Originally Posted by
FCeng84
All of the features of the 737 stabilizer control system that are active when flying manually (i.e., when the autopilot is not engaged) are designed to present the crew with speed and maneuver stable characteristics. An increase in speed results in airplane tendency to pull the nose up (thus to slow down) in a speed stable manner. An increase in AOA results in airplane tendency to push the nose down (thus seeking a lower AOA) in a maneuver stable manner. When the pilot has trimmed the pitch axis via repositioning the stabilizer to allow steady flight at the current condition with zero column force, the system should not interfere by moving the stabilizer such that the pitch axis is no longer trimmed. Instances of the system running the stabilizer turning a steady, trimmed condition into one that is no longer trimmed should be recognized as improper operation of the automatic stabilizer control system. The procedure for improper stabilizer control is to the follow the checklist actions that include disabling electric stabilizer control via placing the stabilizer cutout switches in the cutout position.
It seems clear to me with regard to the Lion Air events of late October that the crew on the second to last flight recognized that the automatic stabilizer control was not functioning properly. They took the correct action of disabling further electric stabilizer control and flying the remainder of the flight employing manual, trim wheel control of the stabilizer as needed. It also seems clear to me that the crew on the last flight repeatedly recognized the need to apply pilot commanded electric stabilizer trim via the wheel mounted pitch trim switches to return to a trimmed condition when the malfunctioning system ran the stabilizer away from the proper trim position. To me there are a number of mysteries surrounding this tragedy:
1. Why the crew on the second to last flight did not mention in their post flight write-up that they flew the entire flight with the stick shaker rattling and that they found it necessary to activate the stabilizer cutout switches to stop the system from repeatedly taking them away from trim. If the crew of the last flight had taken off with that mitigation action in mind we would probably not be having this PPRUNE discussion today.
2. Why the crew of the last flight went through so many cycles of trimming manually only to have the system take them away from trim each time without suspecting that there was something wrong with the automatic stabilizer control and thus it should be shut down.
3. Why when faced with difficulty managing pitch control the crew of the last flight chose to stay at only 5000 feet altitude and chose to fly at such a high speed. Climbing higher and at a slower speed would have been prudent to give them more room and also preserve more elevator pitch control. 737 pilots should know that at higher speeds elevator travel is hinge moment limited and thus their control authority reduces with increased speed.
4. Why the crew of the last flight were successful for several minutes in their correct actions to counter the errant system stabilizer motions with pilot initiated stabilizer motions that re-trimmed the airplane, but did not continue with that during the final 30 seconds to a minute of the flight.
I know that I am only rehashing questions that many of us have been struggling with over the last 3+ months. I sure wish that the CVR information that has now been recovered were available to the public to better understand what the thinking was among the flight deck crew on that last flight. While the cockpit recording will probably not answer all of our outstanding questions, it will likely move us to greater clarity. As I contemplate pressing send on this post I hope that I am not triggering a re-run of PPRUNE volleys that have already been lofted on this topic. In the end, however, I find it helpful to share my lingering wonderments. Lot's of questions and not enough answers. Bottom line is that 189 who should not have died did and it is our collective job to minimize the risk of adding to that number.
My several cents worth,
FCeng84
As to point 3, I think they had the stick shaker going of, and they thought the airspeed indication was unreliable. I guess that made them reluctant to reduce power.
As to point 4, I read that control was handed to the FO about 30 sec before the end, and the FO made much smaller ANU trim inputs than the PIC before him, and each of those small inputs allowed MCAS to add 2.5deg AND, leading to loss of control.
Not suggesting I know more about this than you, as I am sure I don't, just wanted to add some possible relevant info. Also agree very much with everything in your post, just think the MCAS system Boeing implemented should have been more failure resistant, and better explained to the crew.