Originally posted by
DaveReidUK (Post 137):
“All of the above are theoretically possible. None have actually been demonstrated to be the case”.
Originally posted by
ecto1 (Post 139):
“I don't particularly fancy the theory of a mechanical offset because of loose bolt (slipping shaft). People do know how to manufacture square shafts and splines and keyed shafts and safety wire and plenty of other tricks. I don't know how much is one of those vane things, but no less than 3000 dollars for sure. Probably way way more. Two cylindrical shafts with a screw tightening one against the other is not a proper way to do the job of critical torque transmission without slipping (well, maybe in toys)”.
“In other words, I would not expect any connecting part of a rotary sensor with an all round shaft. Proper way: Splined (if it needs adjusting), keyed, or at least with a D shape”.
Originally posted by
JRBarrett (Post 140):
“I have replaced RVDT sensors of similar electrical design (with a sin/cos output), used for elevator or aileron position feedback for the autopilot on various aircraft. They have always used a splined shaft. I’ve never seen one with a round shaft held with a set screw. I would think that the internal mechanical connection in an AOA sensor would be similar”.
In response to the above comments regarding Double07’s posts suggesting that the two B737MAX incidents may have been caused by a defective AoA sensor, please read the recent FAA Emergency Air Worthiness Directive No. 2019-08-51, for the Cirrus SF50 aircraft, dated April 18, 2019. (I can’t post this AD because of my PPRUNE newbie status, but you can search for it on Google).
This emergency AD describes three incidents with the Cirrus Model SF50 aircraft that are astonishingly similar to the incidents with the B737 MAX aircraft. Quoting from the FAA document: “This emergency AD was prompted by Cirrus reporting three incidents on Cirrus Model SF50 airplanes of the stall warning and protection system (SWPS) or Electronic Stability & Protection (ESP) System engaging when not appropriate, with the first incident occurring in November 2018 and the latest in April 2019. The SWPS or ESP systems may engage even when sufficient airspeed and proper angle of attack (AOA) exists for normal flight. The SWPS includes the stall warning alarm, stick shaker, and stick pusher. The ESP includes under speed protection (USP). The SWPS system engaging inappropriately could potentially result in a STALL WARNING crew alert (CAS) message activation, accompanied by an audio alarm and stick shaker activation, followed possibly by either low speed ESP/USP engaging, and/or the stick pusher engaging. The pilot will also observe the dynamic and color-coded (Red) airspeed awareness ranges displaying the stall band, regardless of actual indicated airspeed.”
The AD goes on to state: “Cirrus and Aerosonic (manufacturer of the technical standard order AOA sensor) have identified the probable root cause as an AOA sensor malfunction due to a quality escape in the assembly of the AOA sensor at Aerosonic. Two set screws that secure the potentiometer shaft to the AOA vane shaft may have improper torqueing and no application of thread locker (Loctite) to secure the two set screws. The AOA sensor with this quality escape is labeled with part number 4677-03”.
So, even though the AoA sensors in these two aircraft have different manufacturers and designs, they are sufficiently similar that they can be affected by the same type of defect. And in both cases, this defect can cause an offset in the AoA sensor output leading to a false triggering of the stall warning system.