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Old 5th Apr 2019, 11:51
  #3295 (permalink)  
Join Date: Oct 2004
Location: australia
Posts: 173
Originally Posted by MemberBerry View Post
Maybe I can try, EASA's sentence was: "The increased safety provided by the Boeing design limits on the thumb switches (for out-of-trim dive characteristics) provides a compensating factor for the inability to use the thumb switches throughout the entire flight envelope."

This seems to be in reference to the thumb switches not being able to trim the stabilizer until the mechanical stop limit.

They seem to mean that the safety added by not being able to trim with the thumb switches until you hit the mechanical stops compensates not being able to do that when you actually need it.

For example, if a thumb switch gets stuck, the resulting runaway won't be able to bring the trim to the full nose down mechanical limit, so it decreases the risk of getting the aircraft in an unrecoverable dive, they consider that an advantage.

The disadvantage is that, if you really need to bring the trim full nose down, to the mechanical limit, you won't be able to do that with the thumb switches.

So they are saying the advantages and disadvantages compensate each other.

Also, a lot of people seem to interpret that EASA's statements as saying you are not able to use the thumb switches at all in those conditions. But the trim limit switches are designed to prevent you from reaching the mechanical limits, not going away from them, so electric trim attempts going in the opposite direction, away from the mechanical limit, should still work, even if you are outside the designed trim limits for manual electric trim.

So I think it's unlikely the trim limit switches were a factor in this accident.
My take would be that the aircraft could not comply with the 3 second trim operation, no load, required for electric trims for (CS 25.255(a)(1)).
So they declared that the manual trim wheel was the "main trim'. This only requires a 30 lbf mistrim, but allowed trimming throught the entire flight envelope.
The electric stops were then reset so maximum electric trim deflection was sufficient for the "normal airline operational" envelope, manual trim picks up the rest.
So now we can claim how conservative we are.
Now, where does a stability augmentation system failure morph into a trim runaway?

Last edited by zzuf; 5th Apr 2019 at 12:30.
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