3 ms·

It also didn't mention that you had 5 seconds after the release of the electronic trim switch to flip the CUTOUT before MCAS poured back on the trim, but as I r

It also didn't mention that you had 5 seconds after the release of the electronic trim switch to flip the CUTOUT before MCAS poured back on the trim, but as I recall that took more digging to find out afterward. This was also not pointed out in any type of pilot accessible documentation either; nor did it apparently make it's way to the FAA; as I recall them mentioning it was supposed to at max apply .6 degrees trim at once, but was later changed to ramp up to 2.5 degrees trim repeatedly, without an updated safety analysis. Paperwork update failure: https://www.avweb.com/recent-updates/business-military/mcas-certification-flawed-report/ https://www.avweb.com/recent-updates/business-military/mcas-... Electric stab trim switch reset of MCAS activation timer was mentioned in the final report. https://www.faa.gov/foia/electronic_reading_room/boeing_reading_room/media/737_RTS_Summary.pdf https://www.faa.gov/foia/electronic_reading_room/boeing_read...

WalterBright 5y ago
Both crews repeatedly used the trim switches to bring the aircraft back to normal trim. The cutoff switch was within easy reach, easily set within 5 seconds. And even if it took them 6 seconds, or 7, the trim moving does not mean it's instantly hard over and unflyable. You can also turn off the trim system while using the trim switches giving a 0 second delay. The reason is the circuit turning on the trim is a wire with the trim switch and the cutoff switch in series. Both have to be on for current to flow. Think of it like a light switch in a room and the corresponding breaker in the breaker box. Both have to be "on" for the light to come on. The evidence that this is quite possible is the first LA crew experiencing MCAS malfunction trimmed back to normal, turned it off, and landed safely.
- salawat 5y ago
  Oh, absolutely. I don't doubt that. Though there was a subtle change to the meaning of the switches. https://www.seattletimes.com/business/boeing-aerospace/boeing-altered-key-switches-in-737-max-cockpit-limiting-ability-to-shut-off-mcas/ https://www.seattletimes.com/business/boeing-aerospace/boein... In their eagerness to match the procedure, they actually lost a level of granularity in system isolation. The infamous "electrical short" misunderstanding, I think, may have risen from system description that supported the change. The idea being a short in the yoke switches could cause a runaway, which is what the left switch in the NG is for isolating, and the right switch was for isolating the Autopilot. In MAX, this distinction was lost. Either switch completely isolated both Autopilot and yoke switches. Assumedly, if one was very studious and aware of the switches purpose in the NG, one could get taken by surprise flying the MAX if they expected them to work the same way. This is why I tend to beat into my devs the importance of "if you change the interface, you explain the change, or you offer the ability to fall back to older behavior. I realize that's not practical in aerospace's regulatory environment, but it is nevertheless a helpful guide.

WalterBright 5y ago

Both crews repeatedly used the trim switches to bring the aircraft back to normal trim. The cutoff switch was within easy reach, easily set within 5 seconds. And even if it took them 6 seconds, or 7, the trim moving does not mean it's instantly hard over and unflyable. You can also turn off the trim system while using the trim switches giving a 0 second delay. The reason is the circuit turning on the trim is a wire with the trim switch and the cutoff switch in series. Both have to be on for current to flow. Think of it like a light switch in a room and the corresponding breaker in the breaker box. Both have to be "on" for the light to come on. The evidence that this is quite possible is the first LA crew experiencing MCAS malfunction trimmed back to normal, turned it off, and landed safely.

salawat 5y ago
Oh, absolutely. I don't doubt that. Though there was a subtle change to the meaning of the switches. https://www.seattletimes.com/business/boeing-aerospace/boeing-altered-key-switches-in-737-max-cockpit-limiting-ability-to-shut-off-mcas/ https://www.seattletimes.com/business/boeing-aerospace/boein... In their eagerness to match the procedure, they actually lost a level of granularity in system isolation. The infamous "electrical short" misunderstanding, I think, may have risen from system description that supported the change. The idea being a short in the yoke switches could cause a runaway, which is what the left switch in the NG is for isolating, and the right switch was for isolating the Autopilot. In MAX, this distinction was lost. Either switch completely isolated both Autopilot and yoke switches. Assumedly, if one was very studious and aware of the switches purpose in the NG, one could get taken by surprise flying the MAX if they expected them to work the same way. This is why I tend to beat into my devs the importance of "if you change the interface, you explain the change, or you offer the ability to fall back to older behavior. I realize that's not practical in aerospace's regulatory environment, but it is nevertheless a helpful guide.