Originally Posted by
Callsign Kilo
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Someone mentioned that the MCAS provides 10 secs of trim input when activated. Can that be correct/verified? 10 secs of trim input is massive on the 737.
Was this not the contributing factor in the FlyDubai 738 crash in Rostov?
Yes, that is correct - 10 seconds, or 2 degrees forward, for each operation of MCAS. And after 5 seconds, you get another 2 degrees of trim, until you end up with full forward trim. It was designed for slow speed, but if it happens inadvertently at high speed, you could be in the sh1t. Imagine this happening up in coffin corner, and you suddenly get 10 seconds of pitch - even the quickest of crews disconnecting the stab-trim would still result in an overspeed incident. And I can tell you the aircraft shakes a lot - we had an autopilot failure which pitched the aircraft down by 5 degrees. The recovery was smooth, but the high spoeed buffet was considerable.
But I still cannot imagine why or how this MCAS system was ever certified by the FAA. Even if MCAS operated at the right time, in a stall condition, who on earth would want full trim forward when pulling out of the subsequent dive? We tried it in the sim, and concluded it was impossible to recover from the dive. So the system that is supposed to save you, saves you and then kills you.
Rostov.
I have been wondering about Rostov. Is it possible that the Fly Dubai was fitted with a Max speed-trim computer? I bet they are compatible, because that is cheaper, and Boeing does everything on the cheap. So did an engineer not have the right spare unit, so fitted one from a Max instead? - Not realising that there was a fundamental software difference between the two units?
I wonder because the report said that the Fly Dubai pilots trimmed forward for 10 seconds. And everyone was aghast by that news, because no pilot would ever trim forward for 10 seconds. We were thinking in terms of a health issue, with someone freezing on the controlls. But as it happens, this is exactly what MCAS does - it trims forward for 10 seconds. So was the Fly Dubai fitted with a Max speed-trim computer?
Silver
P.S. If Boeing had fitted a
stick-
pusher, none of this would have happened. A stick push operates on the elevator, not the stab, and is more easily overcome with stick force. More importantly, when a stick-push relents (by either a stall recovery or a cutout), the aircraft is
instsantly in trim. But with a trim-pusher you are way out of trim, and it will take you 30 seconds to trim back to the normal position. You may not have 30 seconds left. (But if course a stick-pusher would have cost money, and lots of certification time...)