Some contributions state that the ADR and the IRU parts of the ADIRU are independant from each other, that you can switch off one of them to let the other operate in a normal way. This lead some contributors to say that a complete anemometric failure does not account for an IRU failure as reported by the ACARS messages.
But what do we learn from the Qantas incident ?
"An A330 aircraft experienced a sudden nose down order while in cruise. This order was preceded by an automatic autopilot disconnection and triggering of the “NAV IR1 FAULT” Electronic Centralised Aircraft Monitor (ECAM) Caution. Investigations highlighted that at time of the event the Air Data Reference 1 (ADR) part of ADIRU1 was providing erroneous and temporary wrong parameters in a random manner. This abnormal behaviour of the ADR1 led to several consequences such as unjustified stall and over speed warnings, loss of attitude information on Captain Primary Flight Display (PFD) and several ECAM warnings."
(source:
http://atis.casa.go.kr/ASMS_AD/file/...008-0203-E.pdf )
is that to say that a faulty ADR can generate a "NAV IR1 FAULT" ?
in such a case, what is the procedure ? :
"Turn off the affected IR.
Turn off the corresponding ADR.
Use AIR DATA switching as appropriate.
Use ATT HDG switching as appropriate."
is that to say that when an ADR is faulty, the corresponding IR should be switched off ? (another directive acknowledges that a faling ADIRU can be difficult to switch off)
can we truely say that the ADR and the IRU parts are independant ?
And the ISIS can fail through its mach meter function.
Given these facts, a failure chain starting from a severe icing of the Pitots probes, statics ports and possibly barometric pressure sensors would seem relevant to explain the sequence of fault reports ?
Jeff