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Stab trim cutout is not the only way to disable MCAS. Extending the flaps any amount also disables MCAS. Turning on autopilot also disables MCAS, though this i

by Cacti 7y ago

Stab trim cutout is not the only way to disable MCAS. Extending the flaps any amount also disables MCAS. Turning on autopilot also disables MCAS, though this isn’t entirely effective since spurious AoA readings may quickly disable the autopilot again.

cjbprime 7y ago
Boeing put out an emergency airworthiness directive after Lion Air. It doesn't tell pilots to lower flaps. AoA sensor failure causes IAS Unreliable warnings, and the checklist for that item demands that flaps be left alone -- if you don't know how fast you're going, lowering flaps could cause a wing stall. It's not reasonable to expect pilots to disobey checklists. We would all be less safe if they did. If pilots are following Boeing's instructions and planes are crashing, that's on Boeing.
metanoia 7y ago
One question I've had is why Boeing changed the design of the stab cutout switches from the 737NG from a pair of switches - the right side switch that disabled the autopilot control of trim and the left side that disabled the trim motors entirely [1] - to a design with a similar pair of switches, but each controlling the primary and backup control motors of the elevator trim [2] instead of controlling the source of the input. (If you look closely at the yoke trim switches, there are two independent switches that are grouped together by a frame so that they move simultaneously. One drives the primary motor, and the other the secondary). Assuming they'd kept MCAS on the switch associated with the automatics, the procedure would have been to throw the AUTO PILOT switch, disabling MCAS, but keeping the MAIN ELEC switch on, allowing them to trim back to neutral for that speed. [1] https://www.airliners.net/photo/Australia-Air-Force/Boeing-737-7DT-BBJ/865930 https://www.airliners.net/photo/Australia-Air-Force/Boeing-7... [2] http://www.b737.org.uk/mcas.htm#stcs http://www.b737.org.uk/mcas.htm#stcs
- metanoia 7y ago
  EDIT: Oops, there's only one trim motor on the NG. Both trim switches must be actuated on the yoke switches though. But the input cutoff switches still work the same way I originally stated. Not sure about the Max's # of trim motors, but the Max definitely doesn't have input source cutoff switches.

by Cacti 7y ago

cjbprime 7y ago

Boeing put out an emergency airworthiness directive after Lion Air. It doesn't tell pilots to lower flaps. AoA sensor failure causes IAS Unreliable warnings, and the checklist for that item demands that flaps be left alone -- if you don't know how fast you're going, lowering flaps could cause a wing stall. It's not reasonable to expect pilots to disobey checklists. We would all be less safe if they did. If pilots are following Boeing's instructions and planes are crashing, that's on Boeing.

metanoia 7y ago

One question I've had is why Boeing changed the design of the stab cutout switches from the 737NG from a pair of switches - the right side switch that disabled the autopilot control of trim and the left side that disabled the trim motors entirely [1] - to a design with a similar pair of switches, but each controlling the primary and backup control motors of the elevator trim [2] instead of controlling the source of the input. (If you look closely at the yoke trim switches, there are two independent switches that are grouped together by a frame so that they move simultaneously. One drives the primary motor, and the other the secondary). Assuming they'd kept MCAS on the switch associated with the automatics, the procedure would have been to throw the AUTO PILOT switch, disabling MCAS, but keeping the MAIN ELEC switch on, allowing them to trim back to neutral for that speed. [1] https://www.airliners.net/photo/Australia-Air-Force/Boeing-737-7DT-BBJ/865930 https://www.airliners.net/photo/Australia-Air-Force/Boeing-7... [2] http://www.b737.org.uk/mcas.htm#stcs http://www.b737.org.uk/mcas.htm#stcs

metanoia 7y ago
EDIT: Oops, there's only one trim motor on the NG. Both trim switches must be actuated on the yoke switches though. But the input cutoff switches still work the same way I originally stated. Not sure about the Max's # of trim motors, but the Max definitely doesn't have input source cutoff switches.