So a plausible scenario for the crew faced based on the Boeing Bulletin is:
- At or soon after rotate, so message like "IAS disagree", possibly other messages and warnings, possibly even stick shaker.
- Manually flying so no autopilot to drop out.
- As they are manually flying, and trimming, the runaway stabilizer fault is masked and dealing with the unreliable airspeed.
- Probably running the unreliable airspeed memory items
- After not trimming for 5 seconds, STS trims nose down, PF counteracts with nose up pitch and trimmed, runaway trim is masked for 5 seconds.
- As they are solving the UA checklist, speed is increasing, air loads on horizontal stabilzer increase.
- A series of sequences of manual trim, 5 sec delay, then runaway stab, manual pitch up and trim, ratcheting down the stabilizer, pilots compensate with
- Eventually the speed increases and air load becomes so large, that both pilots are unable to overcome the nose down stabilizer trim all the way forward becoming unrecoverable as there is a further rapid increase in speed.
Basically in this type of scenario, the crew are confronted with an
intermittent runaway stabilizer in addition to a UA. This
intermittency is a difficult problem to nut out in the cacophony of noise and confusion of an unreliable airspeed scenario. Ironically, it may be possible they may of actually had three valid IAS indications, in close agreement, further heightening the confusion.