in an airplane, how does an AoA sensor failure cause Altitude/Airspeed Disagree

Question

I’m reading about the Boeing 737 Max 8 incidents involving Lion Air Flight 610 and Ethiopian Airlines Flight 302.
In both flights, erroneous AoA data from the pilot side AoA sensor fouled airspeed and altitude measurements for the pilot side FCC which triggered an airspeed disagree and altitude disagree with the correct First Officer FCC.
My question is how is AoA data used to calculate altitude and airspeed, I’m an aerospace engineer and in school we are taught that conventionally, things like altitude and airspeed is measured with static pressure, stagnation pressure etc.
I can’t find any straight forward answers online other than folks on Quora taking a whole essay to say “well AoA data is used to calculate it in the ADIRU” which doesn’t explain anything.

Edit: I just remembered how AoA is used to correct pitot-static pressure and varying angles of attack because the tube isn’t head on into the incoming flow but that still leaves the question of altitude? are they incrementally multiplying the velocity by time to get a displacement and taking the sin(pitch) to get altitude? that’s my guess.

In: Engineering

Answer 1

You need to read r/Admiral_Cloudberg.

I don’t have all the information you want. But, essentially, the static pressure is whatever you would expect it to be at that altitude. The static pressure is from a sensor built into the side of the plane, not facing the wind. The dynamic pressure is the pressure of the air in the pitot tube as the plane is moving. It directly faces into the wind.

The AoA sensor represents the angle at which the plane is facing into the wind. It rotates with the attitude of the airplane. If, for example, the AoA is frozen in place at a 0 degree angle, but the airplane is pitched 20 degrees nose up, the airspeed will be slower (I think) than it would be with the same air pressure inputs if it was level flight, 0 degree attitude.

Typically “disagree” means there’s a difference in the resulting data. For example, if one AoA sensor is moving freely as it should, but the other is frozen in place, one might say 15 degrees nose up, the other is frozen at 0 degrees. The two inputs don’t match, so the computer doesn’t know which one is which. There’s a third sensor, so it takes the best two out of three and goes from there as best it can.