Originally Posted by
Easy Street
Further to GordonR’s reply, I would add that the regulatory assumption of a 3-second response time to trim runaway is highly questionable. The operating data manual for my (military) type assumes a 3.5-second pilot response time for loss of thrust during takeoff, and that’s a readily-diagnosed failure where the pilot can reasonably be assumed to be in a state of optimum vigilance with hand on the throttle, ready to cut. Contrast that with trim runaway on a 737, where the first second or two could easily be rationalised away as speed trim, leaving only another second to complete the diagnosis and instruct PNF to cutout the trim. Then add more time for PNF to process this startling instruction and find, unguard and flip the switches. Then maybe add more time for CRM SOPs (“state the malfunction”... “memory items”...), depending on company culture and crew experience. The Mentour Pilot video from months ago may have exaggerated this grossly (10:10 to 11:25 at the link below), but whichever way you look at it, an allowance of only 3 seconds to isolate the trim implies an
exceptionally optimistic assessment of human startle response.
Video
I only mentioned the most basic issue, and there are plenty of others, some mentioned in the Seattle Times, and others earlier in this thread:
The first is that the FAA seem to have emphasized that it is not acceptable for pilots to be constantly monitoring the FCC, and relying on them as the last resort to promptly diagnose and catch a catastrophic single point of failure, which can and should be detected by software. BTW, this is a very different approach from the "just-fly-the-aircraft" mantra, raised repeatedly several months ago.
Secondly the video and associated discussion assumes there are always two pilots in the cockpit. This would certainly be true during takeoff, but not necessarily during the cruise. If there is only one pilot at the controls, and a runaway stabiliser trim occurs, the chances of recovery are significantly less. (See AF447 et-al). Take that same video, and imagine a single pilot trying to pull back on the yoke, and flip switches at the same time...
Thirdly it has been widely discussed that once the stabiliser trim goes beyond a certain point, the situation may be unrecoverable except via extreme measures, such as the roller-coaster maneuver. This modification on the MAX (in light of runaway MCAS) actually goes some way to fixing an issue which has been present on the NG model for decades, though there seems to be no pressing need to retrospectively fix all of those aircraft.
The B737 is designed and certified to be flown manually, and removing scenarios that interfere with that should be welcome. IMO a good fix is one that removes a whole class of potential errors, rather than applying a band-aid on top of another band-aid.
Originally Posted by
Bend alot
So is the correct response "cut out" or trim to "neutral feel" as per the info given to ET302?
Given a 3.5 second reaction time, after STS seems excessive?
Edit: The check-list for runaway trim is likely to change in view of the proposed changes to the MAX, and probably pilot training too.