Originally Posted by
Takwis
That question is near the top of my list, as well. Why change from individual cutouts for manual and autoplot trim? The higher ups don't seem to be asking it, and Boeing certainly is not giving any meaningful answers.
This was actually covered in a news article (Seattle Times, I think) a month or so back.
As background, there was a time when the 737 had a procedure where the two cutout switches could be used separately to isolate the offending trim system. Quite a few years ago (early 2000’s if I recall correctly), Boeing changed the procedure so that both switches would always be used together. At the time I remember being told that this was part of a general trend in eliminating “troubleshooting” type actions plus the desire to shut down any runaway as quickly as possible.
Even today, if I had a runaway stab malfunction on a 737NG which I knew was being caused by the automatic trim system, I would have no legal procedure to use just the automatic trim cutout switch. That doesn’t mean I wouldn’t pull out the “Captain Emergency Authority” card, but I would be going out on a limb to diverge from the published procedure.
Fast forward to the MAX. Boeing wanted to maintain as much commonality with the 737NG as possible. The NG had two cutout switches, so the MAX would have two cutout switches. This would make the Runaway Trim procedure identical in both aircraft. However, Boeing’s philosophy had changed regarding what these switches should do. Since the original intent to isolate malfunctions had been replaced by a desire to shut down the entire trim systems as quickly as possible, they wired these switches to accomplish just that. Even though procedurally both switches were required, selecting just one switch (i.e. fat-fingered fumbling) would accomplish the job.