AF447 Thread No. 3

[A33Zab]

ADIRU

Went into the ADIRU today:

ADIRU computes AOAi from sensor resolver cos. and sin. and calculates
AOAc as function of AOAi, FLAPSLAT CONFIG and Sensor position(LH/RH)

If CAS < 60 Kts AOAi & AOAc are set to 0° and SSM (System Status Matrix) is set to NCD (No Computed Data),
this is also valid for TAS 0 Kts if CAS < 60

If CAS < 30 Kts it declares itself invalid and outputs 0 Kts and NCD.

These parameters are send to 8 similar ARINC output busses.
Bus 5-8 are reserved for the engines only. (Bus 7-8 are not used on A33).
Note: GE engine provides its own Air Data, A/C ADR is only used as backup.

PRIM 2 & 3 receives data by ADR's bus 2, PRIM 1 by ADR’s bus 3.

Note: In Back-Up Speed Scale (BUSS) equipped A/C AOA is send via IR bus
Note: Couldn't find if a SSM NCD is taken in account by PRIM but most probably it will.

In the 1st BEA report the unreliable speed logic is explained.

The PRIMs trigger a monitoring process when one of the speeds decreases more than 30 Kts (in 1 sec.) compared to the median value.
The PRIM opens a monitoring window during which it operate in ALT 2 Law, the rudder deflection limit is frozen but associated message is inhibited.
At the end of the monitoring window, if the diff. is less than 50 Kts the PRIM returns to normal law.
If not it remain in ALT 2 LAW and at that time the F/CTL RUD TRV LIM FAULTmessage is shown.
Outlier ADR is rejected and remaining control is on median value of the other 2 ADR’s
(Stall warning is generated by highest AOA and not the median value)
This means actual AOA could be indeed 40° while not triggering ABNORMAL ATTITUDE LAW due to a median AOA value below <30° as already mentioned in the BEA ‘leak’.

If 2 ADR outputs are erroneous, but different, and the remaining ADR is correct OR all 3 are erroneous but different:

The AP and A/THR disconnects and if disagree last for more than 10s, the PRIMs trigger the NAV ADR DISAGREEmessage.
ALTERNATE 2 LAW become active and latched for remainder of the flight.
(AP and A/THR can be re-engaged if ADR output was only transient.)

Active PRIM could in this case reject the correct ADR data.
That's why crew need to perform the unreliable speed Indic/ADR check QRH procedure and isolate the ADR in error. (Thus it will not be used for faulty PRIM input & indication).

The PF slight but consistent nose up command is a +G request to the PRIMs, the A/C feedback by means of accelerometers however results in a -G.
PRIM deflects elevator and due to negative result drives the THS all the way ANU.

Unfortunate left unnoticed because any hand on the trim wheel had cancelled the PRIM THS orders.