The elephant in the living room is the source of the erroneous angle of attack inputs. How did two brand new airplanes experience nearly the same angle of attack input faults within six months of each other, particularly when one of them had the AoA vane changed halfway through the four day maintenance history? Is this a hardware problem? Or are we dealing with some form of bad code within the systems somewhere?
As Dave Davies once wrote regarding a runaway stabilizer, “if this should occur at high speed the airplane is bound to be in severe trouble; the only hope is to get the speed off.” Certainly reducing power would have added a nose down pitching moment, but getting rid of the speed would have taken the bite out of the stabilizer, reduced the elevator required and allowed the trim to be moved manually (if that was a problem).
We don’t really know whether the trim could be moved manually or not. It took the FO 8 seconds to conclude that “it’s not working”. I don’t know how hard he tried, or if he even understood how much force might be required. There may be more information within the CVR transcript that we don’t have.
However, EASA Equivalent Safety Finding B-05 discusses the limits of the yoke trim switches, stating that “Simulation has demonstrated that the thumb switch trim does not have enough authority to completely trim the aircraft longitudinally in certain corners of the flight envelope, e.g. gear up/flaps up, aft center of gravity, near Vmo/Mmo corner, and gear down/flaps up, at speeds above 230 kts. (italics added). It then states that “The trim wheel can be used to trim the airplane throughout the entire flight envelope. “
Boeing has always stated in their FCTMs that “excessive” airloads may require the efforts of both pilots, and that in “extreme” cases those airloads may have to be relieved aerodynamically. I don’t know what ‘excessive” airloads are or what kind of case is an “extreme” case, but based on the EASA document as well as the absence in the QRH of any such discussion, I am assuming that I can operate the trim wheel manually in all corners of the envelope. This will be an interesting discussion next month in recurrent training.
It appears to me that they were climbing with the master trim selected off. I don’t know why the captain said “The pitch is not enough”…of course, I don’t really know what he said at all, only how it was presented in the public release. But it seems to me that they were stable and climbing, so one option would have been to just hang on and get some altitude before trying to sort things out.
The other option would have been to turn the thing back on and mash the trim switches until you got a pretty good nose up trim established, then shut it off again. You’d have to understand how the system works, but we’ve had those discussions for years in regard to using some parts of the trim system to reset a runaway stabilizer if it was necessary.
The Lion Air captain trimmed nose up against the MCAS 21 times; the Lion Air FO only tried this twice. The preceding Lion Air crew never came close to the ground, so presumably they trimmed nose up against the MCAS quite a lot, until they shut it off and finally left it off. The Ethiopian captain appears to have trimmed nose up against the MCAS at least 5 times, but when the system was turned back on, he only tried a couple of short nose up inputs. I wonder if he even knew it was back on. For the life of me, I cannot understand why either the Lion Air FO or the Ethiopian captain did not simply mash those yoke switches and hold them there until they relieved the column force, and then mash them again when the MCAS started running the trim nose down. They are hand flying the thing…they certainly would feel the trim start to change.
But like I said, the real elephant in the living room is the source of the erroneous angle of attack data. The airplane isn’t going back to flight status until that gets sorted.