Originally Posted by
JRBarrett
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.
JR - your story about automatic retraction of the inboard leading edges at high AOA on the Falcon 900 reminds me of stall characteristics on the original 777. As an airplane approaches stall you would like to see two things happen. First the center of lift shifting aft will generate a nose down pitch break. Second the wing tips continuing to hold on (i.e., not stall first) will preserve roll control. Both of these objectives are achieved if the inboard portion of a swept wing can be made to stall before the outboard portion of the wing. On 777 the darn thing would not stall! It got up to 30+ degrees without a pitch break and the wing tips would sometimes go first leading to a rather exciting rolloff - beyond 90 degrees in some cases! The solution on 777 was to practically eliminate the leading edge gap on the inboard wing when the slat are extended. This caused the inboard wing to stall first at a much lower AOA and preserved roll control with the outboard ailerons. It sounds like the Falcon 900 by retracting the inboard leading edges at high AOA was probably accomplishing the same thing by forcing the inboard portion of the wing to stall first with the wing tips still flying well and thus preserving roll control.
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