morrisond wrote:Amiga500 wrote:OK, so now you are saying it should be possible to diagnose after several repeated MCAS interventions?
Right.
So if, say, the pilots on ET302 were to engage the cut-off switches after several MCAS activations - but then found they couldn't reverse the stabliser with the manual trim wheel due to adverse loading - so had to reengage the electric trim in an effort to get some authority on stabilizer...
As they were flying at full thrust at over Vmo - well above normal operating speed and the speed limit below 10,000' - it's as bad as no thrust and below stall speed - the trim wheel isn't going to help much there either.
You keep being economical with the facts . . .
When MCAS became alive, their speed was around 250 kts at 8000 ft pressure alt (05:40:00 in the FDR chart)
Vmo was around 340 kts at those conditions.
THEY WERE WAY BELOW VMO, before MCAS run-away.If you study the FDR chart further, you will notice that just before MCAS came alive, two nose-up actions were initiated by the crew: short ANU electric trim, and aft column input. This suggests that they had been building speed by keeping a fairly shallow climb profile. As they hit 250 kts, they put in aft column (climb) and ANU trim (climb).
So it very much seems their game plan to handle the problems in hand (Stick Shaker/Stall Warning/Master Caution Ant-Ice/Unreliable Air Speed) was to build speed (250 kts) at the expense of shallower climb, and then increase rate of climb to maintain speed around 250 kts.
Just at that point when they seem to be changing over from “gaining speed” to “gaining altitude” MCAS hit them full in the face. MCAS could not have chosen any worse moment to put the aircraft in an out-of-trim situation. In just 9 seconds they went from having control over a non-standard (but non-event situation), to finding themselves fighting a severly out-of-trim situation, unable to put it back in trim.
It seems from that point on, their biggest/only concern was to try to stop the plane from diving into the ground. One could also add, that normal stable airplane behaviour is to raise the nose with increasing speed. I think they never realized how their speed was preventing them from doing just that.
Cut Out Switches:
While it is believed a crew should act within 4 seconds to a runaway trim, the first MCAS cycle went totally unanswered by the ET crew in terms of electric trim and cut-out switches.
The same thing can also been seen on the JT accident flight, and likely also with the JT pre-accident flight (though this one the FDR data does not provide sufficient resolution to state such as fact).
It seems the 4 second reaction time is not realistic.
If crews were supposed to react within 4 seconds of a stabilizer run-away, then surely they should also react within 4 seconds to an MCAS activation (valid or bogus),as there is no way for them to distinguish between MCAS run-away and “real trim runaway”.
But hitting the cut-out switches within 4 seconds after MCAS activation, destroys a great deal of the intention of MCAS
So, according to Boeing procedures, MCAS should then never ever reach its intended 9 second cycle if crews are following procedures (kill stabilizer trim within 4 seconds at run-away).
This is a catch 22 situation, as MCAS will not be able to perform its intended function, if crews follow the published procedure of hitting cut-out switches within 3 or 4 seconds of a stabilizer trim runaway. But if they allow MCAS to do its thing for full 9 seconds, they will find the plane in a severly out-of-trim situation.
I wonder how MCAS 2.0 deals with this situation.