@roker Don't mind the news, we all know how 'accurate' they can be.
The problem was that MCAS could detect what it saw as an approach to a stall situation and take remedial action ( uncommanded ) by pitching the nose down. Remember, a lot more than just low airspeed can result in a wing stall.
The MCAS read different variables from different sources which included, wing config., thrust setting, COG., speed and AoA. It read only one of the two AoA sensors ( albeit this alternated on each start-up ) and so without any cross reference from the second AoA sensor the MCAS could inadvertently command a pitch down in an attempt to reduce the AoA. In the early stages of development, accelerometers were employed to measure change in pitch, but proved to be unreliable at low speeds and slow changes in attitude and so were abandoned.
While every 737 pilot has been trained to deal with what is called a 'runaway stabilizer' situation which is a condition that gives similar results to MCAS misbehaving, unbeknownst to pilots Boeing had changed the functionality of the two switches, at the back of the throttle quadrant, which may be used to deal with the above situation.
Before implementing MCAS switching off one ( or both, depending on the circumstances) pitch control could be operated electronically by use of the toggle switches on the left of each control yoke. But with the MAX and MCAS, Boeing insisted that BOTH switches be off without explaining the complete function of each switch. This removed the use of the electric yoke toggle switches and so the only option to change stabilizer trim was to use a hand crank on the trim wheels.
If Boeing had kept the functionality the same as the NG's, then just turning off the right hand stab trim (autopilot ) switch would have disabled the STS ( speed trim system) and MCAS but the pilot would still have electric trim control. Unfortunately, as we know, this was not the case.
The problem was that MCAS could detect what it saw as an approach to a stall situation and take remedial action ( uncommanded ) by pitching the nose down. Remember, a lot more than just low airspeed can result in a wing stall.
The MCAS read different variables from different sources which included, wing config., thrust setting, COG., speed and AoA. It read only one of the two AoA sensors ( albeit this alternated on each start-up ) and so without any cross reference from the second AoA sensor the MCAS could inadvertently command a pitch down in an attempt to reduce the AoA. In the early stages of development, accelerometers were employed to measure change in pitch, but proved to be unreliable at low speeds and slow changes in attitude and so were abandoned.
While every 737 pilot has been trained to deal with what is called a 'runaway stabilizer' situation which is a condition that gives similar results to MCAS misbehaving, unbeknownst to pilots Boeing had changed the functionality of the two switches, at the back of the throttle quadrant, which may be used to deal with the above situation.
Before implementing MCAS switching off one ( or both, depending on the circumstances) pitch control could be operated electronically by use of the toggle switches on the left of each control yoke. But with the MAX and MCAS, Boeing insisted that BOTH switches be off without explaining the complete function of each switch. This removed the use of the electric yoke toggle switches and so the only option to change stabilizer trim was to use a hand crank on the trim wheels.
If Boeing had kept the functionality the same as the NG's, then just turning off the right hand stab trim (autopilot ) switch would have disabled the STS ( speed trim system) and MCAS but the pilot would still have electric trim control. Unfortunately, as we know, this was not the case.
Last edited:
