GrayBeard says
The TCAS gets its altitude from the selected transponder. The transponder gets its altitude from its selected ADC. ADC altitude is separate from ADC airspeed, whose source was apparently flawed. There is no reason for the ADC to fail its altitude output if its airspeed input has failed. Hence, the TCAS Fail is unrelated to pitot problems..
Perhaps drop to another earlier level of air data processing (i.e. the ADM) and reflect that the pitot takes in BOTH RAM (dynamic) and static pressure (the latter being deducted by static port sourced static pressure to derive the CAS). It's the fact that the two sources of static received by their respective ADM's slip outside allowable minor differences that creates the "reject".
.
per.....
.
Each ADIRU comprises an Air Data Reference (ADR) and an Inertial Reference (IR) component.
An ADR (Air Data Reference) fault will cause the loss of airspeed
and altitude information on the affected display.
Air Data Reference
The ADR component of an ADIRU provides airspeed, Mach, angle of attack, temperature and barometric altitude data.
Ram air pressure and static pressures used in calculating airspeed are measured by small Air data modules (ADM) located as close as possible to the respective pitot and static pressure sensors. The ADMs transmit their pressures to the ADIRUs through ARINC 429 data buses.
Complexity in redundancy
Analysis of complex systems is itself so difficult as to be subject to errors in the certification process. Complex interactions between flight computers and ADIRU's can lead to counter-intuitive behaviour for the crew in the event of a failure. In the case of Qantas Flight 72, the captain switched the source of IR data from ADIRU1 to ADIRU3 following a failure of ADIRU1; however ADIRU1 continued to supply ADR data to the captain's primary flight display. In addition, the master flight control computer (PRIM1) was switched from PRIM1 to PRIM2, then PRIM2 back to PRIM1, thereby creating a situation of uncertainty for the crew who did not know which redundant systems they were relying upon.
.
Reliance on redundancy of aircraft systems can also lead to delays in executing needed repairs as airline operators rely on the redundancy to keep the aircraft system working without having to repair faults immediately (the MAS 777 case - 9M-MRG had
graceful degradation failures dating back many years that were intentionally "hidden". A further failure brought that failed accelerometer back into play - and precipitated QF72's wild ride).
.
Precedents:
a. This pre QF72 incident to a QANTAS A330 (QF68 on 12 Sep 06) (see:
this link ) was quite probably an incipient AF447 scenario. No fault was ever found. Why? The pitot ice had melted well prior to landing and the CirroStratus encounter/exposure was likely to have been a mild one (AND the crew took prompt and luckily, correct action).
b. On 07 Feb 08 another QANTAS aircraft (VH-EBC) suffered an identical event while conducting the JQ7 service from Sydney to Ho Chi Minh City, Vietnam.
c. 27 December 2008, Qantas Flight 71 from Perth to Singapore, the same A330-300 registration VH-QPA and the same ADIRU as involved in the Qantas Flight 68 incident, was involved in an incident at 36,000 feet approximately 260*nautical miles (480*km) north-west of Perth and 350*nautical miles (650*km) south of Learmonth Airport at 1729 WST.
d. While examining possibly related events of weather-related loss of ADIRS, the NTSB decided to investigate two similar cases on cruising A330s. On a 21 May 2009 Miami-Sao Paulo TAM Flight 8091 registered as PT-MVB, and on a 23 June 2009 Hong Kong-Tokyo Northwest Airlines Flight 8 registered as N805NW each saw sudden loss of airspeed data at cruise altitude and consequent loss of ADIRS control.
Whether or not the Autopilot and TCAS drop out probably depends upon how fast the situation onsets - and to what extent the differently derived static pressures eventually disagree.... prior to crew intervention.