Originally Posted by
Loose rivets
737 Driver
Quote:
. . . On the NG, the left switch disables the main electric (pilot actuated) trim and the right switch disable all automatic inputs (Speed Trim/Mach Trim/Autopilot Trim). We used to try to identify the offending system and actuate these switches separately in the runaway stab trim procedure depending on the situation. Somewhere along the way, Boeing changed their philosophy on the procedure (probably part of a larger trend of moving away from “troubleshooting” type actions) and now we always use both switches when necessary. I have previously speculated that two switches were retained in the MAX for both redundancy against possible relay welding and to simply harmonize the procedural aspects of runaway trim between the NG and the MAX. That is, both aircraft have two switches (for different reasons) and you always use both switches at the same time.
This is a very revealing quote. I can see how the procedures evolve, or de-evolve. But I can also see how it could lead even attentive crews down a very dark garden path.
This could be a case of compounded unexamined assumptions along the way:
The initial 737NG safety analysis used the availability of separate cutouts (automation and all) to justify smaller wheel/harder to move under high loading since in most cases pilot electrical trim would still be available.
This may also have been a factor in dropping training on the 'unloading' technique.
Over time training was simplified, not by itself a bad thing, but did not take the above into account.
When MAX was designed the fact that training always used both switches was used to justify removing the seperate functions for whatever reason.
Some have suggested that the training was changed to accomodate the upcoming MAX, I highly doubt this was true since there was no technical need to change the functionality to add MCAS.