Illustrations : NY Times
As Investigators and experts are looking into reported technical issues of the Lion Air Boeing 737MAX-8 that crashed last month, Boeing released a bulletin explaining, the recovery procedure, when a 737MAX enters into a “stall”.
When an airplane exceeds its given critical angle of attack and is no longer able to produce the required lift for normal flight, it enters into “stall”
Characteristics of a stall include a sudden (if sometimes gradual) pitch down of the nose of the aircraft, and may accompany by a roll or yaw to one side if the aircraft is uncoordinated. If this happens and recovery procedures are not initiated right away, an aircraft may enter a spin.
Boeing 737MAX8 has a stall recovery system called maneuvering characteristics augmentation system, or M.C.A.S.
According to NY times, citing Boeing’s Bulletin, “On the outside of the yoke in front of both the pilot and the first officer, there is a switch for electrically controlling the trim – the angle of the stabilizers. If the pilot understood what was happening, he could have used that switch for a few seconds at a time to counteract what the M.C.A.S. was doing to the stabilizers. But that would have been only a temporary solution: the pilot has to release the switch or the nose could go too high. But if he releases the switch, the anti-stall system would reactivate a few seconds later”…..
“The crucial step, would be to reach across to the central console to a pair of switches (sometimes protected with covers that must be opened), and flip the switches off. Those switches disable electric control of the motor that moves the stabilizers up and down, preventing the anti-stall system from exerting control over their position.”
“The final step would complete the process for giving the pilots physical control. Cables for manually operating the stabilizers run over a wheel – actually two wheels, one on either side of the console next to the ankles of the pilot and first officer. One of the pilots must rotate the wheel to pull the stabilizer back into the correct position.”
In the case of Lion Air, There are no official findings, that M.C.A.S., was activated before the crash.