Originally Posted by
GarageYears
I understand the intent here, but as revealed by the FDR traces, the crew of the fated flight clearly understood there was a trim issue, they repeatedly applied trim opposite to that applied by the automation (MCAS) - true, they may not have understand 'what' was applying the trim, but clearly they knew it was happening. The question is why did they not hit the cut off switches?
And the crew of previous flight also recognized there was a trim problem, but in their case almost immediately disabled the automated trim and trimmed via the mechanical trim wheels.
- GY
The question as to why did JT610 not run the Runaway Stabilizer NNC all the way to 'STAB TRIM CUTOUT switches (both) ...... CUTOUT' just as JT43 did will remain largely open until the CVR is recovered (hopefully it will have enough of JT43 preserved to compare the two cockpits). Short of that I'd offer two observations:
- The presentation of the problem on take-off goes IAS DISAGREE + L stick shaker first at a couple of hundred feet, MCAS commanded ND trim second on flap retraction. When the IAS DISAGREE + L stick shaker manifested itself on JT43 they would have ran the Airspeed Unreliable memory items and importantly they maintained pitch of 15° and the existing takeoff thrust. That attitude meant that when the MCAS commanded ND trim kicked in on flap retraction control column forces built rapidly (the CFFORCE_PITCHCWS forces on JT43 quickly spiked to levels not experienced by JT610 until the last 40 seconds or so of their flight). JT43 was presented with pressing imperative to do something about the ND trim, hence STAB TRIM CUTOUT switches to CUTOUT.
- Through a confluence of factors (most likely a briefed plan for UAS and an impromptu reaction) JT610 were never presented with the same control column force 'imperative'. There's plenty of evidence to support the contention that JT610 had briefed for a UAS and the plan was to get to 5,000 and establish level flight flaps up at VREF40+70 (per the QRH Flight With Unreliable Airspeed table), stabilse and return to Jakarta. On take-off, lo and behold, the initial IAS DISAGREE + L stick shaker presents. Priming and confirmation bias will now shape the crew's perception of and decisions about everything that happens subsequently. When the MCAS commanded ND trim kicked in on flap retraction JT610's (I suspect impromptu) reaction is to re-extend flap. That action temporarily halts MCAS so they're not subjected to significant control column forces as they climb and because they are levelling off when MCAS bites them next they're not subjected to significant control column forces for most of the rest of the flight. The MCAS commanded ND trim is 'manageable' manually with control column and main electric trim up until the point when it isn't.
I suspect that this might be one of those cases where the guys with the plan (albeit for the emergency that they were expecting, not the one they got) faired much worse than the guys without a plan who just reacted instinctively. If indecision is the key to flexibility, a prepared plan is probably its enemy.