Originally Posted by Svarin #2706
One can notice that ADR DISAGREE does not entail A/P LOST condition.
It seems to me that the distinction between "DOUBLE ADR FAULT" and "ADR DISAGREE" is rather subtle, as both conditions usually occur together. As I understand it, the former results from two ADR's internally faulting (each ADR faults itself when it sees a sudden change in its output), and the latter from the PRIMs comparison of one ADR against the other(s).
Air Caraibes recorded the following explanation for A/P OFF from 'Airbus engineers' (my translation):
<q>The warning "AUTO FLT AP OFF" appears on the first line of the ECAM. The AUTO FLIGHT system compares continuously the values it receives from the three ADR's. If this cross-check is no longer possible after loss of two ADR's, it results in disconnect of FD and automatic pilot. As the AUTO FLIGHT system is conceived, any sudden variation of CAS, Mach, or altitude implies the rejection of one ADR. Thus, due to icing of PITOT and TAT probes, one sees on the two PFD, i.e. the ADR's 1 & 2, a very rapid reduction of these parameters which change respectively from 273 kt to 85 kt, M0.80 to M0.26 and from 35000 ft to 34700 ft within 1m25s. NOTE: thresholds for reject of one ADR: CAS 20 kt during 0.45s, Mach 0.04 during 0.45s, altitude 400 ft during 0.45s. <u/q>
See also Paragraph 1.6.11.2 in BEA's 2nd Interim Report on AF447.
EDIT:: Both explanations don't quite line up with the diagram posted by PJ2. If ADR DISAGREE does not cause A/P disengagement, then it was probably caused by DOUBLE ADR FAULT due to a rapid change of CAS and Mach in at least two ADR's.
regards,
HN39