4 ms·

> MCAS commands nose-down trim. The pilot uses the trim button on the yoke to command nose-up trim. Why isn't that enough to disable MCAS and cause an audible a

> MCAS commands nose-down trim. The pilot uses the trim button on the yoke to command nose-up trim. Why isn't that enough to disable MCAS and cause an audible alert? It does. It's just that MCAS tries again in 5 seconds. That's why the experienced pilots in both the Lion Air and Ethiopian crashes kept the MCAS in check - trimming to neutral column force is second nature to them - and it only crashed when the first officers took over, who allowed it to get to full nose down trim. A constantly incorrectly trimming system is supposed to trigger the runaway stabilizer trim checklist, so Boeing thought a failure in this system was covered by existing training. Unfortunately, a periodic every-5-seconds trim wasn't recognized as such. I'm not a 737 pilot, so I don't know what runaway stab trim normally looks like; maybe it's usually constant?

lisper 8y ago
> I'm not a 737 pilot, so I don't know what runaway stab trim normally looks like; maybe it's usually constant? Yes, exactly. The problem (well, one of the problems) is that during normal operations the trim wheels are constantly moving intermittently, so additional intermittent motion isn't immediately seen as unusual. (Disclaimer: I am not a 737 pilot either, but I am a pilot, I fly a plane with an electric trim (Cirrus SR22) and the info above came from a very experienced 737 pilot named Juan Browne who hosts an excellent channel on YouTube called Blancolirio.)
cameldrv 8y ago
The problem is: 1. MCAS failure produces an intermittent stab trim movement. 2. Boeing claimed that the stab trim runaway checklist should have been run and would have let the crew recover. 3. Boeing's checklist only calls for action on "continuous" uncommanded stab trim movement https://www.satcom.guru/2019/03/taking-next-steps-while-awaiting-on.html https://www.satcom.guru/2019/03/taking-next-steps-while-awai... (second image) 4. Intermittent uncommanded stab trim movement is normal. The Speed Trim System and Elevator Feel Shift system regularly adjust trim, so merely seeing the wheels move by themselves is not cause for alarm. 5. MCAS is not documented at all in the flight manual, and the stab trim runaway checklist was not updated to address how MCAS might cause a failure necessitating its use, presumably to avoid the need for additional transition training. 6. MCAS is unreliable due to its reliance on a single AoA sensor.

lisper 8y ago

> I'm not a 737 pilot, so I don't know what runaway stab trim normally looks like; maybe it's usually constant? Yes, exactly. The problem (well, one of the problems) is that during normal operations the trim wheels are constantly moving intermittently, so additional intermittent motion isn't immediately seen as unusual. (Disclaimer: I am not a 737 pilot either, but I am a pilot, I fly a plane with an electric trim (Cirrus SR22) and the info above came from a very experienced 737 pilot named Juan Browne who hosts an excellent channel on YouTube called Blancolirio.)

cameldrv 8y ago

The problem is: 1. MCAS failure produces an intermittent stab trim movement. 2. Boeing claimed that the stab trim runaway checklist should have been run and would have let the crew recover. 3. Boeing's checklist only calls for action on "continuous" uncommanded stab trim movement https://www.satcom.guru/2019/03/taking-next-steps-while-awaiting-on.html https://www.satcom.guru/2019/03/taking-next-steps-while-awai... (second image) 4. Intermittent uncommanded stab trim movement is normal. The Speed Trim System and Elevator Feel Shift system regularly adjust trim, so merely seeing the wheels move by themselves is not cause for alarm. 5. MCAS is not documented at all in the flight manual, and the stab trim runaway checklist was not updated to address how MCAS might cause a failure necessitating its use, presumably to avoid the need for additional transition training. 6. MCAS is unreliable due to its reliance on a single AoA sensor.