Qoute: "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."
If they were on direct law, they were lucky, IMHO. In alternate law, if IAS goes to a (right or wrong) low figure one has to pull the stick to avoid nose down automatic command (and the contrary for high - right or wrong - IAS reading). The problem seems to be on who decides what law to go to. It is the system’s decision, not the pilot’s. I wish I had a switch I could flip (no push button) into direct law in these cases, avoiding alternate law and its “protection” from too low/high right/wrong speed readings. Then it would be easier to maintain the required pitch.
Another subject, unreliable IAS procedure: I fly both GEs and PWs. GEs have N1 as the main thrust setting, PWs have EPR. For both types the Procedure shows only N1 settings for unreliable IAS, which is good. EPR is unstable and dependant on ice on probes, as well.