Originally Posted by
EDLB
Can you verify this? This would be very crude early 1970th technology and with a Haming distance of 1 only protect against single bit failures. A two bit failure would create the next valid data. And there is no guarantee that any interference will only disturb 1 bit. Equaly likely is, that a sequence of bits are disturbed.
I still wonder where the 20 degrees constant offset in the Lion Air AOA sensor came from. See preliminary report. That’s not a stuck sensor.
If the Ethiopian will show a similar offset then there is another problem burried in their flight control system.
It may not be a case where the Arinc 429 data label is being corrupted in transit (which would be detected by the CRC check). More likely that the A/D converter that receives the analog AC voltage from the AOA vane is, under certain circumstances, generating an incorrect binary representation of the voltage.
Every modern aircraft contains multiple A/D converters for various system parameters. For instance, all of the engine data originates as analog signals. The N1 and N2 sensors on the engine produce a pulsed waveform whose frequency varies with the rotational speed of the spools. The EGT probes directly generate a DC voltage which is proportional to measured temperature. The oil pressure transducer resistance varies with pressure, which also produces a variable DC output voltage. The fuel flow transducer produces an AC sine wave the frequency of which is proportional to flow rate.
All of these analog signals have to be converted to digital values to drive the displays in the flight deck, and to supply other systems like the FMS.
I have seen many cases of A/D converter failures causing loss of engine indications. If I had a squawk that there was no oil pressure reading, I would suspect the transducer on the engine, but if the squawk related to a loss of N1 or N2, I would suspect the A/D conversion, because the engine-mounted speed transducers are so simple and rugged in design that failure would be almost impossible.
USUALLY if an A/D converter fails, it will simply produce no output at all. But, I have seen two instances where the conversion did not fail outright, but resulted in garbage data. In one case, an aircraft was showing exactly half the expected fuel flow rate at any given power setting. This could very well be a case of failure of one specific binary bit in the digital representation of the analog fuel flow transducer voltage. In another case, An aircraft was showing 50 psi oil pressure when the engine was completely shut down, and the pressure rolled backwards to zero when the engine was running.
Both of these instances were caused by a “one-off” failure of the DAU (data acquisition unit), which was permanently corrected by installing a replacement DAU. In both cases, the faulty DAU had been in service for many years.
But if the false AOA reading in the two Max incidents was indeed caused by an incorrect A/D conversion, it is a more serious problem. Both aircraft were almost brand new. If the FDR readout from the Ethiopian flight also shows an exact 22 degree AOA reading on one side, it points to an inherent latent software or hardware bug in the AOA A/D channel that could very well happen again on other aircraft.
I’m sure that Boeing engineers will be looking very closely at the AOA system on the Max in addition to their modifications to MCAS.