PPRuNe Forums - View Single Post - Ethiopian airliner down in Africa
View Single Post
Old 4th Apr 2019, 09:49
  #3058 (permalink)  
ecto1
 
Join Date: Nov 2018
Location: madrid
Posts: 47
[QUOTE=bill fly;10438384]
Originally Posted by ManaAdaSystem View Post


The Boeing NNC regarding runaway trim, or MCAS, has never told us to trim to neutral before placing the switches to cut off. It tells us to stop the trim with the switches if the trim doesn’t stop after disconnecting the autopilot. Then use manual trim. Period.

The armchair experts have now moved from «They should just have placed the cut off switches to off and contained the problem» to «They should just have trimmed neutral and then used cut out switches and contained the problem».
Congratulations! It took only a few weeks to come to this conclusion.
The Lion Air and Ethiopian pilots only had a few minutes.
[/QOTE]
And that highlights a big difference between runaway stab. and MCAS unwanted operation:

With runaway stab you can’t catch it by trimming - it is running away...

With MCAS unwanted (for want of a better description) operation you can theoretically catch (reverse) it by trimming.

Another difference is that in an AoA fault caused unwanted MCAS operation other factors also come into play - even before MCAS operation on flap retraction:

Autothrottle, Instrument anomaly, Stall warning etc. which can cause:

Unwanted rise in speed, initial pilot input nose down response, cockpit confusion.

Therefore the standard drill for stab runaway does not necessarily apply. Trimming to neutral and ATS disconnect might well have priority over stab trim cutoff operation.

Of course we didn’t know this, the poor crews didn’t know it and it is a scenario which Boeing will be looking at hard before submitting a suitable procedure for recertification, I am sure.
You are both right. But Boing introduced a very "clever" sentence in the AD:

"Electric estabilizer trim can be used to neutralize control column pitch forces before moving the stab trim cutout switches to cutout"

Can: means that if you do it and you crash, they would say they didn't tell you to do it, but if you don't do it and you crash they will say that they mentioned it for something.

Not that any amount of words is going to change the fact that they are responsible. If only they were as clever in the design as they are with legal-related documents.
ecto1 is offline