Originally Posted by
row13please
But Why, Mr. Boeing (uh, McDonnell)? What did that improve or solve?
On the first-generation 737s, there were 2 trim motors. One was controlled by the trim switches and ran at a higher speed. The other was used by the autopilot and ran at a lower speed. Each motor was controlled via a pair of wires, one for nose up and one for nose down. On later models, the 2 motors were replaced by a single motor that could run at multiple speeds. The rest of the control system was left unchanged, so there were still high-speed inputs and low-speed inputs.
During MAX development, it was found that in order for MCAS to serve its purpose, it would have to be able to trim at a high speed. Therefore the Flight Control Computers were connected to both wire pairs. They would still use the lower speed for the autopilot, but MCAS would use the high speed inputs. If the switches were left unchanged, moving just the AUTOPILOT switch to cutout would've still allowed MCAS to operate. To allow the AUTOPILOT cutout switch to disable all automatic trim, like before, would've required an additional pair of wires. By that time, the procedure already specified that both switched should be used in tandem, not individually. So there was no benefit to adding more wires and instead, both switches were changed to have the same function (disabling all electric trim).
Originally Posted by
row13please
But wasn't the reason for the original 2 switches that the pilots can at least try to use a simple motor to fix the trim without going straight to pointing the plane down to trim it up?
Autopilot issues were common in the 50s and 60s, so it was considered useful to be able to disable autopilot trim inputs while retaining pilot-controlled electric trim. In case of a runaway, the column-actuated trim cutout switches would stop the motor long before the aircraft could become severely out of trim. The rollercoaster technique wouldn't be needed. As far as I can tell, the only scenario in which the rollercoaster technique could've become necessary would be a trim brake failure. In that scenario, the motor wouldn't be operating but the stabilizer would move freely and would react to aerodynamic loads. The only way to regain control would be to grab the trim wheel(s), bring it to a stop, and then hold onto it for the remainder of the flight (or at least be ready to grab it again if needed). This is the reason for the final step on the Runaway Stabilizer checklist (Trim Wheel...Grab and hold).
MCAS was the only input that could bypass the column cutout switches, so it did create an additional risk. It was still assumed that pilots would use the trim switches to interrupt MCAS and keep the aircraft in trim. The Lion Air captain did exactly that. He flew successfully for about 5 minutes, interrupting each MCAS input within a second or two and not allowing the aircraft to get out of trim. He called for the Unreliable Airspeed memory items, but the FO didn't know them. The FO started flipping through the QRH, but paused several times respond to ATC calls. After 5 minutes of that, the captain handed over control to the FO, presumably so he could look for the checklist himself. The FO didn't interrupt the MCAS inputs. He let it trim nose down for the full 9-10 seconds each time, then he would make a single 1-second nose-up input... just enough to re-arm MCAS, but not nearly enough to get back into trim. He did express that he was having difficulty controlling the airplane, but the captain appeared to ignore him until it was too late to recover.
Originally Posted by
row13please
Didn't one of the Max8 crash crews try turning the switches back on? And if they could have used just one without it force-pairing the functions, they would have saved the plane?
(somewhere said if they'd held the yoke trim switch all the way it would eventually have trimmed up even with MCAS and both cutoff switches on, but I assume not as quickly as earlier 737s?)
That was the second crash, Ethiopian Airlines. Yes, if they'd actually made significant nose-up inputs after moving the switches back to NORMAL, they would've regained control. Pilot input always overrode automatic inputs (including MCAS), and each pilot input would disable all automatic inputs for 5 seconds. I'm not sure what you mean by "not as quickly as earlier 737s". The trim would've run at the normal flaps-up speed. It would've taken around 20-30 seconds of continuous input to get back into trim.
The Ethiopian Airlines captain essentially repeated the actions of the Lion Air FO. Even after re-enabling electric trim, he made enough nose-up inputs to reactivate MCAS but not enough to get into trim.