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
so, not an engineer, but my understanding is that a their will be dynamic pressure changes brought on by the planes motion, as the plane pushes though the air, so to properly eliminate those you need to know the planes angle of attack. to quote wiki:
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in flight, the air pressure varies slightly at different positions around the aircraft’s exterior, so designers must select the static ports’ locations carefully. Wherever they are located, the air pressure that the ports observe will generally be affected by the aircraft’s instantaneous [angle of attack](https://en.wikipedia.org/wiki/Angle_of_attack).[^([12])](https://en.wikipedia.org/wiki/Static_pressure#cite_note-12) The difference between that observed pressure and the actual atmospheric pressure (at altitude) causes a small [position error](https://en.wikipedia.org/wiki/Position_error) in the instruments’ indicated altitude and airspeed
so, to properly account for that change, you need to know AoA
Altitude is typically a separate measurement, from the altimeter, but you might be referring to rate of climb, which is something the air data computer can calculate.
The pitot tube doesn’t swivel, so it only points directly into the path of motion at the trim AoA. Other AoAs cause it to read low, and the AoA indicator is used to calculate a correction.
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.
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