Sorry for the pilot speak, but I found a brief explanation of the issue from a U.S. pilot flying the airplane (Southwest or American). It also explains the engine issue mentioned by someone.
Brief explanation of terms:
MCAS...Maneuvering Characteristics Augmentation System...designed to minimize a stall
AOA...Angle of attack...in layman's terms, the climb angle of the aircraft.
QRH...Quick Reference Handbook...a handbook of emergency procedure checklists...available at each pilots seat
AFM...aircraft flight manual
Subject: Re: The MCAS
I just flew the Max yesterday. I understand why MCAS was added to MAX. The larger engines created a greater pitching (up) moment at high AOA. It runs the trim nose down at a rate slightly faster than speed trim if the airplane is flying very slowly with high AOA.
2 issues I have with the system are that:
1). Boeing deleted ANY mention of MCAS and how it works in the AFM. Nobody knew about it before LionAir.
2). It looks and feels like speed trim....BUT, the column switches that stop speed trim DO NOT stop MCAS.
If you remember Speed trim, it Trims nose down as you accelerate (autopilot off) to maintain a specified column force (FAR 23 requirement). If you apply aft column force against the (nose down) speed trim, switches in the base of the control column cut out the speed trim. This is not the case with MCAS.
In the LionAir (Indonesia) crash, a faulty AOA indicator told MCAS that the AOA was high and the trim started running forward as soon as the flaps were raised. The faulty AOA also caused a continuous stick shaker at rotation. The continuous shaker going off most likely contributed to their “helmet fire”.
I think the airplane is fine. Boeing is supposed to issue a software fix to add the column switches to override MCAS trim.
It’s unfortunate that the LionAir crew did not run the “runaway trim” QRH procedure.
If they had simply turned off the two switches below the flap handle or selected flaps 1 or more, they would have been fine.
Brief explanation of terms:
MCAS...Maneuvering Characteristics Augmentation System...designed to minimize a stall
AOA...Angle of attack...in layman's terms, the climb angle of the aircraft.
QRH...Quick Reference Handbook...a handbook of emergency procedure checklists...available at each pilots seat
AFM...aircraft flight manual
Subject: Re: The MCAS
I just flew the Max yesterday. I understand why MCAS was added to MAX. The larger engines created a greater pitching (up) moment at high AOA. It runs the trim nose down at a rate slightly faster than speed trim if the airplane is flying very slowly with high AOA.
2 issues I have with the system are that:
1). Boeing deleted ANY mention of MCAS and how it works in the AFM. Nobody knew about it before LionAir.
2). It looks and feels like speed trim....BUT, the column switches that stop speed trim DO NOT stop MCAS.
If you remember Speed trim, it Trims nose down as you accelerate (autopilot off) to maintain a specified column force (FAR 23 requirement). If you apply aft column force against the (nose down) speed trim, switches in the base of the control column cut out the speed trim. This is not the case with MCAS.
In the LionAir (Indonesia) crash, a faulty AOA indicator told MCAS that the AOA was high and the trim started running forward as soon as the flaps were raised. The faulty AOA also caused a continuous stick shaker at rotation. The continuous shaker going off most likely contributed to their “helmet fire”.
I think the airplane is fine. Boeing is supposed to issue a software fix to add the column switches to override MCAS trim.
It’s unfortunate that the LionAir crew did not run the “runaway trim” QRH procedure.
If they had simply turned off the two switches below the flap handle or selected flaps 1 or more, they would have been fine.
