[bodas]

Here we have an Indonesian company with a terrible safety record and an aircraft that hasn't been maintained properly.

However, if the computer is found to have initiated the pitch downwards, then the same failure could happen to any aircraft of this type, even if well maintained, if the sensors or computer simply begin to malfunction while the plane is in the air. Absent pilot error, the worst that should happen is a reversion to manual control.

What concerns me is that any software flaws with the aircraft will not be addressed until the problem reoccurs with another more reputable airline in the future, because Lion Air appear so guilty in this case.

[raverbashing]

I'm not sure how it works on that specific model, but it's usual that a disagreement between sensors doesn't trigger a direct action like pitch down.

It will more likely trigger a 'fallback' action like turning autopilot off (or changing flight mode which is what happened on AF447)

[VBprogrammer]

It sounds like a trim runaway incident. This has been a known issue since the earliest days of electronic trim controls.

For those who don't know, trim is the system by which the neutral position of an aircraft's controls is set. In this case in pitch. An aircraft will fly at more or less a fixed speed for a given trim setting, slowing down will cause it to pitch down unless a correcting force is applied to the controls, and speeding up will cause it to pitch up. The problem is that in a severe out of trim condition it can take tens of kilograms of force to maintain the desired pitch.

Trim runaway is when the trim motor for whatever reason doesn't stop moving, in the simplest systems this is sometimes caused by faulty switches. Most aircraft actually have a switch which is split down the middle but is naturally pressed as if it were one. This requires two of the switches to fail to get this situation. There are usually trim in motion indicators and alarms if it's in motion for too long which are intended to help pilots avoid this situation.

I suspect the computer drove the trim heavily nose down, as a result of the envelop protection trying to avoid a stall. The pilots probably tried to intervene or the autopilot handed them the aircraft back knowing something wasn't right. At that point the trim was mis-set enough that they failed to recover correctly.

Mostly conjecture but it fits what I've read.

[sokoloff]

It’s also possible for the flight crew to have been fighting the AP (often inadvertently), resulting in auto-trim continuing to trim against them.

If the crew applies sustained pitch up inputs, above what the flight director is commanding, the auto-trim will apply trim down to compensate. This can result in pilot applying even more pitch up input, resulting in more trim down. Solution of course is to kill the AP and trim motors, the multiple means of how to do so is a memory item in every aircraft.

Trim runaway has caused accidents to be sure, but is still a flight crew error IMO, unless every means to kill the trim was tried and failed.

[Brockenstein]

I'm quite sure Boeing is not going to ignore this because Lion Air is a crummy airline...