Originally Posted by
FCeng84
The UAS on Lion Air resulted from the left and right airspeed signals not tracking each other. This was most likely caused by the the left /right sensed AOA difference, but declaring UAS did not involve a direct comparison of the two AOA signals. On 737 the difference between measured static pressure and actual static pressure is a function of AOA. In order to account for this, measured static pressure is compensated based on measured AOA to minimize the resulting static pressure error that would result from a change in AOA. This AOA compensation for static pressure is done independently by the two air data systems that use the sensors on the two sides of the airplane. As a result, if the left and right static pressure ports measure identical pressures but AOA as sensed by the vanes on the two sided of the airplane differ, the compensated static pressure signals will between left and right. This difference causes the left and right barometric altitudes to differ slightly and the computed left and right airspeeds to differ slightly. It is the difference in airspeeds left to right that leads to the system issuing its UAS indication.
AOA sensors are standard on almost every transport category aircraft. They are the primary sensor for the stall warning system (along with indicated airspeed), and may (or may not, depending on model) be used as part of static source error compensation.
I am an AME, not a pilot, so cannot comment on the operational utility of having an AOA display in the cockpit. Two aircraft models I have worked on that do have such displays as “standard kit” are the Beech 400A - which uses a military-style lighted chevron indexer on top of the glareshield, and the Gulfstream GIV, which has an relative AOA index scale on the pilot and copilot PFD displays, immediately to the left of the airspeed tape.
On both models, I assume flight crews are trained in the proper use and interpretation of the AOA readings.
On every other model in my experience, AOA is “there” in the background, as part of the overall stall warning and air data system, but not displayed to the pilots directly.
On the GIV, the AOA correction to altitude and airspeed can be significant in certain flight configurations. It is something we test during the required 24-month re-certification of the air data system. In landing configuration, with full flaps and the AOA vane rotated to a specified high alpha setting, the correction to indicated altitude can be as much as 600 feet.
The only aircraft model I am familiar with where the AOA system can cause an automatic aircraft configuration change is the Falcon 900. If both AOA vanes sense an impending stall, and the leading edge slats are deployed, the inboard slats will automatically retract. I believe this is done to prevent instability in the roll axis from occurring when close to to a stall.
But unlike the 737-8 MCAS - the Falcon autoslat retraction is not a “hidden mystery”. Every Falcon pilot is trained in the existence and operation of the system, and there is are specific preflight tests performed prior to taxi to insure it is working properly. If either AOA vane fails in flight, the autoslat system becomes inoperative with a warning light displayed to alert the crew.