Boeing advice on "aerodynamically relieving airloads" using manual stabilizer trim

[Centaurus]

Reference Boeing 737 Classics and NG FCTM under the chapter Non-Normal Operations/Flight Controls and sub heading Manual Stabilizer trim.

Edited for brevity one paragraph states: "Excessive air loads on the stabilizer may require effort by both pilots to correct mis-trim. In extreme cases it may be necessary to aerodynamically relieve the air loads to allow manual trimming. Accelerate or decelerate towards the in-trim speed while attempting to trim manually."

What control movements are needed to "aerodynamically" relieve the airload? This is not amplified in the FCTM and from experience I believe few pilots know what Boeing mean by "aerodynamically relieve the airload".
In fact I wouldn't be surprised in the two recent Boeing 737 MAXI accidents and the apparent inability for their crews to recover from unusual attitudes (nose down) could be traced in part to absence of knowledge on how to aerodynamically relieve airloads if using manual stabilizer trim.

My understanding of the meaning of "aerodynamically relieving" is best illustrated as follows: An aircraft suffers a severe nose down runaway stabilizer trim.causing the aircraft to initially dive and rapidly lose height. Any delay caused by surprise factor further compromises flight path control. Both pilots haul back hard on the elevators while attempting back trim using manual stabilizer trim. Due to increasingly heavy aerodynamic airloads against the stabilizer the effort to manually rotate the trim wheels in this condition is considerably higher than normal.

To relieve these airloads so that manual stabilizer trim can be used to wind off the forward position of the stabilizer which has caused the problem in the first place, it may be necessary for the crew to first attempt to raise the nose well above the horizon. With the nose high, the control column is immediately released from all back pressure. This action momentarily "aerodynamically relieves the airloads that in turn allows rapid unimpeded manual operation of the stabilizer trim control; to return the stabilizer to mid-range and thus permit more effective elevator effectiveness.

This "yo-yo" technique may be the only effective way of overcoming the difficulty of using the manual stabilizer trim during an attempted recovery from a high speed dive where electrical operation of the stabilizer is unavailable. To my knowledge this technique is not covered during simulator training for manual stabilizer trim operation

[Bergerie1]

Centaurus,

The 'yo-yo' manoeuvre you describe was the same for the 707. I never had to do it for real, only in the simulator. But I have heard, at second hand, of a crew who had a runaway stabiliser and who used that method to restore normal trim. I understand it was exceedingly hard work. And stressful, as it was done at low altitude over Toronto, but they saved the day.

[finncapt]

Was on a subsequent conversion course with one of the guys (p2?) on that.

He claimed they went past a 76 gas station below the level of the sign.

Very frightening and the forces required were enormous.

[Chris Scott]

In fact I wouldn't be surprised in the two recent Boeing 737 MAXI accidents and the apparent inability for their crews to recover from unusual attitudes (nose down) could be traced in part to absence of knowledge on how to aerodynamically relieve airloads if using manual stabilizer trim.


Hi Centaurus,
From what I remember of the B707-320 over 40 years ago, the possible scenario of the HS refusing to move when under high aerodynamic load could apply to electric trimming (using the piccolo switches) as well as manual trimming (cranking the large trim wheels on each side of the centre pedestal). Is the B737 similar, and what about the dash-800 MAXI?

Trouble is that, when the a/c is already pointing downwards at a very low height, releasing the yoke to unload the elevators is not an option...

[Centaurus]

Hi Centaurus,
From what I remember of the B707-320 over 40 years ago, the possible scenario of the HS refusing to move when under high aerodynamic load could apply to electric trimming (using the piccolo switches) as well as manual trimming (cranking the large trim wheels on each side of the centre pedestal). Is the B737 similar, and what about the dash-800 MAXI?

Trouble is that, when the a/c is already pointing downwards at a very low height, releasing the yoke to unload the elevators is not an option...

That is true. The pilot would have to know exactly what he is doing and act real fast and hope he could pull out in time.

[safetypee]

Centaurus,
A sound and well reasoned position #1.
Questions for clarification; does the yo-yo manoeuvre involve reducing the manual control force to, or only towards zero; not withstanding the likely hood of a crew doing this at low altitude, more-so if they had not been specifically trained for the event.

Is this problem similarly described as a ‘jack stall’ in other aircraft.

Do these versions of the 737 have an automatic cross-cockpit control force disconnect to alleviate a jammed control; thus with a trim problem, if one pilot pulls hard and the other doesn’t, then what …
If the pitch control system is ‘split’, would an ‘inactive’ elevator provide sufficient relaxation on the tail forces to enable manual trim input.
Conversely can a single elevator generate sufficient nose-up change to allow a load alleviating manoeuvre.

Does the 737 Max FCTM consider this situation - different aerodynamics, but presumably a similar physical control mechanism and air loads.

[gums]

Salute!

Absolutely stellar post, Centaurus . Outstanding and timely.

Guess being able to manually fly a plane is a valuable skill, as is recognizing what the problem is. I say this as many posters on the relevant threads are kissing off the trim implementation on the 737 by saying, :"no big deal. Just turn off the electric trim". Unlike those of us that flew single engine and had engine failure, the MCAS malfunction is not as easy to recognize. Especially true if you have never heard of the thing and exactly how it works.

Gums sends...

[Chris Scott]

It may be worth mentioning for the benefit of non-aviators and new ones that the "yo-yo" expedient described by Centaurus would most probably be used after a runaway HS, in order to unload the HS sufficiently to restore its ability to be moved. If the runaway took place following a nose-down trim adjustment - as typically required during flap retraction and/or a go-around in a contemporary jet airliner with engines mounted under the wings - the first priority for the pilot(s) may be to select enough up-elevator to stop the aircraft nose diving into terrain.

Yes, safetypee, the failure of a serviceable trimming system to move an aerodynamically-overloaded HS is generally referred to as a "stall" of the HS jack.

[Chu Chu]

I read the quote from the FCTM as telling you how to aerodynamically relieve loads: "Accelerate or decelerate towards the in-trim speed." Perhaps that won't be feasible or sufficient in all cases. But it seems to be the technique the FCTM advises.

[Centaurus]

The 'yo-yo' manoeuvre you describe was the same for the 707. I never had to do it for real, only in the simulator. But I have heard, at second hand, of a crew who had a runaway stabiliser and who used that method to restore normal trim. I understand it was exceedingly hard work. And stressful, as it was done at low altitude over Toronto, but they saved the day.
Tried to Google the 707 incident to get more explanation of the Yo Yo technique taught in those days but could not find any reference. Date, location and brief description of incident would be greatly appreciated.

[finncapt]

May help.

The company was BOAC, I thought the incident was out of Dorval, Montreal (but could be Toronto), probably a -436.

The conversion course we did together was in 1978 and I had the impression the incident was some years previous.

The chap was a captain on the 707 by then, so, if in the incident he was a P2, it possibly happened in the 60s.

I joined BOAC in 1970 and don't recall it happening in my time, but that may be because I was on the VC10.

I know that D P Davies, of AAIB fame, was closely involved in the investigation (my acquaintance was not very enamoured with him).

He stated that the trim even ran away with the brake on (wasthere a brake?) and his hand trying to stop it.

Never flew the 707 but I think it may have been something to do with the servo tab working in the wrong direction.

I know the name of the person involved (we shared a house in New Zealand on the ANZ DC10 conversion course) but I will not reveal it here.

[Centaurus]

that the "yo-yo" expedient described by Centaurus would most probably be used after a runaway HS, in order to unload the HS sufficiently to restore its ability to be moved

After some research in my aviation library I think I have found the answer to my original questions of the meaning of the Boeing term "relieving aerodynamic loads."
Extract from the Boeing 737-200 Pilot Training Manual February 1982 page 04.80.31. Edited for brevity
Runaway and Manual Stabiliser - Recovery from Severe Out-of-Trim
"In an extreme nose-up out-of-trim condition, requiring almost full forward control column, decelerate, extend the flaps and/or reduce thrust to a minimum practical setting consistent with flight conditions until elevator control is established. Do not decrease airspeed below the minimum maneuvering speed for the flap configuration. A bank of 30 degrees or more will relieve some force on the control column. This, combined with flap extension and reduced speed should permit easier manual trimming.

If other methods fail to relieve the elevator load and control column force, use the "roller coaster" technique. 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. Repeat this sequence until the airplane is trim.

If nose-down trim is required, slowing down and extending the flaps will account for a large degree of nose-up pitch. If this does not allow manual trimming then the reverse "roller coaster" can be performed to permit manual trimming." (I read somewhere it was called the Yo Yo manoeuvre)

Boeing "Airliner" magazine published in May 1961 discussed the above subject as it applied to the Boeing 707 by stating: "To trim the stabilizer manually while holding a high stick force on control column. As the airplane changes altitude, crank in the desired trim change. Correct airplane attitude after a few seconds with elevators. Relax stick force again and crank in more trim. Repeat this procedure as necessary until proper 'trim' position of stabilizer is established."

We learned all about these maneuvers in the 1950-60s. Yet, for some inexplicable reason, Boeing manuals have since deleted what was then - and still is - vital handling information for flight crews.
Finally, author D.P.Davies comprehensively covers the subject of large trim changes, failure cases and Mach number effect on stabilizers, at pages 38 to 42 in his fine book "Handling the Big Jets," A good case for current airline pilots to buy his book as it is still the best on the market, IMHO

[VinRouge]

Does anyone know whether the 737 Max MCAS is an aerodynamic crutch to a design issue discovered at flight test or was it designed in from the off to allow for increased aerodynamic/fuel efficiency reasons?

[fireflybob]

May help.

The company was BOAC, I thought the incident was out of Dorval, Montreal (but could be Toronto), probably a -436.

The conversion course we did together was in 1978 and I had the impression the incident was some years previous.

The chap was a captain on the 707 by then, so, if in the incident he was a P2, it possibly happened in the 60s.

I joined BOAC in 1970 and don't recall it happening in my time, but that may be because I was on the VC10.

I know that D P Davies, of AAIB fame, was closely involved in the investigation (my acquaintance was not very enamoured with him).

He stated that the trim even ran away with the brake on (wasthere a brake?) and his hand trying to stop it.

Never flew the 707 but I think it may have been something to do with the servo tab working in the wrong direction.

finncapt, I also joined BOAC in 1970 (as a young Second Officer) and started on the 707-436 and I recall this incident being talked about. Your version of events concurs with mine and I believe the aircraft had departed from Montreal. The cloudbase was low and radar assisted then to avoid higher terrain as they had initially reduced power to maintain pitch control. Despite the fact that the stab trim brake was on the trim continued to run away slowly and I believe the F/E put both hands on either side of the wheels to prevent further runaway. I believe the cause of the runaway stab was incorrect grease being used on at least one of the clutches on the stab trim control.

[PEI_3721]

It is surprising that such a feature, requiring significant piloting skill to manage a rare and surprising failure, is allowed in modern aircraft - 737 NG.
However, noting later 737 designs still claim the ‘grandfather rights’ in certification approvals from previous versions, we may not be so surprised.

The NG information above implies that the effect of aerodynamic and control system changes present no greater risk than previous versions, but the ‘hidden’ assumption is that crew performance, knowledge and training, is similarly adequate.

A cursory view suggests that this particular failure mode and recovery action is not widely known, nor been trained for with simulation. If so, the risk assessment for this failure case would change - because the extent which a crew can mitigate the failure and continue safe flight could be significantly less than originally assumed.
Aircraft have ‘grandfathers’, but pilots might not !

If … IF, the 737 Max has been certificated on the basis the previous approvals, and the effects of failure, and required crew skills are the same, then an argument of equivalence could be made.

Was such an assessment made for the 737 Max, did the certification process consider changes in piloting experience and skill over the lengthy time scale of 737 operations. Additionally, are the stick forces with trim malfunction in the Max similar to those already approved, and if such stick forces exist, did the need for a skilled manoeuvre occur at a similar position in the control range, specifically not earlier.
If no such assessment was made … recorded … overseen … approved in certification … then …
Any 737 Max operator able to clarify or comment; FCTM.

Based on current knowledge of the Lion accident, the questions above are more significant in that a severe mis-trimmed condition could be encountered more often than that assumed in the certification assessment of trim runaway alone, because of additive contribution of MACS.
Also, that with erroneous, incremental trim activity from MCAS, higher, possibly limiting stick loads could similarly be encountered; i.e. with AoA failure - MCAS activity, recognising the the complex situation and need for alleviating trim inhibition, spans a very narrow time frame.

Setting this in the context of an apparent unreliable airspeed and stick shake due to AoA failure at take off, then varying control force due to erroneous MACS trim activity after flap retraction, the crews mental and increasingly physical effort could easily be overwhelmed. Following on is the need for a very demanding and coordinated procedure to maintain and effect some control in order to manually operate the trim and achieve some semblance of normal control.

A step too far - a giant leap in the realms of safety and certification.

[finncapt]

PEI

Whatever the conclusion of the investigation into the recent accidents, I am somewhat surprised to discover that a very difficult (I know that is subjective) procedure which was required some 40+ years ago is, essentially, required in a modern day airplane.

[alph2z]

There was a test flight around the 70s that intentionally brought a large airliner jet (don't remember exact model) to near Mach 1 in a descent. They couldn't trim nose up so they had to move the stick forward, and then they were able to trim nose up. Don't know if they tried using the trim wheel with their hands.

A major crash occurred near Montreal when a DC8, after takeoff, pitched up, then stalled, and crashed. It was blamed on trim switch. DC 8s were known at the time to have switch problems. I believe they didn't have a recorder. This was in 1963.

[PEI_3721]

finncapt,
it’s like the boiled frog problem, we don’t notice small changes over time, but when thrown into boiling water (accident) it is a big surprise - painful. Many problems remain hidden because we have forgotten, failed to record, or haven’t questioned the original assumptions against those now accepted in the industry.
See double loop learning.
But then, we don’t question our parentage or grandfathers; perhaps it’s time for a DNA check on the industry

[LeadSled]

Centaurus,

The 'yo-yo' manoeuvre you describe was the same for the 707. I never had to do it for real, only in the simulator. But I have heard, at second hand, of a crew who had a runaway stabiliser and who used that method to restore normal trim. I understand it was exceedingly hard work. And stressful, as it was done at low altitude over Toronto, but they saved the day.

Folks,
I don't know where this comes from, but with a jammed/frozen stab on the B707, you simply trimmed by splitting the spoilers, and at lower speed, splitting the flaps ---- one of the lesser advertised benefits of a swept wing.
There was no need for any so called "yo-yo" maneuver.
It is true of the B707 that full elevator deflection could stall the stab, and you had to reduce the aerodynamic load on the stab by reducing the elevator deflection, which you wouldn't want to do close to the ground, if it meant lowering the nose.
So: On the overhead panel, about the Captain's right eyebrow, Spoiler Switch UP, and pull the Speedbrake as required. The rule was: Switch UP, pitch Up, Switch Down, pitch Down ( except on G- registered aircraft, where D.P.Davies buggered it up, as usual).
I will be fascinated to find out what the real problem is with the speed stability system on the Max --- anybody who flew them cast your minds back to the A310 and A300-600, and aircraft losses.
Tootle pip!!

[Bergerie1]

finncapt,

You are probably right - Dorval, not Toronto. It happened in the early 1960s and I believe the wrong grease was the cause. I knew the captain, a good pilot and a good bloke.

[Chris Scott]

Folks,
I don't know where this comes from, but with a jammed/frozen stab on the B707, you simply trimmed by splitting the spoilers, and at lower speed, splitting the flaps ---- one of the lesser advertised benefits of a swept wing.
There was no need for any so called "yo-yo" maneuver.
It is true of the B707 that full elevator deflection could stall the stab, and you had to reduce the aerodynamic load on the stab by reducing the elevator deflection, which you wouldn't want to do close to the ground, if it meant lowering the nose.
So: On the overhead panel, about the Captain's right eyebrow, Spoiler Switch UP, and pull the Speedbrake as required. The rule was: Switch UP, pitch Up, Switch Down, pitch Down ( except on G- registered aircraft, where D.P.Davies buggered it up, as usual).


Yes, doubt any pilot or F/E rated on the B707 could forget the splitting of spoilers (and, for the approach, the flaps) for pitch control in the jammed-stabiliser case. But I'm not familiar with the layout of spoilers on the wing of the seven-THREE, or if they can be split in the same way. We need a rated B737 pilot to comment...

I will be fascinated to find out what the realm problem is with the speed stability system on the Max --- anybody who flew them cast your minds back to the A310 and A300-600, and aircraft losses.

Not sure what you are specifically referring to. I did (only) two years P1 on the A310 35 years ago, and my main recollection of significant trim (THS) contingencies relates to the G/A situation - with or without AP. The auto-trim with AP engaged was, IIRC, slower than manual THS trimming, so the yoke remained forward for a longer period. But I'm not aware that the large amount of down-elevator required in either case ever led to the THS stalling.

It seems unfortunate that, faced with unexpectedly strong competition from the A320 family (first certificated in 1988), Boeing decided to exploit the grandfather certification rights of the B737 rather than introducing an all-new type.

[LeadSled]

Chris Scott,
Specifically the trim pitch up (instead of reducing power by retarding the thrust levers) to prevent exceeding limit speeds, and pilots fighting the pitch up trying to maintain assigned altitude, with autothrottle engaged.. When it all came apart, violent pitch ups exceeded 35 or more degrees, from memory.
There is a spectacular video of a A 310 over Paris, the pilot had to roll it on its side to get the nose down, that actual aircraft was later lost in what was thought to be similar circumstances some time later. At least he understood he could never just get the nose to pitch down by pushing.
There was an A 310 lost on approach at Nagoya, that was a close one for Qantas, there was some discussion about an immediate t/o for QF, they got it, and the A310 finished up about where the QF B747 has been holding.
I well remember, at the time, ANG (PX)) banning any coupled missed approaches ---- but that was back in the day when we could ALL fly --- and were expected, so to do. I thought it was a very wise decision on behalf of m' old mate, their boss of training. the best way of eliminating mode confusion is eliminate the modes as required.
What I do know about the B737 (up to NG) is that the stab trim cutout switches are in exactly the same position as every other Boeing aircraft I have flown, and that is a few. I would bet the Max is the same.
The crew of the Lyon Air "the night before" from Bali demonstrated what was possible with their particular set of defects.
Tootle pip!!

[kenparry]

We learned all about these maneuvers in the 1950-60s. Yet, for some inexplicable reason, Boeing manuals have since deleted what was then - and still is - vital handling information for flight crews.

The "inexplicable reason" is, I understand, input from Boeing lawyers as a reaction to Product Liability law. Lots of good stuff was removed from the B737-200 manuals in the 80s, not just what Centaurus describes, but also advice on (for example) landing with partial gear. I was flying the 737-200 at the time and was unimpressed by the deletion of useful advice.

[PEI_3721]

Re high control forces and EASA certification document - 737 Max.

This might not generate a warm feeling about a runaway trim in any 737 version. If the text below also relates to 737 NG, does the FCTM identify these points, why there is an inability to trim, and a special procedure.
Also, is the thrust level considered; discussions re Max question ‘what if’ the AT remains engaged and is in a thrust / climb mode during the ‘dive’.

This document adds further light on the MCAS problems in the Max. Also, if the Max is similar or worse than other 737s then it might not meet the requirements due to a special procedure, if recoverable at all. #15

“This annex to the EASA TCDS IM.A.120 was created to publish selected special conditions / deviations / equivalent safety findings that are part of the applicable certification basis:”
Re line 1 typo? ‘switches’ should read ‘wheel’, consistent with ‘wheel’ later in the page.


STATEMENT OF ISSUE

The aisle stand trim switches can be used to trim the airplane throughout the flight envelope and fully complies with the reference regulation.

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. In those cases, longitudinal trim is achieved by using the manual stabilizer trim wheel to position the stabilizer.

The trim wheel can be used to trim the airplane throughout the entire flight envelope. In addition, the autopilot has the authority to trim the airplane in these conditions. The reference regulation and policy do not specify the method of trim, nor do they state that when multiple pilot trim control paths exist that they must each independently be able to trim the airplane throughout the flight envelope. Boeing did not initially consider this to be a compliance issue because trim could always be achieved, even during the conditions where use of the aisle stand trim switch was required. Subsequent to flight testing, the FAA-TAD expressed concern with compliance to the reference regulation based on an interpretation of the intent behind “trim”. The main issue being that longitudinal trim cannot be achieved throughout the flight envelope using thumb switch trim only.

EASA POSITION

Boeing set the thumb switch limits in order to increase the level of safety for out-of-trim dive characteristics (CS 25.255(a)(1)). The resulting thumb switch limits require an alternative trim method to meet CS 25.161 trim requirements in certain corners of the operational envelope. The need to use the trim wheel is considered unusual, as it is only required for manual flight in those corners of the envelope. The increased safety provided by the Boeing design limits on the thumb switches (for out-of-trim dive characteristics) provides a compensating factor for the inability to use the thumb switches throughout the entire flight envelope. Furthermore, the additional crew procedures and training material will clearly explain to pilots the situations where use of the trim wheel may be needed due to lack of trim authority with the wheel mounted switches. The trim systems on the 737Max provide an appropriate level of safety relative to longitudinal trim capability.



https://www.easa.europa.eu/sites/default/files/dfu/IM.A.120%20Boeing737%20TCDS%20APPENDIX%20ISS%2010.pdf
Page 15

[Loose rivets]

I posted this on R&N, but would probably be better here.

When we gave the Americans the flying tailplane :-), was it hinged at the front?

Pivoted at the rear, or nearer to, it is inherently unstable. The nut failure, or some such would mean it thwacking over full deflection, but it hasn't happened, has it? Okay, so we've not had a wing detaching g force that I can remember, but now we've got a new reason to pivot at the front. The loads would be smooth and progressive, and even if it had been cranked all the way, hand-winding it back would now be aided rather than opposed.

The first scenario we can discount because of history. The second issue a major change in design philosophy, but not a huge change in pilot handling and seemingly needed. I'd never dreamed of having to unload it in a series of switchback rides. It would be so easy to crank it with a front fulcrum and I'd guess a lot cheaper than gum's "rehanging the engines".

Where to put the Jack? I'd have cut a slot out of the rudder before I'd have put the fulcrum at the rear. But seriously, a horizontal jack would only need a redesigned lever and the loads would be less and far less consistent.

[Dave Therhino]

Pivoted at the rear, or nearer to, it is inherently unstable. The nut failure, or some such would mean it thwacking over full deflection, but it hasn't happened, has it? .

Alaska Airlines MD-80 Flight 262

[Clandestino]

Yes, doubt any pilot or F/E rated on the B707 could forget the splitting of spoilers (and, for the approach, the flaps) for pitch control in the jammed-stabiliser case. But I'm not familiar with the layout of spoilers on the wing of the seven-THREE, or if they can be split in the same way. We need a rated B737 pilot to comment...Thanks for bringing up this interesting topic, Takwis beat me to answer. Just to add that the NG spoiler switches are same class as on KC-135: maintenance purpose only and flaps can't be split as all the high lift devices are green... pardon my French... HYD B powered. It's nice to have another option of pitch control with splitting of high lift and drag devices but its utility has to be weighted against the nuisance of pitching moments with hydraulic system failure. I would guess it's a reason behind spoilers' hyd sources being GYBYG on 320 and AABABA on NG.

Whatever the conclusion of the investigation into the recent accidents, I am somewhat surprised to discover that a very difficult (I know that is subjective) procedure which was required some 40+ years ago is, essentially, required in a modern day airplane.

It is surprising that such a feature, requiring significant piloting skill to manage a rare and surprising failure, is allowed in modern aircraft - 737 NG.However, noting later 737 designs still claim the ‘grandfather rights’ in certification approvals from previous versions, we may not be so surprised.


Well, it's modern in the sense it is flying nowadays and still being produced but it's really 1960s design, leaning a lot on its 1950s bigger brother with some fancy modern stuff grafted on. Up to NG, the grafting was successful and proven safe in use, even if its results were not always quite ergonomic.


He stated that the trim even ran away with the brake on (wasthere a brake?) and his hand trying to stop it.


No brake anymore on 737 NGs (no mention of it on MAX conversion course ether, so I assume it's the same trim sys :E) but there is similar failure mode; last recall item for Runaway Stabilizer, after cutting of electrical power to it, is "Grasp and hold the trim wheel". I can't speculate about stab hinges or if it is something aerodynamic, mechanical or pure magic that can make them still run away after (supposedly) all electric power is cut off, yet the possibility is in the QRH.


So: On the overhead panel, about the Captain's right eyebrow, Spoiler Switch UP, and pull the Speedbrake as required. The rule was: Switch UP, pitch Up, Switch Down, pitch Down ( except on G- registered aircraft, where D.P.Davies buggered it up, as usual).
Could you please provide more details on the parenthesed part? As an avid aviation history buff, I'm really interested in it. Thanks.

[infrequentflyer789]

SOMEWHERE on this great big site, buried within one of several threads now applicable to the two MAX crashes, SOMEONE made the statement that the trim wheel was made smaller, because of a "chokepoint" (or "choke point") between the trim wheel mechanism and the new instrument panel/screens.

Remembered that post because it was a reply to one of mine, the phrase used was "pinch point" - until we have HAL to do searches looking for choke point won't find it :)

Link: https://www.pprune.org/showthread.php?p=10438165

I find they don't always work that well - gets you near the post not on it - so the relevant text is (and it's unattributed - copied from another forum):

I agree it's a flawed design. And I used to work there. I'm glad I don't now.

Regarding the trim wheels: When the NG was being introduced, I happened to be the Lead Engineer in charge of them and a whole lot of other stuff. There were some issues. The new display system created a pinch point between the dash and the wheel. We had to make the wheel smaller. And the new trim motor resulted in the wheel, which is directly connected to the stabilizer by a long cable, springing back when electric trim was used. It was an undamped mass on the end of a spring. We had to add a damper.
Result: Depending on the flight conditions, the force to manually trim can be extremely high. We set up a test rig and a very fit female pilot could barely move it.
As I said, I'm glad I'm no longer there.

Hope that helps.

[Roger_Murdock]

Does anyone know whether the 737 Max MCAS is an aerodynamic crutch to a design issue discovered at flight test or was it designed in from the off to allow for increased aerodynamic/fuel efficiency reasons?

I don't know at what stage MCAS was added to the design, but it has nothing to do with increased efficiency. It's supposed to only operate on the fringes of flight envelope, near stall. Ideally it would never activate. So there's nothing to be gained by it.

[ve3id]

Was on a subsequent conversion course with one of the guys (p2?) on that.

He claimed they went past a 76 gas station below the level of the sign.

Very frightening and the forces required were enormous.
Something funny here, we never had '76' Gas Stations in Toronto!

[megan]

A question if I may as SLF. To what speed is the 737 controllable with full nose down trim? Boeing in its analysis of the Braniff 707-220 accident cited the aircraft controllable (full up elevator) up to 480 knots in 1"g"flight. Beyond that speed "g"reduces ie nose drops as horizontal stab overpowers elevator.

[Good memories]

Question if I may. Does the manual trim wheel adjust the HS by means of cables?

[severidian]

http theaircurrent.com/aviation-safety/vestigal-design-issue-clouds-737-max-crash-investigations/

[PJ2]

Question if I may. Does the manual trim wheel adjust the HS by means of cables?

Yes, the manual trim wheels use cables to adjust the HS. This is why either wheel can be held by a crew member to stop any undesired trimming.

The trim wheels have a retractable handle to make life easier when using the wheels to trim. Anyone who doesn't remember to put the handle back after use in the sim, certainly remembers the next time to do so...!

I am informed from a post on R&N, (DaveReidUK (https://www.pprune.org/rumours-news/619272-ethiopian-airliner-down-africa-166.html#post10439502)), that the full trimmable range for the HS takes "250" turns of the wheel. Since the full range is 17°, (AMM ATA27 for the -400), each turn of the wheel yields a 0.07° change in trim and to change the trim 1°, it takes approximately 14.7 turns of the wheel.

Earlier in this thread, with regard to stalling a highly-loaded HS drive system, a DC8 accident out of Montreal, (Dorval) airport was mentioned; https://en.wikipedia.org/wiki/Trans-Canada_Air_Lines_Flight_831 . Link to the final report: https://reports.aviation-safety.net/1963/19631129-0%20CF-TJN.pdf

A few months later, and Eastern Airlines DC8 appeared to have suffered the same issue: https://en.wikipedia.org/wiki/Eastern_Air_Lines_Flight_304

PJ2

[GordonR_Cape]

PJ2
each turn of the wheel yields a 0.07° change in trim and to change the trim 1°, it takes approximately 14.7 turns of the wheel.


Thanks for cross posting that. So, for every 10 second 2.5 degree nose down incremental trim by MCAS, you need roughly 35 turns of the wheel. Good luck with that while flying at 250kts and 1000ft elevation!? God help any crew placed in that situation.

[Derfred]

PJ2


Thanks for cross posting that. So, for every 10 second 2.5 degree nose down incremental trim by MCAS, you need roughly 35 turns of the wheel. Good luck with that while flying at 250kts and 1000ft elevation!? God help any crew placed in that situation.

Why wouldn't you just trim it back with electric trim?

[737 Driver]

Why the crew wasn't more aggressive with electric trim when MCAS first kicked in is one of the great mysteries of this accident. As to why they didn't use more electric trim in the final moments, see this article for some informed speculation at Bjorn's Corner/Leeham News in the article "Bjorn's Corner: ET302 crash report, the first analysis." (I would have normally included a link, but apparently the forum rules don't allow me to as a newbie member)

[GordonR_Cape]

Why wouldn't you just trim it back with electric trim?

Good point, but I was referring to the stage where the crew decided they had enough of MCAS, and hit the cutoff switches in the nose down position, as suggested by the (ambiguous) emergency AD procedure. I was purely discussing the number of turns, if that was the choice they made, and the procedure specifically says not to turn electric trim back on again.

BTW, parallel discussion in the other thread, so I won't say any more. See: https://www.pprune.org/rumours-news/619272-ethiopian-airliner-down-africa-180.html#post10441496

[PJ2]

Why wouldn't you just trim it back with electric trim?

Hi Derfred;

Reading your posts on the Boeing and your discussions with other members with interest, thanks.

I posted the info purely because it didn't seem to be anywhere to be found in any of the usual manuals crews have access to. It seemed like it was good to know this about the B737's manual (wheel) trim system so it sets one's awareness of a rarely-used system and sets expectations of the rate one is turning the wheel so one doesn't give up just because "nothing seems to be changing".

PJ2

[Derfred]

Great point PJ.

In addition to awareness of the number of manual turns required per unit of trim, I noted with interest in one of Mentour’s YouTube videos the difference in ability between him and the RHS pilot when it came to rotating the wheel.

He did it easily and rapidly while simultaneously pulling control column back pressure. But the RHS pilot had considerable difficulty.

There is obviously “technique” involved, which may come with practice.

Manual trimming is not regularly practiced in my airline... I think I’ve tried it once in the type rating.

[India Four Two]

737 Driver,

Here's the link you referred to:

https://leehamnews.com/2019/04/05/bjorns-corner-et302-crash-report-the-first-analysis/

[infrequentflyer789]

Great point PJ.

In addition to awareness of the number of manual turns required per unit of trim, I noted with interest in one of Mentour’s YouTube videos the difference in ability between him and the RHS pilot when it came to rotating the wheel.

He did it easily and rapidly while simultaneously pulling control column back pressure. But the RHS pilot had considerable difficulty.

There is obviously “technique” involved, which may come with practice.

Manual trimming is not regularly practiced in my airline... I think I’ve tried it once in the type rating.

As an engineer I would note that unless two hands are used (which was tried in the now deleted video) the LHS pilot is going to be using their right hand while the RHS pilot is using their left - if both pilots are right handed, then LHS will find it easier.

However, in the Mentour video I think you are referring to, there is probably another factor at work:
- when they cut the switches they are out of trim, so it's hard to turn due to airload
- it becomes easier when both pilots join in
- as they get back in trim you can see it moves more easily
- when Mentour is trimming on his own it is after they have got back in trim and in fact he says "just minor adjustments"
- this is easily done because they are back in trim (or nearly)

But like I say, that's an engineering take on it, pilot opinions may vary.

[Derfred]

789, thanks. I don’t have a link to the video I was recalling - I’ll have to search for it again. You might be right.

The aspect I was considering was that Mentour is obviously a Sim Trainer who may have had the opportunity for extensive practice at manual trimming in the Sim, whilst the rest of us don’t.

I’ll try to find the video again.

EDIT: Found it. This is the video I was looking at:
https://youtu.be/xixM_cwSLcQ

It’s positively irritating for pilots as these videos are obviously made for non-pilots.

So FFW to 19:00 minutes

It’s obviously not set up for a highly elevator loaded stab, it’s just a runaway stab simulation in which the stab is not really even running away for long.

I just thought the F/O vs CPT technique interesting. But you are right, when the Capt joins in, they are doing it together.

Cheers

[clarkieboy]

I’m not sure if it helps your very interesting discussions, but I had to be on an old 737-400 as well as a Max today. The trim wheel on the 400 was 10 inches in diameter, the Max was 9 inches. I tried the Max, and yes, it took approx 15 turns of the wheel per unit of trim.
As I said, hope it helps. Cheers.

[Artisan]

The B737stab trim system has two speed modes for the autopilot and main electric trim, I.e. high rate trimming with flap extended and low rate trimming with flap up.

I am wondering if the MCAS was programmed to always input high rate trimming? Hence if you are flying a MAX with flap up and the MCAS puts in a high rate AND burst for 10sec, then the pilots available ANU counter response on the main electric trim will only be at the low rate.

[DaveReidUK]

The B737stab trim system has two speed modes for the autopilot and main electric trim, I.e. high rate trimming with flap extended and low rate trimming with flap up.
I am wondering if the MCAS was programmed to always input high rate trimming? Hence if you are flying a MAX with flap up and the MCAS puts in a high rate AND burst for 10sec, then the pilots available ANU counter response on the main electric trim will only be at the low rate.

Looking at the FDR trace, the longest application of ANU trim (about 9-10 seconds) appears to have moved the stab about two units. That's almost, but not quite, the rate that MCAS had been moving it in the opposite direction.

[YYZjim]

From (only) a private pilot. I find prose descriptions of technical systems hard to follow, and prefer diagrams, schematics and flowcharts. I still do not fully understand the pitch control on 737 NGs and MAXs. The following diagram helped me, but likely contains errors and misunderstandings. Comments would be appreciated.

1. Does the autopilot operating in "normal" flight mode (excluding its STS and Mach Trim trimming) move the elevators or the stabilizer?

2. Does flipping the CUTOUT switch terminate the autopilot's control of the elevators or just its control of the stabilizaer jackscrew?

3. I have read about a "hidden" switch somewhere on the back of the center console that did something on the NG, but has been removed on the MAX. What is it?

4. I have read that the First Officer's trim switch is actually a pair of switches. If true, do the separate switches do different things?

5. I have read that the MAX has an automatic Elevator Jam Landing Assist system which manipulates the spoilers to assist in pitch control. The EJLA looks after things when the flaps are deployed; the MCAS looks after things when they're not. Hmmm. I wonder if there is an unintended software interaction?

[GordonR_Cape]

From (only) a private pilot. I find prose descriptions of technical systems hard to follow, and prefer diagrams, schematics and flowcharts. I still do not fully understand the pitch control on 737 NGs and MAXs. The following diagram helped me, but likely contains errors and misunderstandings. Comments would be appreciated.

1. Does the autopilot operating in "normal" flight mode (excluding its STS and Mach Trim trimming) move the elevators or the stabilizer?

2. Does flipping the CUTOUT switch terminate the autopilot's control of the elevators or just its control of the stabilizaer jackscrew?

3. I have read about a "hidden" switch somewhere on the back of the center console that did something on the NG, but has been removed on the MAX. What is it?

4. I have read that the First Officer's trim switch is actually a pair of switches. If true, do the separate switches do different things?

5. I have read that the MAX has an automatic Elevator Jam Landing Assist system which manipulates the spoilers to assist in pitch control. The EJLA looks after things when the flaps are deployed; the MCAS looks after things when they're not. Hmmm. I wonder if there is an unintended software interaction?

We need a FAQ for this topic. I am not a pilot, but these are what I have read.
1. The autopilot can move any control it likes, within limits.
2. The trim cutoff switch only controls the horizontal stabiliser, not any other flight controls.
3. There is an underfloor cutoff switch at the bottom of the control column. It is still there, but its function changed on the MAX.
4. Each pilot's electric trim consists of two thumb-switches mounted close together. For each pilot, both have to be pressed for any action to occur.
5. No answer.

[Australopithecus]

This system description is from the 737NG FCOM.

It explains the control column (under-floor) cutout switches which have been modified in the MAX to prevent them from disabling MCAS stab trim movement. The “hidden switch” on the centre console (actually called the aisle stand) is in plain view. Its a black cover-guarded switch called the STAB TRIM OVERRIDE that will override the underfloor cutout switch. It is there to restore normal trim operation if one of those switches has failed. I don’t know if it is still fitted to the Max, but that would be moot in any event.


Stabilizer Trim​
Stabilizer trim switches on each control wheel actuate the electric trim motor ​
through the main electric stabilizer trim​ circuit when the airplane is​ flown ​
manually. With the autopilot engaged, stabilizer trim is accomplished through the ​
autopilot stabilizer trim​ circuit. The main electric and autopilot stabilizer trim ​
have two speed​ modes:​ high speed with flaps extended​ and low speed with flaps ​
retracted. If the autopilot is​ engaged, actuating either pair of stabilizer trim ​
switches automatically​ disengages the autopilot. The stabilizer​ trim​ wheels rotate ​
whenever electric stabilizer​ trim​ is​ actuated.​
The STAB TRIM MAIN​ ELECT cutout​ switch and the STAB TRIM ​
AUTOPILOT cutout switch, located on the control stand, are provided to allow ​
the autopilot or​ main​ electric trim​ inputs​ to be disconnected from the stabilizer ​
trim motor.​ ​
Control column actuated stabilizer trim cutout switches stop operation of the main ​
electric and autopilot trim when the control column movement opposes trim ​
direction. When the STAB TRIM override switch is​ positioned to​ OVERRIDE, ​
electric trim can be used regardless of control column position.​
Manual stabilizer control is accomplished through cables which allow the pilot to ​
position​ the stabilizer​ by rotating​ the stabilizer​ trim wheels. The stabilizer​ is held​ ​
in position by two​ independent​ brake systems. Manual​ rotation​ of the trim​ wheels ​
can be used to override autopilot or main​ electric trim. The effort required to ​
manually rotate the stabilizer trim wheels may be higher under certain flight ​
conditions. Grasping the stabilizer trim wheel will stop stabilizer motion

[Judd]

Grasping the stabilizer trim wheel will stop stabilizer motion.
I believe this action is only if the stab trim wheel keeps on "coasting" - that is turning slowly under aerodynamic forces if power is removed. Try stopping an electrical runaway stab trim with bare hands and you risk injury.

It seems to me that regardless of the two accidents involving EMCAS the Boeing QRH procedure to counter a runaway stabilizer trim has not changed significantly from back in the days where the 737-200 began operating.
The key is prompt recognition that "something" has caused an un-commanded stab trim operation - whether up or down. The pilot action should be to immediately disconnect electrical power to the stab trim before it moves any further. That is by selecting both stab trim cutout switches to Cutout. Providing there is no delay the stab trim will normally be stopped from moving any further. In turn, operation of the manual trim should be controllable without significant effort.

It is only if the stab trim is allowed to run to its limits (in either direction) because of slow pilot reaction, will the situation become serious. Even then, use of the roller coaster method to relieve aerodynamic forces, is an emergency fall-back technique and assumes the crew are aware of this technique as an essential part of their training. Clearly excessive airspeed needs to be avoided if the roller coaster method is to be successful in regaining manual control of the stabilizer trim position.
The point being made is IMHO Boeing are entitled to assume the crew are technically competent when faced with the instructions contained in the QRH. We know from reading accident reports this is not always true..
It is this writers experience that very few pilots are aware, for example, that rolling the aircraft towards the nearest horizon may be necessary in order to get the nose to drop in event of an extreme nose up wings level unusual attitude. This should be part of training for the PPL and is elementary airmanship. Certainly it should be taught during unusual attitude recoveries as part of instrument flying. It is even described in airline FCTM of some types - including the Boeing series.
The ever increasing accent towards more automation has left basic airmanship in its wake.

[Uplinker]

How can it be that the THS trim motor can be stalled by elevator input? Wouldn’t you make the motor strong enough to overcome the aerodynamic forces caused by full opposite elevators?

We were not told about this, or the yo-yo recovery, and I did a 737 type rating last year.

[HowardB]

There is an article in Aviation Week dated 10/5 titled "Ethiopian MAX Crash Simulator Scenario Stuns Pilots" which talks about testing this situation in a simulator and the problems encountered.

[safetypee]

HowardB,

Link -
https://aviationweek.com/commercial-aviation/ethiopian-max-crash-simulator-scenario-stuns-pilots?NL=AW-05&Issue=AW-05_20190513_AW-05_525&sfvc4enews=42&cl=article_1&utm_rid=CPEN1000002344541&utm_campaign=19626&utm_medium=email&elq2=32716dd5cd31477aa8b3175d28a7b54a

Highlights for the non registered (free).

‘A simulator session flown by a U.S.-based Boeing 737 MAX crew that mimicked a key portion of the Flight 302 accident sequence suggests that the crew faced a near-impossible task of getting their 737 MAX back under control, and underscores the importance of pilots understanding severe runaway trim recovery procedures.’

‘What the U.S. crew found -. Keeping the aircraft level required significant aft-column pressure by the captain, and aerodynamic forces prevented the first officer from moving the trim wheel a full turn.
They resorted to a little-known procedure to regain control.’ (YoYo Roller Coaster)

The excessive descent rates during the first two steps meant the crew got as low as 2,000 ft. during the recovery.

The article continues discussing aspects of the recent accidents, worthy of inclusion in previous (closed) threads; failing that - register with AvWk for info.

Concluding:-
‘The simulator session underscored the importance of reacting quickly to uncommanded stabilizer movements and avoiding a severe out-of-trim condition, one of the pilots involved said. “I donʼt think the situation would be survivable at 350 kt. and below 5,000 ft,” this pilot noted.’

“This is the sort of simulator experience airline crews need to gain an understanding of how runaway trim can make the aircraft very difficult to control, and how important it is to rehearse use of manual trim inputs,”

[DaveReidUK]

I believe this action is only if the stab trim wheel keeps on "coasting" - that is turning slowly under aerodynamic forces if power is removed.

My understanding is that the trim wheel moves if, and only if, the motor/screwjack/stab are moving. If those are moving, it won't be due to aerodynamic forces.

Try stopping an electrical runaway stab trim with bare hands and you risk injury.

Previous posts suggest that it's doable provided that you grab it firmly enough. Same principle as grabbing a spinning bicycle wheel - do it half-heartedly and your skin will suffer. Bear in mind that you're not actually trying to overcome the trim motor, as that won't attempt to drive the stab if resistance is detected at the cable drum.

[Takwis]

Foot works better.






I find it hard to "Grasp and Hold" with my foot.

Previous posts suggest that it's doable provided that you grab it firmly enough. Same principle as grabbing a spinning bicycle wheel - do it half-heartedly and your skin will suffer. Bear in mind that you're not actually trying to overcome the trim motor, as that won't attempt to drive the stab if resistance is detected at the cable drum.

To Grab (or "Grasp") implies encircling something. The edge of the wheel is less than a quarter inch thick. You can grasp it, but if your thumb gets a tiny bit under that edge, when the handle housing comes around (1/2 a second or so), you're going to really wish you hadn't. More like grabbing the tire and getting your thumb down between the spokes. Except with less give.