Ethiopian airliner down in Africa

[Derfred]

Originally Posted by Airbubba

Here's an explanation of the EASA position on the observation that the yoke trim switches on the MAX don't work throughout the entire speed envelope (with some highlighted text):



From: https://www.easa.europa.eu/sites/def...20ISS%2010.pdf

The EASA document is being mis-interpreted by many.

It notes that once you get to a certain point of AND trim (somewhere between 3 and 4 units from memory, varying by model), you cannot trim further AND with the yoke switches (flaps up). If you ever needed to, for an unusual reason (eg jammed flight controls), you would need to use manual trim. In normal ops, you would never need to.

This point is irrelevant to this discussion because it doesn’t prevent the yoke switches from trimming ANU, even if the aircraft has been trimmed full AND. It is a one directional limit switch only. It doesn’t prevent trim in the other direction.

Note: I’m referring to the limit switch in the forward trim range above. There is another limit switch aft which works similarly in reverse.

EASA have made it confusing by referring to it as trim “authority”, which could be interpreted as “the motor is not powerful enough”.

Ironically, the limit switch was implemented to help prevent either runaway trim or pilot yoke switch trim into the “danger zone” which might result in an unrecoverable dive. EASA, in recognition of this “safety improvement”, approved the implementation on the basis that it improved safety rather than reducing it.

But Boeing’s MCAS implementation has actually allowed MCAS to trim past this limit switch, which was very unwise and has turned out to have had tragic consequences.

I hope this clears it up.