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Old 10th Nov 2018, 18:55
  #963 (permalink)  
wiedehopf
 
Join Date: Jul 2014
Location: Germany
Posts: 343
Originally Posted by Gysbreght View Post
According to the FAA Emergency AD, an erroneous Angle of Attack (AOA) input, among other symptoms, can result in indications of:

IAS DISAGREE alert.

ALT DISAGREE alert.

I'm wondering why an erroneous Angle of Attack (AOA) input would cause those alerts. Is it because the pitot-static position error corrections require an AoA input?
If you read the thread carefully you will come across this post:

Originally Posted by Denti View Post
From my own experience on the NG a wrong AoA input will result in UAS, unreliable altitude, vertical speed, wind information and ground speed display. It is a basic correction factor into the ADIRU that does effect all resulting air data related information and surprisingly the non air data related information of ground speed as well. It might be different in the MAX, but somehow i doubt it. As different airflow over the fuselage results in huge position error values for static and pitot tube values, the AoA vane corrects those very different position errors, therefore a wrong AoA indication will result in a completely unreliable air data set.
Then there is also this comment.

Flutter speed
8th Nov 2018, 15:20
"So, for a 737, what would be the largest difference between IAS, uncorrected for AoA, and CAS, within the range of AoA that could be reasonably expected in flight? The large airplanes I've flown didn't correct IAS for AoA, and without looking them up, I don't recall that the airspeed calibration charts having a correction larger than 5 ish knots. Seems if it came down to a choice between an IAS that differed from CAS about 5 kt at low airspeeds/high AoA and no usable airspeed indication at all if the AoA fails, I know what I'd prefer. Especially if my takeoff and landing charts were referenced to IAS, whcih makes CAS a relatively uninteresting number. (except in cruise, where IAS and CAS are usually fairly close anyway) "

Just ran some numbers, in very general terms, depending on sensor position and factors like Mach number, in extreme cases... expect up to 5kts difference. Probably enough to trigger an IAS and ALT disagree on a modern airliner. For a plane flying at moderate speed the difference will be more towards 2kts.

The archive version of this forum can be searched a little bit better with CTRL-F
https://www.pprune.org/archive/index...14857-p-4.html

In case you want to dig around the thread some more.
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