German "NSTB" report/SR on Airbus pitots from 1999
A report dating from January 1999 (nr. 5X002-0/98) by the German NSTB (the Bundesstelle fur Flugunfallundersuchung BFU) is a very interesting read vis-à-vis pitot icing on Airbus a/c.
The report concretely deals with a serious incident on an A320-200 in a holding pattern near Frankfurt (with PROBE/WINDOW HEAT on AUTO), when, in heavy icing, rain showers and turbulence, the speed indications on both Primary Flight Displays as well as on the stby-indicator disappeared. The A/P and A/T disconnected and several associated warning and failure messages were generated. If I understand things correctly, the a/c had gone into Direct Law, since the report mentions that the PIC stabilised the a/c at 10000 ft using horizon and engine power (PITCH and POWER). Shortly after stabilisation at that altitude all three speed indications returned. For the landing A/P and A/T were again available.
It is interesting that the report notes that the AIRPLANE OPERATING MANUAL states that when comparison between both ADR systems is impossible (ADR-DISAGREE) and the stby system isn't available either, as was the case here, the flight crew should act on their own discretion and experience.
Even more interesting the report (I repeat from 1999) notes that "disappearing speed indications in certain weather conditions have been notified by several A320 operators". And that "in 1993 Airbus had issued SB 34.10.00.011, addressing that specific problem", and had "informed all operators of A320, A321, A330 and A340". The report continues to note that "with the information note and with the experiences in daily operations it is a principle fact that all operators and maintenance organisations were bearing knowledge of weather related anomalies in the speed indication systems of certain Airbus types". However, the report also states that maintenance organisations were not given any indications for further measures.
The report concludes that because of the particular construction of the probes, unlimited flight of these a/c types in heavy rain and icing conditions is not possible. Reason why the BFU has issued the following safety recommendation:
01/99: The specification for the pitot probes should be modified in such a way that unlimited flight in heavy rain and icing conditions is possible. The application of the already developed improved pitot probes for all aircraft types (A320, A321, A330, A340) addressed in SIL 34-047 should be made mandatory.