A function of the flight control computer, the MCAS activates when an aircraft’s angle of attack (AOA) exceeds a certain threshold based on airspeed and altitude readings, pushing the nose down to reduce the risk of a stall. The system operates only when the autopilot is off, flaps are up and during steep turns. It resets when the AOA falls below the threshold or pilots provide manual horizontal stabilizer trim commands.
Not part of previous 737 designs, the system is a feature of new MAX series airliners, Boeing has acknowledged. But the MCAS is not covered in the MAX flight crew operations manual (FCOM) or in difference training instructions for pilots of earlier 737NGs, operators have noted.
Boeing has issued an FCOM bulletin to 737 MAX operators warning that erroneous AOA data trigger automatic nose-down inputs and emphasizing that pilots follow specific procedures to keep the aircraft from descending uncommanded. The FAA mandated the action in a Nov. 7 emergency airwo
From Boeing: The AOA is used to drive stall warning, stick shaker, stall margin info on airspeed indicators, pitch limit indicator.
It comes from this: (this file wont take photos). Anyway, an AOA Vane on the side of the A/C, just behind that is a transducer which changes analog movement to a to an electrical signal. That signal is fed to both Air Data Computers which drives all the above.
Failure of this unit (transducer) would create extremely dangerous conditions; stick shaker, stall horn (very loud), erratic airspeed, climb and dive, etc. Daytime VFR would be demanding (probably the source of the previous write-ups). Night IMC would most likely be fatal.
That A/C required a maintenance check flight.
Very easy to do. Even though most airlines REQUIRE the localizer be dialed in, it doesn't happen. Saw it many times at ELP/Biggs (we called Biggs 22R) and Ellyson Field and Pensacola Regional. Ellyson was a helicopter base, 1500' runways, saw an Eastern 727 almost touchdown there. Ellyson is an industrial park now. Also Corpus Christie and NAS.
Fortunately it usually just hurts egos.
ICAO FHSH, Runway 02/20, 15 degrees South. VOR, GPS ground augment to ILS precision. Nice what $350m can make. Always nice to have options, especially in the middle of nowhere.
Took a P3 into Ascension for two weeks back in the '70's. Nice people, nice island, nice airport but SO remote.
