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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 14th Mar 2019, 22:11
  #21 (permalink)  
 
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Originally Posted by LeadSled View Post
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...

Originally Posted by LeadSled View Post
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.
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Old 15th Mar 2019, 05:07
  #22 (permalink)  
 
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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!!

Last edited by LeadSled; 15th Mar 2019 at 05:19. Reason: minor text change.
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Old 18th Mar 2019, 17:40
  #23 (permalink)  
 
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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.
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Old 3rd Apr 2019, 14:08
  #24 (permalink)  
 
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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/def...20ISS%2010.pdf
Page 15


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Old 4th Apr 2019, 03:09
  #25 (permalink)  
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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.
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Old 5th Apr 2019, 05:50
  #26 (permalink)  
 
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Originally Posted by Loose rivets View Post

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
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Old 5th Apr 2019, 07:54
  #27 (permalink)  
 
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Originally Posted by Chris Scott View Post
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.

Originally Posted by finncapt View Post
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.
Originally Posted by PEI_3721 View Post
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.

Originally Posted by finncapt View Post
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 ) 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.

Originally Posted by LeadSled View Post
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.

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Old 5th Apr 2019, 21:44
  #28 (permalink)  
 
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Originally Posted by Takwis View Post
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/rumours-news/619272-ethiopian-airliner-down-africa-post10438165.html

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.
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Old 5th Apr 2019, 23:48
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Originally Posted by VinRouge View Post
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.
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Old 6th Apr 2019, 02:03
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Originally Posted by finncapt View Post
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!
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Old 6th Apr 2019, 04:07
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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.
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Old 7th Apr 2019, 09:24
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Question if I may. Does the manual trim wheel adjust the HS by means of cables?
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Old 7th Apr 2019, 09:54
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http theaircurrent.com/aviation-safety/vestigal-design-issue-clouds-737-max-crash-investigations/
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Old 7th Apr 2019, 10:10
  #34 (permalink)  
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Originally Posted by Good memories View Post
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), 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-...nes_Flight_831 . Link to the final report: https://reports.aviation-safety.net/...0%20CF-TJN.pdf

A few months later, and Eastern Airlines DC8 appeared to have suffered the same issue: https://en.wikipedia.org/wiki/Easter...nes_Flight_304

PJ2

Last edited by PJ2; 7th Apr 2019 at 10:31.
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Old 7th Apr 2019, 11:26
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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.
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Old 7th Apr 2019, 13:08
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Originally Posted by GordonR_Cape View Post
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?
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Old 7th Apr 2019, 13:31
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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)
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Old 7th Apr 2019, 13:56
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Originally Posted by Derfred View Post
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: Ethiopian airliner down in Africa
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Old 7th Apr 2019, 18:12
  #39 (permalink)  
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Originally Posted by Derfred View Post
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".

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Old 8th Apr 2019, 03:19
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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.
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