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Indonesian aircraft missing off Jakarta

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Indonesian aircraft missing off Jakarta

Old 24th Nov 2018, 08:00
  #1601 (permalink)  
 
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A couple of AOA and MCAS details to consider:

1. AOA vanes measure the angle of the airflow at the location of the sensor. The fuselage distorts the flow such that the vane angle does not match the free air AOA seen by the airplane. There is a conversion from degrees of vane to degrees of body AOA. Typically one degree of vane motion corresponds to about half a degree of body AOA change. I'm not sure if the AOA data we have seen here in PPRuNe is in vane degrees or body degrees.

2. When reviewing flight data and trying to determine the status of MCAS and the cutout switches it is important to realize the following:
- MCAS has an authority limit of at most 2.5 degrees of stabilizer (less as speed/Mach increase).
- MCAS will not move the stabilizer an increment more than its authority unless it has been reset by either pilot manual stabilizer trim command or engagement followed by disengagement of the autopilot.
- MCAS would not be reset as a result of mechanical manual stabilizer trim (i.e., pilot physically turning the trim wheel by hand)

Some of the time periods where seeing no automatic nose down trim might lead one to think that the cutout switches have been activated, it could actually be the case that MCAS has already put in its allowed stabilizer motion increment and has not be reset via manual trim and therefore will not move the stabilizer any more regardless of how long the time period is.

Another possibility is that during some of the time periods where there are no automatic trim increments the crew may have been able to engage the autopilot in which case MCAS would be disabled. There would, however, likely be some automatic stabilizer trim with autopilot engaged as fuel burn would require.

With the cutout switches activated, the only means of trimming is via the mechanical trim wheel. It is likely that pilot activated trim via the wheel would not be at the same rate as electric motor trim (and might not be at a continuous, constant rate). Knowing that it might be possible to figure out which stabilizer motions were likely not via electric motor and thus must have been via pilot use of the mechanical trim wheel.
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Old 24th Nov 2018, 09:43
  #1602 (permalink)  
 
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Having followed this thread from the start, there are a couple of questions which have arisen in my mind.

1. Earlier in this thread it was stated that Boeing introduced the MCAS system because, without it, the B737 MAX would not meet the requirements for passenger aircraft certification in respect of longitudinal stability. If this is true, should a fully functional MCAS be a specified item in the MEL as a mandatory requirement for release of the aircraft to revenue flights? Surely, if it was mentioned in the MEL both flight crew and engineering staff at Lion Air would have asked "What's this MCAS all about?". Is it normal for a system deemed necessary for certification of an aircraft not to be referenced in the MEL?

2. The maintenance record for the previous flight states that the crew thought that the STS was working 'in reverse', which we now know was due to MCAS (a system whose existence they were unaware of at that time). The crew of the flight that crashed would have read the maintenance report before departure and also were unaware of the existence of MCAS. Is it likely they would have been predisposed/alerted to the idea that the STS might 'go rogue' and operate in reverse? After repeated manual NU trim inputs, which were negated by MCAS ND trim, do the shorter manual NU trim inputs just before the aircraft dived towards the sea suggest that the crew were beginning to think their manual NU trim inputs were the cause of their problems?
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Old 24th Nov 2018, 10:31
  #1603 (permalink)  
 
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Originally Posted by FCeng84 View Post
Another possibility is that during some of the time periods where there are no automatic trim increments the crew may have been able to engage the autopilot in which case MCAS would be disabled.
Could they have engaged the AP with the CAPT's stick shaker active?
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Old 24th Nov 2018, 10:57
  #1604 (permalink)  
 
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FCEng84: the MCAS descriptions I've seen state that 2.5 degrees is the maximum incremental change that will be commanded, with a 0.27 deg/sec movement. After each such increment, if the high-AoA condition persists, another incremental change is commanded. I didn't see any statement that manual trim "resets" the mcas. On the FDR plots, we see three trim elements: manual electric command, elevator trim position, and automatic command, which includes STS, MCAS and A/P. At some point on the previous flight, we see both command channels cease correlating to trim movement. That decoupling, rather than any lack of movement, was what I was pointing to.
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Old 24th Nov 2018, 11:21
  #1605 (permalink)  
 
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Apologies if this has been mentioned before (I can't find a direct post...). On an 737NG Ground School Slide Show I see the following:

45. Control column actuated stabilizer trim cut out switches stop operation of the main electric and autopilot trim when the control column movement opposes trim direction. i.e. If the pilot is pulling nose up nose down trim is inhibited. When the STAB TRIM override switch is positioned to OVERRIDE, electric trim can be used regardless of control column position. The control column activated cut out switches are bypassed. [my emphasis]

Anyone know whether the Max is more-or-less the same? I.e., you can have tripped the cut-out switches, but their cut-out effect can be overridden?
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Old 24th Nov 2018, 11:28
  #1606 (permalink)  
 
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Originally Posted by Avionista View Post
1. Earlier in this thread it was stated that Boeing introduced the MCAS system because, without it, the B737 MAX would not meet the requirements for passenger aircraft certification in respect of longitudinal stability. If this is true, should a fully functional MCAS be a specified item in the MEL as a mandatory requirement for release of the aircraft to revenue flights? Surely, if it was mentioned in the MEL both flight crew and engineering staff at Lion Air would have asked "What's this MCAS all about?". Is it normal for a system deemed necessary for certification of an aircraft not to be referenced in the MEL?
The MEL only lists items that may be inoperative before dispatch, provided certain limitations and procedures are observed. It does not list items that must be serviceable. The MEL or DDG typically includes a statement that says (in bold capital letters, as below):

"ALL ITEMS WHICH ARE RELATED TO THE AIRWORTHINESS OF THE AIRCRAFT AND ARE NOT INCLUDED ON THE LIST ARE AUTOMATICALLY REQUIRED TO BE OPERATIVE".
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Old 24th Nov 2018, 12:01
  #1607 (permalink)  
 
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Originally Posted by AGBagb View Post
Apologies if this has been mentioned before (I can't find a direct post...). On an 737NG Ground School Slide Show I see the following:

When the STAB TRIM override switch is positioned to OVERRIDE, electric trim can be used regardless of control column position. The control column activated cut out switches are bypassed. [my emphasis]

Anyone know whether the Max is more-or-less the same? I.e., you can have tripped the cut-out switches, but their cut-out effect can be overridden?
Indeed it is not quite desirable to have the aircraft trim against the pilot input. But isn't the override an emergency function to be able to fly the aircraft using the trim buttons when having a jammed elevator? In that situation the last thing you'd want is MCAS taking over the 'elevator'. So hopefully MCAS ignores STAB TRIM OVERRIDE.

Last edited by StuntPilot; 24th Nov 2018 at 12:27.
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Old 24th Nov 2018, 12:02
  #1608 (permalink)  
 
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Some random observations.

On the previous flight they disconnected the stab trim system, which stopped the MCAS inputs. So it is possible to use the trim cutout switches on this system. They then reinstated the trim system, to test it, got more MCAS trim inputs, and cut it out again. Sensible idea. But still had to operate the whole flight with the stick shaker going.

The next crew were more unsure about the problem - as if they had not been briefed. They appear to have focused on the stick shaker, by putting the flaps back down. Obviously wondering if the stick shaker was correct, and they were close to the stall. And the stick shaker did stop for a while, which may add to the confusion. “Are the stall warnings correct?”, they are wondering...

Then they get all these MCAS trim forward commands, which would be a bit of a mystery. And they reverse these MCAS inputs every time, until the end. Questions: Why not use the cuttout switches? Did this not seem like a real trim runnaway, because the trim started and stopped? And why at the end stop countering the MCAS trim inputs? Only they will know.

People here saying full forward trim can be countered with elevator? Sure about that? Even if this flight had a far-forward CofG? Only time I have experienced runnaway trim in the sim, the controls were getting uncontrollable as the cuttouts were operated - and we were nowhere near max forward. (Like this incident, we were doing 250 kt.)

So we come back to the Boeing design philosophy. Was there one? Or was the MCAS stitched together without any oversight. For instance:

Do Boeing really design things, so that a single sensor failure can lead to multiple problems? Was there really no way to have an AofA oversight system, that could detect a malfunctioning AofA indicator? Could they not have a comparitor system, that detected a sensor conflict and disabled MCAS? Single sensor input and actioning was ok, when it merely operated a stick-shaker - pilots can deal with that. But now the single malfunctioning sensor is operating the flight controls....! Did anyone think of the consequenses?

Do Boeing really design a stall-trim system that can operate to full-forward trim? Really? When do we EVER give full forward trim? Should not the system have operated until a certain (safe) forward limit was reached, and stopped there? Why allow MCAS so much authority over the controls?

Did Boeing ever try the system at 320 kts, instead of at the stall? MCAS may seem like a good idea, when approaching the stall, but just what might happen at 320 kts? Did they ever test that scenario? And this is not a case of 20/20 hindsight. This is what design testing is all about - testing out all the scenarios and all the possible (and seemingly impoossible) failure modes. Was this system rushed through, so the Max could compete with the A320..?

And why did Boeing think that a stall-trim system was a good idea at all? Traditionally, stall-pushers operate on the elevators, and for good reason. A traditional stick-pusher will push, but when it relents you are still in trim, and stick forces return to normal immediately. Conversely, a stall-trimmer will push you forwards, and then leave you way out of trim in the dive - making the recovery difficult. And remember, MCAS can trim you full forwards. Have you ever tried recovering from a stall, with the trim on the forward stop...??? Me neither...!!

So where is the Boeing QRH advice, for stall recovery with the trim fully forwards? How many pilots performed that exercise in the sim, on conversion to the Max? Answer - nil. And what would happen? Anyone tried it...? I’ll give it a go next time.

And finally, how much knowledge did the next pilot have, after the incident the previous day?? Aviation authorities have reduced preflights from 60 min to 45 min, without any thought to safety. And then the same aviation authoriites placed security in the way, so instead of the crewbus taking you to the aircraft in 3 minutes, it now takes 20 minutes. So preflight time at the aircraft can be reduced from 30 minutes to 10 minutes (plus 10 minutes starting).

So what briefing did the next pilot get? There would have been no Deferred Defect (HIL), as the problem the previous day was cleared - even though we all know that a cleared defect is not a cleared defect. Swapping a box has never cleared a technical problem. ‘Tested Found Serviceable’ has never cleared a technical problem. So the Tech Log is a book of half-truths and lies. Whatever happened to the days gone by, where such problems were left open, and marked ‘please report further’?

So a cursory glance at the tech log will show no problems. Did they take off, with no idea of the problems on the prievious flight? This is the problem with the new Politically Correct tech logs - they look wonderful and clean, but may conceal a host of problems. Intermittent defects are cleared, as if they have been rectified, but eveyone knows that the problem is still lurking. But you are not allowed to alert the next crew by having an open Deferred Defect - oh, no, that would be far too logical and safe - and would attract the attention of the greedy lawyers, who are just waiting to pounce on a single incorrect pen-stroke.....

Silver.

Edited Post Script. Regarding stalls, the QRH says, quote:

“Excessive ​use of pitch trim may aggravate​ the condition, or​ may​ ​result in loss of control or​ in high structural loads.​“

So why would MCAS want to put in full forward trim, if the QRH says this can lead to a loss of control....?

.


Last edited by silverstrata; 24th Nov 2018 at 12:56.
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Old 24th Nov 2018, 12:08
  #1609 (permalink)  
 
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Originally Posted by AGBagb View Post
Anyone know whether the Max is more-or-less the same? I.e., you can have tripped the cut-out switches, but their cut-out effect can be overridden?
The Stabilizer Trim Override Switch and the Stab Trim PRI Cutout and Stab Trim B/U Cutout Switches accomplish two different things. The Override Switch relates to the control column actuated stabilizer trim cutout switches; these aren't manually selected, they are activated by movement of the control column such that when that movement opposes the automatic trim direction it stops the operation of the trim. It's called breakaway. When the STAB TRIM override switch is positioned to OVERRIDE, the control column actuated stabilizer trim cutout switches are disabled and electric trim can be used regardless of control column position. Just by the bye, we now know that breakaway won't interrupt MCAS (which makes sense when you know about the purpose of MCAS but which would be initially confusing if you weren't aware of MCAS).

The Stab Trim PRI Cutout and Stab Trim B/U Cutout Switches are provided to allow the autopilot and main electric trim inputs to be disconnected from the stabilizer trim motor. They essentially revert the stabiliser trim to only manual inputs via the trim wheels.
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Old 24th Nov 2018, 12:26
  #1610 (permalink)  
 
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Originally Posted by MickG0105 View Post

Just by the bye, we now know that breakaway won't interrupt MCAS (which makes sense when you know about the purpose of MCAS but which would be initially confusing if you weren't aware of MCAS).
Clearly, MCAS ignores the force switches (otherwise this accident would not have happened) but in the 'override' case (jammed elevator) I think MCAS should be disabled?
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Old 24th Nov 2018, 14:54
  #1611 (permalink)  
 
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Originally Posted by AGBagb View Post
Apologies if this has been mentioned before (I can't find a direct post...). On an 737NG Ground School Slide Show I see the following:

45. Control column actuated stabilizer trim cut out switches stop operation of the main electric and autopilot trim when the control column movement opposes trim direction. i.e. If the pilot is pulling nose up nose down trim is inhibited. When the STAB TRIM override switch is positioned to OVERRIDE, electric trim can be used regardless of control column position. The control column activated cut out switches are bypassed. [my emphasis]

Anyone know whether the Max is more-or-less the same? I.e., you can have tripped the cut-out switches, but their cut-out effect can be overridden?
Not the same switch. Stab trim CUTOUT shuts electric trim off. Stab trim OVERRIDE lets you trim in the opposite direction of stick movement.
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Old 24th Nov 2018, 16:07
  #1612 (permalink)  
 
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Was there really no way to have an AofA oversight system, that could detect a malfunctioning AofA indicator? Could they not have a comparitor system, that detected a sensor conflict and disabled MCAS? Single sensor input and actioning was ok, when it merely operated a stick-shaker - pilots can deal with that. But now the single malfunctioning sensor is operating the flight controls....! Did anyone think of the consequenses?

Do Boeing really design a stall-trim system that can operate to full-forward trim? Really? When do we EVER give full forward trim? Should not the system have operated until a certain (safe) forward limit was reached, and stopped there? Why allow MCAS so much authority over the controls?
Silver

Thats about it in a nutshell.That will be in the report 100%.Leaving only a)how/why the crew didnt have enough cognitive function to disengage those trim switches and b)strong focus on Lionairs maincontrol procedures and competency.Answer to (a) can only be startle factor caused by the false alarms(stick shaker).If the Birgenair crew had been able to quickly cut out the two opposing alarms(shaker vs overspeed) they would not have crashed.No doubt in my mind.Answer to (b) we've visited many times before in accident investigation....
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Old 24th Nov 2018, 16:27
  #1613 (permalink)  
 
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Re the EAD, paras g and h; ‘AOA DISAGREE’ alert (if option is fitted), presumably referring to the optional modification to display AoA on EFIS (not applicable to Lion).
When the alert shown, as would be with an abnormal AoA vane input, is the EFIS display of AoA removed, or does a potentially misleading indication remain in view without any third system to resolve the disparity?

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Old 24th Nov 2018, 16:53
  #1614 (permalink)  
 
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As I posted previously I wonder if something broke or forces were beyond their design limit. (Ability of the tailplane to produce sufficient down force at that speed & setting)
However, would the aircraft have been controllable if the crew had carried out the QRH (including grabbing the trim wheel) to the letter. Methinks that is where the lawsuits will end up should no other cause be found. The final report will be an education I am sure.
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Old 24th Nov 2018, 16:56
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Having read through the posts and some of the observations, these are my (an old grease monkey) answers to the basic questions on Boeing aircraft and its maintenance procedures, that seems to have confused many in this thread.
Screw-jack: From the early aircraft (B707 onward and may be earlier) all screw jacks have a brake, and it is called 'No-back brake'. As the term implies, it prevents the air-load pushing the control surface in the opposite (up) direction. This brake normally has two discs, one stationary and other rotating that are kept in contact by springs and axial force of air load acting through screw-jack. The control surface is moved against this brake force. In flaps this is the commonly used brake. When flaps are lowered, the drive motor has to move it against the air-load on flap. So there is a ratchet mechanism that slips and brake is not applied. In the opposite direction the brake is active and the axial force by the air-load on screw-jack makes the two discs firmly rubbing against each other and the braking force increases. Why this brake? You have consider control (Drive tube) dis-connection. In which case the flap can move by air-load in opposite direction without a brake. What applies to flaps are also there in most stab screw-jacks with some variation.
MCAS: This aircraft model requires this for it's certification by FAA, as it has reduced pitch stability. So there is no way this can be on MEL. If something is not in MEL it automatically implies that it cannot un-serviceable. There are some MEL items that have a statement 'Must be operative'. But some aircraft MELs do not mention the mandatory items this way.
Autopilot: Some one mentioned, if the autopilot was engaged by pilot, this crash may not have happened. See Boeing's FCOM Bulletin after this crash. If there is a AoA or IAS disagreement, auto pilot disengages and cannot be re-engaged. So that is out of question.
AoA or IAS disagree, which one?: It is not like in old gen aircraft (B737 classic)where the pitot probe and static port are directly connected to the airspeed instruments. At high angle of attack of the wing the airflow near the pitot gets distorted and this affects the SPEED sensing. To correct these errors, the pitot sensed speed is modified or corrected as a function of AoA in the ADIRU. Hence if you have a very large error in the AoA sensed, there will be an error in the IAS for the same side. So this can result in both indications showing a disagree. That is only my inference from the way the system is designed to work.
Why this crash: All sort of theories are advanced in this forum. But my take is that it should not crashed at all, as it should not have left the ground in the first place! The airspeed defect has repeated 4 times, and it keeps flying? I have posted this before. Any airline worth it's salt would not let go of these defects without confirming rectification. In the airlines I have worked, after the second report of such a significant defect, the crew will refuse to accept without positive identification of defect. This is a LCC - the pressures are different. As an engineer, I would not have dared to release the aircraft without :
1. Using an air-data tester and taking the aircraft through a full range of altitude and airspeed, just to confirm instruments operate correctly. 2. Call for a test flight if I cannot be sure to have rectified the defect.
From the day one when I was certifying, one thing that was drilled into me by my seniors, was there are ordinary defects and tricky ones. Never treat two defects in the same way. Any defect in the basic T instruments and flight controls, come in this critical list. This airline has a poor maintenance management (sorry to say that). I cannot see how an aircraft that had a stick-shaker in operation through out its flight (I am assuming from what is posted here), can be released for the next flight without confirming the defect is rectified by just changing a AoA sensor.
ONCE AGAIN WE ARE SPECULATING IN THIS THREAD -THOUGH MOST OF THE POSTS ARE FROM VERY EXPERIENCED AVIATORS.
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Old 24th Nov 2018, 17:17
  #1616 (permalink)  
 
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Originally Posted by PEI_3721 View Post
When the alert shown, as would be with an abnormal AoA vane input, is the EFIS display of AoA removed, or does a potentially misleading indication remain in view without any third system to resolve the disparity?
The consensus appears to be Lion Air did not have the (optional) AoA display on the PFD.

Originally Posted by Hi_Tech View Post
I cannot see how an aircraft that had a stick-shaker in operation through out its flight (I am assuming from what is posted here), can be released for the next flight without confirming the defect is rectified by just changing a AoA sensor.
The sequence of events was the other way round - the FDR trace that was released for the flight (Denpasar-Jakarta) during which the stick-shaker operated continuously took place after the AoA sensor had been replaced at DPS.
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Old 24th Nov 2018, 17:37
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Salute!

I wish to "revise and extend" my comment re: machanical failure of some type.

1) I did not count the pilot application of trim, so the real count of trim commands to the jackscrew motor is twice my estimate of 25 within 6 minutes,or less.

Did Boeing, in their wildest dreams ever test the mechanism by reversing it from full MCAS authority to manual reposiiton and then back to full MCAS authority limits 25 times in 6 minutes? Looks like the 5 second delay is clearly there after each up trim command from the pilot, then whammo! Nose down and repeat the drill. You know, "STS opposes me and I usually apply opposite trim cmd, but I do not realize this new system is fighting me and my stick shaker is telling me I am stalling but I am not."
2) From what I understand, the mechanical connection to the stab is some sort of drum requiring "x" rotations by the wheel to move the stab.
Is the "drum" connected to the jackscrew?

3) It appears that there is overlap of the STS envelope with MCAS. i.e. 10 seconds after takeoff, speed above 100 knots, 5 seconds to pause after pilot trim command, A/P off.
We see no UP trim cmds after flaps are up, but several before then. So I assume those were STS trying to provide the artifical speed stability mentioned elsewhere.
With increasing speed, STS should be commanding up trim, right? And MCAS is active with flaps up, so it sees high AoA and puts in down trim. But no up trim in the first upset at T=flaps up. Just down trim, so looks like the MCAS rules due to the high AoA, as STS should be commanding up trim as speed increases in the dive!!! I see no AoA criteria for the STS, so MCAS must rule and when/where is not in the FCOM, best I can tell.

4) If the crew is holding or moving the manual trim wheel and the "system" keeps applying trim, as the traces show, then can the trim motor overheat and jam fighting the manual command?

I'm scared, and do not unnerstan why giving "hints" to the crew that trim is required cannot be done with the exising "feel" inplementation versus moving the largest, most effective pitch control surface without notice. And then not making the new system well-known and maybe demonstrated to the crews before their first flight with the new doofer. GASP!

Gums exclaims...

Last edited by gums; 24th Nov 2018 at 18:53. Reason: added comment;typo
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Old 24th Nov 2018, 18:47
  #1618 (permalink)  
 
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737 Max Stab

Is the 737 stab powered by hydraulics (in mechanical and/or electrical control ). I just dont understand the stab cut out switches.. in every other post 70's Boeing they isolate the hydraulics so irrellavent of any input the stab doesn;t move. On the 737 does the stab have manual reversion?
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Old 24th Nov 2018, 18:50
  #1619 (permalink)  
 
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@gums
4) If the crew is holding or moving the manual trim wheel and the "system" keeps applying trim, as the traces show, then can the trim motor overheat and jam fighting the manual command?
The traces for manual trim only show the manual inputs for the ELECTRIC trim via the switches on the yoke.

If you move the trim wheel manually it is only shown via the Trim position. (You can see that on the FDR readout from the previous flight where they apparently used the CUTOUT switches)

So manual trim basically has two very distinct meanings. Maybe the nomenclature is a little awkward. One would probably call the trim inputs on the yoke the electric trim. But as they are manual inputs into the electric trim system that's how the FDR trace is labeled.

As far as i understand moving the trim wheel moves a drum with steel cable on it. That steel cable runs all the way back to the tail and turns the jackscrew.
So when the electric trim is used the trim wheel is backdriven via the steel cable.
The electric trim motor is located at the jackscrew.


I'm scared, and do not unnerstan why giving "hints" to the crew that trim is required cannot be done with the exising "feel" inplementation versus moving the largest, most effective pitch control surface without notice. And then not making the new system well-known and maybe demonstrated to the crews before their first flight with the new doofer. GASP!
If an electric system was fighting for trim commands how long would you have counteracted it with switches on your yoke before using the CUTOUT switches?
My bet would be you would have gotten annoyed after less than 30 seconds and asked the copilot to trim for you with the wheel.


As someone probably also wrote when the stick shaker started going off they extended the flaps to 5 while the speed was around 300 knots.
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Old 24th Nov 2018, 19:03
  #1620 (permalink)  
 
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Salute wiede!

Thanks for the explanation, as I was sure someone else had mentioned the wheel-to- cable- to- drum-to-jackscrew implementation versus a pulley-to-stab connection.

Also, I understand that the traces only show the electrical commands, and suspected as such. Thanks for confirming.

Looking at the stab position carefully, there might be manual input just as the two control forces diverge before the dive. Maybe pilot using only left arm and other to turn the wheel. I am trying to be gentle, but I am looking at Boeing and the system design more than anything.

ADDED:
From wiede....
If an electric system was fighting for trim commands how long would you have counteracted it with switches on your yoke before using the CUTOUT switches?
My bet would be you would have gotten annoyed after less than 30 seconds and asked the copilot to trim for you with the wheel.
As another pilot said over on Tech Log, and with years flying the beast and not likeing STS, and not knowing about MCAS on my plane becasue they didn't tell me, I would have trimmed with the switches thinking it was the STS working in reverse. Does that sound like the gripe from the previous flight?

And if i finally applied all the procedures and landed, then found out about MCAS I would have been pissed!

Gums...

Last edited by gums; 24th Nov 2018 at 19:09. Reason: added comment
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