Free Republic
Browse · Search
General/Chat
Topics · Post Article

To: libertylover
Yes, an obstructed static port can also cause an erratic altimeter reading also. But I know from my own repair experience that a dirty AOA probe will cause WILDLY erratic readings.

There are two types of AOA sensors. Supersonic aircraft, like the F-111A that I worked on, usually have the pressure differential type, and subsonic aircraft including most Airbus and Boeing aircraft, including the MAX, use the vane type.

I suspect that in your Air Force days, like mine, that the pressure differential type of AOA sensor was used, and the air pressure was also used somehow in the altimeter.

The MAX uses the vane type, and cannot possibly affect the altimeter.


46 posted on 03/11/2020 6:15:39 AM PDT by Yo-Yo ( is the /sarc tag really necessary?)
[ Post Reply | Private Reply | To 43 | View Replies ]


To: Yo-Yo

Thanks for the diagram. Yes, the F-4D had a pressure differential type of AOA like the one in the diagram.

I wasn’t trying to imply that the MAX crashes were caused by faulty altimeter readings, rather my comments about the wild altimeter readings were intended to point out that AOA information can be wildly inaccurate at times.

From what I’ve read, the MAX uses AOA information fed into a system which then controls the flight surfaces. I believe that the MAX crashes may have been caused by faulty AOA readings that made the system “think” it was in a stall situation, when in fact, it was not.

I remember the F-4’s AOA system because we had so much trouble with it - the altimeter problems. But I also worked on some Boeing built planes, the KC-135, the RC-135, and the EC-135 when I was at Offutt for 6 months. I don’t even remember the AOA system on these aircraft so they were probably pretty reliable but probably had the vane type of AOA probe. But I think the vane type would be just as susceptible to corrosion as the probe type because the problems were inside the housing and not in the probe itself (unless the heater wasn’t working).

FYI: The static ports on the F-4D were located on the side of the Radome and this may be why they had to correct the raw static information for changing AOA. Also, the flushness of the static port itself was crucial. The F-4E models (and I think the C models) had a probe that stuck out in front of the planes, but I never worked on those model so I don’t know whether or not these other models changed the static information based on AOA inputs.

— libertylover


47 posted on 03/11/2020 8:19:18 AM PDT by libertylover (Socialism will always look good to those who think they can get something for nothing.)
[ Post Reply | Private Reply | To 46 | View Replies ]

Free Republic
Browse · Search
General/Chat
Topics · Post Article


FreeRepublic, LLC, PO BOX 9771, FRESNO, CA 93794
FreeRepublic.com is powered by software copyright 2000-2008 John Robinson