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

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

Old 11th Mar 2019, 11:52
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Boeing advice on "aerodynamically relieving airloads" using manual stabilizer trim

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

Last edited by Centaurus; 11th Mar 2019 at 12:03.
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Old 11th Mar 2019, 12:02
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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.
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Old 11th Mar 2019, 12:19
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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.
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Old 11th Mar 2019, 12:24
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Originally Posted by Centaurus View Post
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...
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Old 11th Mar 2019, 13:08
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Originally Posted by Chris Scott View Post
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.
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Old 11th Mar 2019, 13:52
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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.
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Old 11th Mar 2019, 14:47
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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...
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Old 11th Mar 2019, 14:49
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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.
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Old 12th Mar 2019, 00:02
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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.
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Old 12th Mar 2019, 06:56
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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.
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Old 12th Mar 2019, 09:10
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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.



Last edited by finncapt; 12th Mar 2019 at 10:14.
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Old 12th Mar 2019, 10:46
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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
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Old 12th Mar 2019, 10:47
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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?
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Old 12th Mar 2019, 16:07
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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.

Last edited by fireflybob; 13th Mar 2019 at 21:12.
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Old 12th Mar 2019, 16:51
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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.
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Old 13th Mar 2019, 17:59
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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.
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Old 13th Mar 2019, 19:43
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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.
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Old 13th Mar 2019, 22:14
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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
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Old 13th Mar 2019, 23:55
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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.
That's because noone wants to build a new design short haul airliner.

In terms of aerodynamic design the 737 is about 60 years old, the A320 about 40.

Designs that old are limited by the manufacturing technology at the time, hence the menagerie of systems and compromises on the 737.

In the cold light of day I suspect Boeing are going to wish they just went ahead and built a brand new aeroplane rather than the mess they have now made for themselves.

The dumb part of all this is the that the whole issue with MCAS can be aerodynamically designed out so it is not required in the first instance.

It could be a very sobering cross examination for some poor Boeing executive if this mess ends up in court.

Last edited by neville_nobody; 14th Mar 2019 at 00:31.
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Old 14th Mar 2019, 10:45
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Originally Posted by Bergerie1 View Post
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!!

Last edited by LeadSled; 15th Mar 2019 at 04:16. Reason: typo
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