Originally Posted by
BOAC
- a question I have been pondering.
3) In view of MFS's post (2 Mar 12:50) I have to say we may not actually know what the crew were seeing for IAS, and, indeed apparently we cannot assume it to be correct?
Actually, since it's stated to be CAS and is (I assume) the ADC output/display input speed (it's most likely the FDR is picking up the ADC output) I'd consider the most likely interpretation of the CAS values to be:
1. The output from the ADC, accurately measured (given sample rate issues etc)
2. Most likely also the value presented to the crews provided (a) they had selected the appropriate source for display (most (all?) multi ADC multi display a/c allow crew to reconfigure the ADC sources for display, to allow for ADC failures) and provided (b) that the values were not interpreted as "out of range" by display software and displayed at "limit of range" values - for example, some a/c won't display airspeeds close to zero, and instead apply a lower limit of about 20-30 knots, since the airpseed is meaninglessly nonsense at such values anyway.
3. With the a/c at high (but perhaps also erroneously sensed) AOAs, any ADC corrections which are a function of AOA may be incorrect. Therefore the CAS output by the ADCs may not be representative of the pressures actually sensed by the ADCs.
4. Also, with the a/c at high AOA (and perhaps also sideslip) the air data probes themselves may not have been accurately sensing the freestream pressures any more, so even if the ADCs had no correction errors, the raw pressure data, and thus any derived speeds, are suspect.
In other words, the data probably are a decent representation of what the crew saw, but may have little relationship to the true aerodynamic conditions of the time.