nsaspook said:
Was it something like the several second timer that repeats the erroneous trim adjustment? Was it a design control law problem or a problem with some like the software PID implementation of a control law? Typically with a PID control loop the error term has the integral term gain limited (equivalent to a one time adjustment here) to only be able to give X amount of feedback (to combat mechanical system windup to control limits) to adjust the total error signal to balance the control set-point. One of the problems that prevented recovery was the pilot would correct the pitch error but MCAS would just push the nose back down again and again. The pilots were able to counter the nose-down movement multiple times but eventually they ran out of airspace. Obviously the repeated adjustment mode was 'fixed' to one time only now.
The main thing to avoid in airplane stability & control, is an aerodynamic nose up moment that is not commanded by the pilot. The uncommanded nose-up moment would not auto-stabilize, but rapidly get progressively larger with increasing angle of attack, and run away to a stalled airplane.
During certification of a passenger airplane, many tests are carried out to check if the airframe does not start to have a mind of its own.
- If the pilot does not provide a control input, the airframe must return to the trimmed position.
- Forces and inputs to move the airplane away from trimmed position must be such that there is an ever increasing force required to achieve an ever increasing nose-up position. The nose-up position must always be commanded by the flight control surfaces, elevator and stabilizer, in a predictable way.
One of the tests to be performed during certification is stick-force-per-g. Bank the airplane and start turning while pulling the stick back in order to maintain altitude. Then bank more and pull back more, in ever tightening turns. It must be progressively harder to pull back on the stick to maintain altitude, never easier.
It was during this wind-up turn that due to the engine configuration of the MAX an aerodynamic nose-up moment appeared, which would cause the pitch stick force to suddenly become less than expected. Not as bad as a runaway pitch, but still an undesirable situation when the pilot is still straining to maintain the maneuver. This is the situation that MCAS was originally designed for, to auto-compensate for this situation only.
The pilots of ET302 needed to unload the stabilizer, and you would only need to unload the stabilizer if you've allowed the trim runaway to progress to a point at, or very near, the full nose-down limit AND you failed to reduce thrust and your airspeed is excessive. The Ethiopian flight's indicated airspeed reached 390 knots.
Neither accident crew accomplished the steps on either the runaway stabilizer or unreliable airspeed checklists. Both of which have as either their 2nd (IAS) or 3rd (trim) step to disengage the autothrottles.
They did not.
1. Control Column, hold firmly
2. Autopilot (if engaged), disengage
3. Autothrottle (if engaged), disengage
4. Choose one:
... The runaway stops after the autopilot is disengaged:
... End of procedure
... The runaway continues after the autopilot is disengaged:
... ... STAB TRIM CUTOUT switches (both), CUTOUT
... ... ... If the runaway continues:
... ... ... ... Stabilizer trim wheel, Grasp and hold
The ET Captain engaged the A A/P (the one on the side with the invalid data) while still well below the minimum A/P engagement altitude.
They never disengaged the autothrottles and that allowed the airspeed to build to around 390 knots indicated.
They didn't use the stab trim cutout switches until the stab trim was already near the full nose-down position. Then they took the switches back to normal and allowed MCAS to drive the stab the rest of the way to the full nose-down position.
The backup trim system on all 737s is the same as what was used on the B707, B720, and B727 which all operated with that system for many decades. The system is designed to work in stages depending on the amount of force required in a given situation.
The trim wheels have fold-out handles. When the airplane is close to in-trim, the flying pilot folds out his handle and can operate the trim with his inboard hand while flying the airplane with his outboard hand.
As the out-of-trim condition increases, the forces are higher and the pilot-monitoring will operate the trim wheel on command of the flying-pilot. i.e. "Trim down", "Stop trim", etc.
As the out-of-trim condition increases further, the two pilots work together with their inboard-hands turning the wheels together. The two fold-out handles are located 90 degrees of rotation apart. This is so that one pilot has his handle in a position which provides good leverage at any point in the wheel's rotation.
When you get to a situation where the trim is at the full nose-down stop, and your airspeed is around 150 knots faster than it should be, the stabilizer loads are too high to manually move the stabilizer which require alternate periods of unloading and trimming with periods of regaining altitude.
The key to successfully handling any runaway stabilizer event is to accomplish the runaway stabilizer checklist in a reasonably prompt manner so that the runaway is stopped before the trim reaches the full nose-down limit. If you don't, you have made recovery significantly more difficult.
The additional training that these crews needed was not in MCAS, how MCAS works, or even in handling a runaway stabilizer. The training they needed was in the proper prioritization of tasks in an emergency. 1. Fly the airplane, 2. Silence the warnings, 3. Confirm the emergency. This process is how you move past the distractions to find, confirm, and correctly action the emergency.
None of which would have been necessary if they had continued to fly the airplane by retrimming each time the nose got heavy from the MCAS activations. The Lion Air Captain did exactly that through 21 MCAS activations. The F/O, to whom he transferred control, did not.
As for the Ethiopian accident, the excessive airspeed over the stabilizer in the full nose-down position far outweighed the nose-up moment from the high power setting.
You can't fly the airplane at 390 KIAS (Vmo is 340) and full nose-down stab trim. That would be true in every transport jet.
If they had followed the correct procedure, the trim never would have reached full nose-down, the airspeed never would have reached 390 KIAS, and they would have been able to trim manually just as the crew of the Lion Air incident flight, which landed safely, did.