Revelation wrote:aerolimani wrote:Somewhere in this thread, somebody shared a procedure for operating the hand-crank manual trim. I believe it was from the operating manual of a 737 classic. It had to do with the situation where the wheels were near to impossible to turn by hand. I believe the advised procedure in the manual was push the yoke forward to to relieve some pressure on the stabilizer, and to allow the trim wheels to be more easily turned by hand. So, it would seem that the issue (of the the manual trim being difficult to operate) goes back a long ways in the design of the 737, if not to the very beginning.
Of course, the problem gets even worse if an erroneous MCAS activation starts flying you into the ground at a relatively low altitude. Add to that the airspeed disagree, and its corrective procedure which apparently results in flying the plane faster, thus increasing the difficulty of operating the trim by hand. I'm guessing there's an altitude below which one cannot even hope to use the above described procedure for allowing the operation of the manual trim.
So… I come to two questions:
1) Is this procedure for operating the manual trim still in the MAX manual?
2) Have all the previous generations of 737 simply been fortunate enough not to need to need to operate trim manually, at a low altitude, to recover from a nose-down, out-of-trim situation?
The ST article I linked above, Seattle Times: Why Boeing’s emergency directions may have failed to save 737 MAX, has the answer to both your questions.
Here's some relevant fair-use quotes from the article:His scenario is backed up by extracts from a 1982 Boeing 737-200 Pilot Training Manual posted to an online pilot forum a month ago by an Australian pilot.
That old 737 pilot manual lays out a scenario where a much more elaborate pilot response is required than the one that Boeing outlined in November and has reiterated ever since. The explanation in that manual from nearly 40 years ago is no longer detailed in the current flight manual.
Hmm, wonder which forum Dominic is reading for clues (or which ones he isn't, lol!).
It goes on to say:More detailed instructions that conceivably could have saved the Ethiopian plane are provided in the 1982 pilot manual for the old 737. As described in the extract posted by the Australian pilot, they require the pilot to do something counterintuitive: to let go of the control column for a brief moment.
As Lemme explains, this “will make the nose drop a bit,” but it will relax the force on the elevator and on the jackscrew, allowing the pilot to crank the stabilizer trim wheel. The instructions in the old manual say that the pilot should repeatedly do this: Release the control column and crank the stabilizer wheel, release and crank, release and crank, until the stabilizer is swiveled back to where it should be.
The 1982 manual refers to this as “the ‘roller coaster’ technique” to trim the airplane, which means to get it back on the required flight path with no force pushing it away from that path.
“If nose-up trim is required, raise the nose well above the horizon with elevator control. Then slowly relax the control column pressure and manually trim nose-up. Allow the nose to drop below the horizon while trimming (manually). Repeat this sequence until the airplane is trim,” the manual states.
Like I wrote above, it's a darn good summary of the current situation, perhaps with some inspiration from this forum, and perhaps even from this thread, but in any case, a lot better written.
It has even more info related to your questions, so go give it a read. And no, I am not Domenic...
Thanks for that. Somehow, I skipped past this article, despite having followed this thread all the way through.
It seems like Boeing's procedures for runaway trim may be inadequate, as it was stated in manuals from the classic series onwards. The roller-coaster technique could still be necessary, in the event of a "normal" runaway trim scenario, and yet, the technique has not been published in a manual, or trained for, since the Jurassic series.
If everything in this article is true, it seems like a matter of mere good fortune that a runaway stab trim didn't cause a serious problem on any 737-300 through 900 series aircraft, with a crew being unable to operate the manual trim wheel. Of course, with ET302, the final nail in their coffin was reactivating the electric trim, and thus MCAS, after most likely being unable to turn the trim wheels manually. Even so, given their relatively low altitude, I wonder if the roller coaster technique would have been possible.
So… will the roller coaster technique have to be added to the Classic, NG, and MAX manuals? If there are possible scenarios where manual trim could be required, then I would think that this technique may be needed. In other words, Boeing's troubles could go a lot further than just the MAX series. If airlines were required to do additional training for all their Classic and NG pilots, that could be quite an expensive undertaking. Would Boeing end up having some responsibility for that cost?
I'm of the belief that this will all get straightened out, and that the MAX will be ungrounded. I believe that the plane will be safe to fly, eventually. However, it seems unfortunate that this manual trim problem is not something which was ever improved upon from the original design, apart from the electrical systems simply becoming more reliable. Especially, it is shocking that this problem was not corrected (or anticipated) in light of the fact that manual trim became the de facto cure for trim problems caused by MCAS.