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737 quick reference handbook: "Runaway stabilizer. Condition: uncommanded stabilizer trim movement occurs continuously." Source: https://i.imgur.com/K7El4K4.jpg
by mopsi 7y ago
737 quick reference handbook: "Runaway stabilizer. Condition: uncommanded stabilizer trim movement occurs continuously." Source: https://i.imgur.com/K7El4K4.jpg https://i.imgur.com/K7El4K4.jpg
It advises turning off the autopilot after first uncommanded movement and pulling the yoke firmly (triggers trim hold), and hitting trim cutoff switches after the second. That's what they did, but by then MCAS had moved the stabilizer so much that they were struggling to maintain control. Aerodynamic forces from their speed rendered manual trim ineffective. Checklist offered nothing beyond this point.
When they restored electric trim three minutes later in an attempt to trim the nose up, everything was normal for 30 seconds, but then MCAS kicked in for the third time and put the aircraft into a 40-degree nosedive.
All they needed was a bit of electric trim.
It's possible they confused 737MAX with older 737NGs, considering the virtually non-existent retraining. Older ones have two cutoff switches, one for pilot-controlled electric trim and the other for automated systems. MAX retains two switches for commonality, but they have the same function: both turn off all power to stabilizer trim, it's impossible to isolate automated systems without losing all electric assist.
- WalterBright 7y ago> "Runaway stabilizer. Condition: uncommanded stabilizer trim movement occurs continuously." Thank you for the reference. Having it repeatedly come on and run is runaway trim. Trying to interpret the meaning of "continuous" makes for great courtroom drama, but is unhelpful when you're faced with a crisis. Besides, if the runaway trim was caused by an intermittent electrical short, would you dismiss that as well, too? Or would you turn the sucker off and save your life and let the mechanics sort it out on the ground? > it's impossible to isolate automated systems without losing all electric assist. The column electric trim switches override all other trim inputs. That's why the procedure is as follows: Boeing Emergency Airworthiness Directive "Initially, higher control forces may be needed to overcome any stabilizer nose down trim already applied. Electric stabilizer trim can be used to neutralize control column pitch forces before moving the STAB TRIM CUTOUT switches to CUTOUT. Manual stabilizer trim can be used before and after the STAB TRIM CUTOUT switches are moved to CUTOUT." https://theaircurrent.com/wp-content/uploads/2018/11/B737-MAX-AD-1107.pdf https://theaircurrent.com/wp-content/uploads/2018/11/B737-MA...
- mopsi 7y ago> Having it repeatedly come on and run is runaway trim And they recognized it as such and completed the runaway trim checklist, but they couldn't use manual trim (as advised) to recover from the situation, because aerodynamic forces acting on the stabilizer overpowered them. Two Swedish pilots demonstrated this scenario in a 737NG simulator: https://youtu.be/aoNOVlxJmow?t=606 https://youtu.be/aoNOVlxJmow?t=606 Ethiopian pilots were in even more stressful situation, because they also had broken AOA sensor with unreliable airspeed indication, active left stick shaker, occasional terrain proximity warnings and overspeed clicker. No procedures or training existed for the situation they were in. Pilots of early 737s were taught the so-called rollercoaster maneuver to temporarily relieve the load to allow manual trimming, but that was scrapped decades ago.
- kayfox 7y agoManual in this case is the manual controls for the electric trim system, as opposed to automatic trim like the speed trim system.