Old 2nd Jan 2019, 16:29
  #63 (permalink)  
Join Date: Feb 2009
Location: Seattle
Posts: 379
AOA vane angle is converted to body angle

Originally Posted by Machinbird View Post
Conso, either find a picture of the nose of a NG/MAX 737 and count the screw holes or go to the airport and make your own picture. I doubt if it has more than 6.
They can limit the interchangeability of probes by playing with the screw hole spacing, the mounting plate size, the electrical connectors, etc. There is an indexing pin involved as well, and that can control proper installation too.
With 9 holes you you would have a 40 degree error. That doesn't fit the scenario.
When considering the potential for AOA sensor installation to have played a role in this event, it is important to recognize the difference between AOA vane angle and AOA body angle. The local flow that the vane measures is distorted by the fuselage. As a result, the vane angle changes much more than one degree for each degree change in airplane AOA. Within the AOA signal processing logic is conversion of the vane angle back to the corresponding body angle. This conversion has a gain factor of just about 0.5 to get from vane to body.

The data that we have seen for the Lion Air event shows an AOA bias of approximately 20 degrees body between left and right signals. That could result from the difference in vane angles being approximately 40 degrees.
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