Originally Posted by
Takwis
In that instance, L39 and I were talking about the penultimate Lion Air flight... they managed to land safely, and most likely never let the trim get that far forward. We have no data on it.
Actually, we do have some data on it, the FDR traces from the preliminary report, but they are so poorly done that they ofuscate the actual values for the trim position, they even seem to use different scales:
https://reports.aviation-safety.net/...RELIMINARY.pdf
Assuming the accident Lion Air flight reached almost 0 degrees of trim before impacting water, we can deduce that the trim scales start at 0.0 degrees, it seems that on the trace for the accident flight each major division is one unit, and for the previous flight each minor division represents one unit.
This would mean on both flights the takeoff trim was between 6.5 - 7.0 units, and on the accident flight the trim reached close to 0.0 units before impact. Assuming that is correct, it means the previous Lion Air flight experienced MCAS bringing trim down to about 2.0 units after about 2 minutes of fighting it, then they fought it for about 3 more minutes, then they used the cutout switches for the first time.
Also, we can deduce that the aircraft was in trim at about 5.0 units on the previous flight. This means MCAS took them out of trim by a maximum about 3.0 units at the worst point, to 2.0 units, and that the stabilizer was not more than 1.5 units out of trim when they used the cutout switches. The trim position seems to oscillate between 3.5 and 6.5 units for most of their fight with MCAS, if we exclude the spike where it seems MCAS was able to temporarily bring the trim down to around 2.0 units.
With the same assumptions, when they re-enabled the electric trim later, MCAS was able to bring the trim down to about 3.0 units. Then, after they brought the trim electrically to about 3.5 units, they used the cutout switches again to turn electric trim off for the rest of the flight. After that it seems they used the trim wheels to bring the aircraft back in trim from about 3.5 units to about 5.0 units.
In any case, assumptions aside, it's clear that the previous flight didn't reach 0.0 units of trim at any point, this is how the trace from the previous flight looked:
One thing to consider is that MCAS can be much worse than a classical stabilizer runaway caused by stuck or fused thumb switches. With the flaps up, there are limits to how much the main electric trim can bring the nose down. Depending on the 737 model, the limit is around 4.0 units from full nose down trim.
Also Satcom Guru recently tried to find 737 runaway stabilizer incidents in NTSB's database, and he wasn't able to find any:
https://www.satcom.guru/2019/05/737-...incidents.html
So the 737 runaway stabilizer procedure was rarely if ever tested in real incidents. And in any case, an MCAS runaway can be much worse than a classical runaway. With that in mind, Boeing insistence that the existing stabilizer runway procedure is enough to deal with MCAS misbehaving is questionable.