Originally Posted by
Smythe
Yes, the sensor vane is integral to the internals.
It is indexed, according to the individual aircraft angle of attack of the wing to fuselage angle. I believe that if you consider the fuselage horizontal or at 0, the wing chord line is at an angle of 2.5 degrees on the 737?
What is unclear to me is why it is necessary for the AoA sensor to be a vane outside of the ac. It is assumed that airflow is horizontal, and the fuselage/wing combination is at an angle to horizontal. Why is the AoA sensor not internal like the IRU gyro?
Internal sensors can measure pitch angle and acceleration, but that would be of no use for AOA. The AOA sensor measures the angle between the relative wind and the fuselage, and that can only be achieved by a sensor attached externally and exposed to the air stream.
As a maintenance engineer, I have never worked on any model of the 737, but in my career I have had occasion to change AOA vanes several times on various aircraft models. Only once was the change-out caused by a malfunction of the actual position transducer. The more common malfunction is a failure of the anti-ice heater.
In any case, I have never seen any AOA vane model that did not have either a guide pin, or an offset bolt pattern that would make it physically impossible to install it in the wrong orientation.
The actual position is typically measured in one of two ways. On smaller business jets which have DC electrical systems, the AOA transducer will consist of a precision potentiometer that outputs a variable DC voltage corresponding to position. On larger aircraft with AC electrical systems, the AOA transducer will consist of a RVDT transformer which outputs an AC waveform with position corresponding to the amplitude and phase of the output waveform.
https://en.m.wikipedia.org/wiki/Rota...al_transformer
In any case, the raw position data is an analog signal. At some point downstream, the analog signal will be digitized by an AD (analog-to-digital) converter.
The analysis by “Satcom Guru” is intriguing. He has found that the digital representation of AOA in the 737 is expressed as a 26 bit binary word, and if the 26th bit becomes incorrectly set (goes from binary zero to binary 1), it will correspond to an AOA of exactly 22 degrees, which is what the FDR on the Lion Air flight recorded for the left AOA sensor. It remains to be seen if this was also the case on the Ethiopian flight for one of the two sensors.
I strongly suspect that there was nothing wrong with the AOA sensor on LionAir - but rather some kind of intermittent hardware or software fault in the downstream conversion of the position data from analog to digital. If this also happened on Ethiopian, then the cause of this possible data corruption is going to have to be found and corrected on the Max in addition to any changes made to the MCAS system.