debris showed that the aircraft hit the water with a bank angle close
to zero and a positive pitch angle. The aircraft would therefore have
been able, in the last seconds at least, to transmit an ACARS message.
* end of the flight between 2 h 14 min 26 and 2 h 15 min 14.
My thoughts:
Going another 40nm appears feasible at around 600 mph (521 knots) (and also the CAS > 530 in ACARS); but declining from FL350 within 4 minutes would make that rate close to 9,000f/m, which appears not conclusive with a bank eagly close to zero and a positive pitch angle. This would more likely be the case if the a/c had been at a (ACARS-) assumed max altitude of 14,000f at this point in time, making the decl. rate around 3,500f/m, making the ditch interpretation more likely. Wouldn't the conditions of total pressure and and airspeed be met with the first quoted ACARS?
Denis