737 Stuck Manual Trim Technique

[Derfred]

Originally Posted by Capn Bloggs

:


In mine, it's 80% in both cases, same deck angle.

Ok, I’m quoting 737. Cheers

[Porto Pete]

Originally Posted by meleagertoo

Pity, I thought I'd asked a straightforward question, I'll try again.

How many have and have not seen this demonstrated in the sim?

I had 10 years on type at an operator with a reputable training department and this is the first I've heard of this technique.

[OldnGrounded]

In the real world . . .

. . . has anyone actually used the "roller coaster" or "yo-yo" procedure on a revenue flight?

[astonmartin]

Originally Posted by OldnGrounded

. . . has anyone actually used the "roller coaster" or "yo-yo" procedure on a revenue flight?

I’ve done it during testflights and it is doable. Calling it ‘Yoyo’ would be an exaggeration.

[Roger_Murdock]

Ostrower has a copy of the 1982 FCTM with a description of "roller coaster" technique:

https://theaircurrent.com/wp-content...-1240x1903.jpg

(main article)

[GordonR_Cape]

Originally Posted by Roger_Murdock

Ostrower has a copy of the 1982 FCTM with a description of "roller coaster" technique:

https://theaircurrent.com/wp-content...-1240x1903.jpg

(main article)

I find it moderately interesting that the case chosen for evaluation is runaway nose up trim, which is an urgent stall hazard. Presumably nobody in 1982 could visualise the hazards of runaway nose down trim, perhaps due to the limited range of trim travel allowed by the autopilot in the flaps down condition. As the article notes, assumptions made five decades ago, are still propagating through today's 737 flight controls.

[OldnGrounded]

Originally Posted by astonmartin

I’ve done it during testflights and it is doable. Calling it ‘Yoyo’ would be an exaggeration.

Thanks. How much air would want below you to have reasonable confidence in recovering in time?

[ProPax]

Originally Posted by meleagertoo

Recovery was for both pilots to pull the column back to give an appreciable pitch up, if only a few degrees, and then relax the pull. The resulting controlled pitch down released enough load on the stab to allow a bit of trim - perhaps less than half a turn initially, to be achieved. The procedure was repeated and with each repetition more and more manual trim was achieveable until a point was reached where normal though very stiff operation became possible.

How long did it take to reach that point? Doesn't sound like a quick procedure.

[dingy737]

Originally Posted by bzh

First bring the power back.... 94% power with the nose down will make it impossible to use manual trim, for unreliable airspeed 4deg nose up and 80% power will give you strait and level with flaps out 10deg/75%

4 Set the following gear up pitch attitude and thrust:

Flaps extended . . . . . . . . . . 10° and 80% N1

Flaps up . . . . . . . . . . . . . . . . 4° and 75% N1

[Double Back]

Not active any more but I never forgot the Convair accident due to a reversed trim:
https://www.flightglobal.com/news/ar...cables-326251/
The last weeks It crossed my mind I might have tried to trim even more nose down, in a desperate last attempt to see if the nose would come up...Which in this case would only have sped up the outcome of course...
Remember, those guys were loosing it big time, they were desperate...
Many accidents I have played back in my mind ( a kind of survival procedure for me to deal with terrible accidents), with me making the right decisions and saving the day.
But there are a few cases where I cannot find a realistic better outcome. Like JAL123. I think this MCAS affair is close to that, although technically it was survivable. But You would have to get immediately into the right direction of thinking.
How many times in YOUR career the SIM had to be stopped because Your crew was going down the drain, following a completely wrong lead? I was stopped at least once if I remember correctly....

[FlexibleResponse]

fdr said

What is described is also a design that is not compliant with the requirements of Part 25. There is no justification that can be made that an aircraft needs to be handled in such a manner with the failure of a system that is both possible and part of the design requirement to have no adverse effects on failure... Forget about MCAS, how on earth is the procedure in the FCTM reasonable or acceptable. This is not an acro, it is a transport category aircraft.

Forget about MCAS, how on earth is the procedure in the FCTM reasonable or acceptable.

The design defect of immovable manual trim wheel in the pitch control system under some conditions was obviously acceptable to the FAA 50 years ago to allow certification of the B737 aircraft type.

Grandfather rights on existing certification of the B737 aircraft type has somehow allowed this defect in the flight control system to continue to exist in the modern era.

The high authority automatic MCAS system combined with the low authority manual electric trim system for the pilot to counteract the MCAS, has now shown up this "hidden" design defect in a most dreadful way.

The design defect of the immovable trim at high aerodynamic loads has been festering for 50 years awaiting a system that would reveal it to be a fatal design flaw.

[meleagertoo]

Originally Posted by ProPax

How long did it take to reach that point? Doesn't sound like a quick procedure.

Not too long. It was a while ago but I'd think 4 or 6 repeats from full stab deflection before manual trim became usable without it.
It certainly was neither violent nor extreme manoeuvering, you're only pulling to as much of a normal climb attitude as you can reasonably acheive and then relaxing back to level flight. Pax would find it a bit of an undulationperhaps like flying through wave but to call it "yo-yo' is a big exaggeration. Even roller-coaster greatly overstates it although it illustrates the shape of the thing, if you like.

Next time you're in the sim ask to try it. It's quite a gentle flightpath as you're pulling big forces on the yoke at first.
I'm surprised it disappered from the FCOM though. Mr Boing's gonna regret that decision...

Min height to do this at? 500 feet if you had to - not that you'd choose to do of course. There should be no diving or descent involved, just undulations up from and back to whatever altitude you started from. If you had height to spare ypu might use a bit of descent but it wouldn't be to my taste, I'd not want the nose below the horizon with extreme down-trim simply because of the recovery, and also to avoid airspeed buildup which would surely be obvious to most.

[HalinTexas]

Has anyone read the preliminary report here? They used the electric trim 30 seconds before the crash. MCAS responded again. Airspeed was about 350 KIAS.

So, it would seem to me that if the electric trim could move the wheel, and that it can be stopped and overridden by hand power, that manual trim should have worked. 350KIAS should NOT be too fast for manual trim. Since they followed and summarily rejected the QRH and improvised, then who is at fault, or at least share fault? There was no reason to turn the electric trim switches back on, but since they did, they should have used it! If they had, they could have saved the airplane. I'll give the Lion Air guys a bit of a pass, but not much. They have a horrible reputation in the training department.

But what do I know. Six airlines in four countries on the B737 and I teach on it. Read the reports. Read them with a skeptical eye. They are surprisingly incomplete.

[Capn Bloggs]

Originally Posted by HalinTexas

Since they followed and summarily rejected the QRH and improvised, then who is at fault, or at least share fault?

The report clearly states, after they turned off the stab trim, "At 05:43:04, the Captain asked the First Officer to pitch up together and said that pitch is not enough."
Then there were momentary electric trim inputs; I think they realised that they didn't have enough pitch control and decided to try the electric trim again. What would you have done? Just sat there with the switches off because that was the end of the QRH drill and slowly speared in?

There was no reason to turn the electric trim switches back on, but since they did, they should have used it! If they had, they could have saved the airplane.

I think it was a great idea switching them back on. A great pity though that they didn't follow through by trimming against the continued MCAS trim.

[HalinTexas]

I would have recognized a runaway trim. Held the electric trim back until I didn't need so much control force. At the same time I'd call for the "Runaway Stab Trim" QRC/QRH. Used to be a memory item.

Both accidents used the trim to stop the motion, but NOT to reverse the trim. Anyone that hand flies the airplane much knows that you have to trim the hell out of Boeings, and trim some more. I suspect some serious training deficiencies, along with lack of experience.

I'm not buying that they could trim manually. They were doing less than 350KIAS. I suspect they didn't pull the handle out to crank it.

[Capn Bloggs]

Originally Posted by HalinTexas

Anyone that hand flies the airplane much knows that you have to trim the hell out of Boeings

Therein lies a major part of the problem. Some operators actively discourage hand flying, others virtually prohibit it.

Crews with little general aviation experience just aren't getting enough practice to survive these types of events, and it's not their fault.

[LNAV VNAV -]

Originally Posted by meleagertoo

Pity, I thought I'd asked a straightforward question, I'll try again.

How many have and have not seen this demonstrated in the sim?

i did my 737NG type rating at Boeing in Seattle in 2004 and I was not shown this technique.

[DaveReidUK]

Originally Posted by HalinTexas

I'm not buying that they could trim manually. They were doing less than 350KIAS. I suspect they didn't pull the handle out to crank it.

Trim force required isn't just dependent on IAS.

[Cows getting bigger]

Originally Posted by HalinTexas

Since they followed and summarily rejected the QRH and improvised, then who is at fault, or at least share fault? There was no reason to turn the electric trim switches back on, but since they did, they should have used it! If they had, they could have saved the airplane.

I'll subscribe to your shared responsibility argument. The legal people will split that down into percentages from the following:

1. Aerodynamic development that led to poor static stability at high AOAs.
2. Inadequately deigned software add-on (MCAS) to hide comply with static instability issue.
3. Single point of failure (AOA gauge) in safety critical flight control system.
4. Inadequate training and documentation.
5. Poor Boeing assessment of systematic failures that led to Lion Air crash - initial AD.
6. Failure by Lion Air #1 to articulate and capture problem after first, non-fatal event.
7. Regulatory oversight, lack of?
8. Two slightly dull/average/unlucky crews faced with multiple inputs.

I'll wager that the crew 'percentage' will not be the biggest number.

[GordonR_Cape]

Originally Posted by DaveReidUK

Trim force required isn't just dependent on IAS.

Time to repost this (or similar) article link every time someone also asks about:
- how do manual trim wheels, cables and the jackscrew work
- why the horizontal stabiliser is loaded via opposing elevator aerodynamic forces
- alternative techniques for unloading the HS jackscrew (roller coaster)
- why didn't the pilots do X or Y?
- can the plane fly inverted?
- why was this certified?
See: https://theaircurrent.com/aviation-s...nvestigations/

Edit: +1 for Cows getting bigger