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Indonesian aircraft missing off Jakarta

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Indonesian aircraft missing off Jakarta

Old 4th Dec 2018, 16:57
  #1961 (permalink)  
 
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gums - I mostly agree. The one point I would add is that the gradual net nose down stabilizer motion prior to the last minute is likely that required to maintain pitch trim as speed increases. They were going rather fast already prior to the last minute. It looks like the crew was getting back to near zero column force with each cycle up until that point.
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Old 4th Dec 2018, 17:09
  #1962 (permalink)  
 
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Originally Posted by gums View Post
Salute!

So I would like to review the bidding, huh?
- stick shaker activates as pilot rotates and lifts off. I can see this happening with an abrupt stick input at the normal speed and maybe a bit more AoA than normal. Stick shaker continues. A caution indication or two is displayed, but altitude is insurance, and plane is climbing.
- MCAS, aka "Hal", sees the high AoA, flaps are up, and speed being way less than the mach washout limit, bumps the stab maybe to the max limit for one "bump" and stick forces go up.
- crew feels this and beeps trim with the stick switch.You can see the increased stick force correlating with the MCAS down trim cmd and resulting stab movement. Stab reverses, stick forces go down and the 5 second timer starts. Hal is waiting, and the high AoA is there.
- Hal again cmds stab to move another increment, and crew feels this, as shown on stick forces, then makes manual up trim inputs.
- The plot shows a gradual increase in down trim position of the stab because everytime crew beeps the trim Hal stops, but stab not trimmed all the way back up to original degrees. So the slope of stab position values is slightly negative.
- Hal waits 5 seconds and the process repeats two dozen times until "the event"
- At "the event", control input force from each pilot diverge, Hal continues to trim nose down and crew briefly beeps trim but mainly pulls back harder and harder.

Is that how most here see it?

Gums asks....
Yep, hard to argue it any other way.

It is still hard to understand why they didn't hit the trim cut out switches... The more I think about this, the more obvious that action seems. Trim is running, Control force is getting higher. Manual trim opposite way. Force goes away. Trim runs again. Force goes up. Manual trim. Force less. Rinse and repeat. Hindsight is 20/20, true enough.

Dang it, where is the darned CVR?

- GY
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Old 4th Dec 2018, 17:11
  #1963 (permalink)  
 
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Originally Posted by gums View Post
Salute!

So I would like to review the bidding, huh?
- stick shaker activates as pilot rotates and lifts off. I can see this happening with an abrupt stick input at the normal speed and maybe a bit more AoA than normal. Stick shaker continues. A caution indication or two is displayed, but altitude is insurance, and plane is climbing.
- MCAS, aka "Hal", sees the high AoA, flaps are up, and speed being way less than the mach washout limit, bumps the stab maybe to the max limit for one "bump" and stick forces go up.
- crew feels this and beeps trim with the stick switch.You can see the increased stick force correlating with the MCAS down trim cmd and resulting stab movement. Stab reverses, stick forces go down and the 5 second timer starts. Hal is waiting, and the high AoA is there.
- Hal again cmds stab to move another increment, and crew feels this, as shown on stick forces, then makes manual up trim inputs.
- The plot shows a gradual increase in down trim position of the stab because everytime crew beeps the trim Hal stops, but stab not trimmed all the way back up to original degrees. So the slope of stab position values is slightly negative.
- Hal waits 5 seconds and the process repeats two dozen times until "the event"
- At "the event", control input force from each pilot diverge, Hal continues to trim nose down and crew briefly beeps trim but mainly pulls back harder and harder.

Is that how most here see it?

Gums asks....
And the pointer to the problem is in this quote.
I think MCAS was a rudimentary attempt to prevent a stall developing.

It has been the pilots job to prevent a stall developing. Obviously we accept that pilots no longer are able to understand stalls and let HAL do the stuff. Problem is, pilots who do not understand stalls and their causes do not understand the raction of HAL either.
So what are we goeing to do next?
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Old 4th Dec 2018, 17:11
  #1964 (permalink)  
 
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Originally Posted by FCEng84
All of that on top of why the crew allowed the system to take the airplane away from trim so many times without recognizing they should disable automatic stabilizer control ....
Maybe, just maybe, they were unsure of the proper procedure or forgot where the switches are located?
Remember, we are talking about people under great stress, trying to work in a cacaphony of noise created by the stick shaker.
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Old 4th Dec 2018, 17:25
  #1965 (permalink)  
 
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Salute FC! and 'bird, and Garage and Retired!

Nice to have discussion with pilots, although many folks on these forums have a lot to contribute. That is, except that "there I was" experience we fliers have had, heh heh.

Yep. Without data records for all the entities, I cannot plot basic regression curves and look at record by record values, find correlations with crew inputs and aircraft responses. Hell. I would love to see attitudes - roll, pitch and yaw. Altitude and speed with greater resolution, and the beat goes on.

I can't see a significant speed increase after they tried to level off, and the minor ( my term) changes do not seem to be reflected in stab position commanded by Hal or the crew.

Nevertheless, I see a crew that did not turn off the stab trim like the previous flight, but valiantly fought to keep control with stick shaker going, some warning displays on something or other. As you point out, FC, they never trimmed back to the "original" stab position, but only enuf to reduce Hal's down trim cmd and then his 5 second waiting period before we start the drill over. So an ever increasing back stick on the whole until "the event", whatever the hell that was.

To be honest, I would rather have Hal keep putting in trim once I released the switch on the control yoke and not wait a few seconds. So I then might analyze the problem as "runaway trim" !! Voila!! But I didn't know about the 5 second delay or the down trim by Hal when not flying a profile that STS would be using to trim nose down. The basic comment, "STS working back wards" that the crew likely saw in the log will be discussed in the final report, I assure you.

Gums sends...

Last edited by gums; 4th Dec 2018 at 17:28. Reason: added accolades to contributors
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Old 4th Dec 2018, 17:32
  #1966 (permalink)  
 
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I sure hope neither I nor anyone I care deeply about ever rides in the back of an airplane being piloted by crew who don’t recall the stabilizer cutout seitches, where they are, or when to use them
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Old 4th Dec 2018, 17:46
  #1967 (permalink)  
 
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Originally Posted by FCeng84 View Post
I sure hope neither I nor anyone I care deeply about ever rides in the back of an airplane being piloted by crew who don’t recall the stabilizer cutout seitches, where they are, or when to use them
I do agree with you, however, in reality they can be used as rarely as once every 9 or 12 years or just during the type rating in the simulator. All ATA chapters rotate through a three years simulator cycle in EASAland (at least for many companies). However, the trim system is part of flight controls which has a lot more failure cases, so all the major failures have to be trained, and a trim runaway is not deemed a major failure necessarily, depending on preference of the companies training department and their approval from their local NAA. I bet next simulator cycle there will be a lot of trim runaway training in many companies, and then it will quiet down again.
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Old 4th Dec 2018, 17:49
  #1968 (permalink)  
 
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Originally Posted by LEOCh View Post
[...] FAA 25.203 describes (and forbids) what can happen to an aircraft loaded to static instability in the stall, the tail operating at higher AoA stalling before the wing causing pitch up.
No, not at all. This has nothing to do with the tailplane stalling under positive lift. If the tailplane were so heavily loaded (with an inconceivably far rearward CoG) that it might be close to stall under positive lift, the aircraft would be completely uncontrollable, and not be able to even start any certification tests. Or even take off, for that matter.

What is the most common reason for aircraft to pitch up during the stall is that swept wings tend to stall first at the wing tips, which are further aft than the wing roots, increasing pitch up moment. It is a common trait of many modern highly refined and efficient transonic wings.

Unless it was designed by Dr. William Christmas (look up "Christmas Bullet"), it is inconceivable that an aircraft would be built which, even under the most adverse weight and balance configurations and then some, would have a positively highly loaded tailplane. The only situation, in which a tailplane can stall is in severe icing or with an unbelievably forward CoG at low speeds and violent pitch-up maneuvers (early Cessna 177 Cardinal models suffered from that), and then it would stall under downward "lift", result in an aircraft nose-down moment.


Bernd
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Old 4th Dec 2018, 19:55
  #1969 (permalink)  
 
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Originally Posted by GarageYears View Post

It is still hard to understand why they didn't hit the trim cut out switches... The more I think about this, the more obvious that action seems.
- GY
Training. And the fact that you have had time to "think about this".

When I was on the Renton tractor (a few years ago now), the stab runaway event was trained by making the stab, er, run away. So it would, for some reason, go into a continuous clatter in one direction, which would do two things:
  • Make the aircraft either climb or descend in an unwanted fashion, along with a cacophony of continuous trim wheel noise. It was all very obvious.
  • The continuous noise (etc) would make you think: aha: stab trim runaway, and go for the switches.
This event is slightly different, and requires just 1 step of lateral thinking. Easily done here, but not so easy when all hell is breaking loose in the FD. There is no continuous clatter, so the training is not triggered. This is more "I just can't seem to get the thing in trim for pitch/power/etc, and I can't work out what's wrong". Being in a trim/thrust/pitch mess in a 737 is not uncommon (try a stall recovery using the old technique) and so the first thought is that you are once again in some kind of trim/thrust mess, rather than dealing with periodic unhelpful input from a genie in the trim system.

I have huge sympathy for these pilots, who I believe were confused by something they knew nothing about. The operator should not have dispatched, and the manufacturer should have disclosed the new system. I also have a suspicion that the sudden nose down is to do with the elevator breakout mechanism coming into play. But that's something for the specialists.
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Old 4th Dec 2018, 21:09
  #1970 (permalink)  
 
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Originally Posted by HundredPercentPlease View Post
Training. And the fact that you have had time to "think about this".

When I was on the Renton tractor (a few years ago now), the stab runaway event was trained by making the stab, er, run away. So it would, for some reason, go into a continuous clatter in one direction, which would do two things:
  • Make the aircraft either climb or descend in an unwanted fashion, along with a cacophony of continuous trim wheel noise. It was all very obvious.
  • The continuous noise (etc) would make you think: aha: stab trim runaway, and go for the switches.
This event is slightly different, and requires just 1 step of lateral thinking. Easily done here, but not so easy when all hell is breaking loose in the FD. There is no continuous clatter, so the training is not triggered. This is more "I just can't seem to get the thing in trim for pitch/power/etc, and I can't work out what's wrong". Being in a trim/thrust/pitch mess in a 737 is not uncommon (try a stall recovery using the old technique) and so the first thought is that you are once again in some kind of trim/thrust mess, rather than dealing with periodic unhelpful input from a genie in the trim system.

I have huge sympathy for these pilots, who I believe were confused by something they knew nothing about. The operator should not have dispatched, and the manufacturer should have disclosed the new system. I also have a suspicion that the sudden nose down is to do with the elevator breakout mechanism coming into play. But that's something for the specialists.
Precisely.
They were not aware of what ultimately appears to have been the problem. That the regulator allowed the manufacturer decline to tell them and that the airline appears to have wanted the aircraft back in service is scandalous. It is easy with hindsight to point at the stabiliser trim.
Confronted with a cacophony of noise mostly erroneous, aural cautions and warnings and an aircraft doing things that got increasingly harder to correct leaves one with a lot of empathy for the individuals concerned.
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Old 4th Dec 2018, 22:12
  #1971 (permalink)  
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So an ever increasing back stick on the whole until "the event", whatever the hell that was.
Well, it was something with a massive fuel flow followed by a captain's display of a high ground-speed peak.
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Old 4th Dec 2018, 22:19
  #1972 (permalink)  
 
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Loose Rivets - please review last couple of pages of comments and explain your latest. How does fuel flow have anything to do with this event???
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Old 4th Dec 2018, 22:39
  #1973 (permalink)  
 
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Originally Posted by Rated De View Post
Precisely.
They were not aware of what ultimately appears to have been the problem. That the regulator allowed the manufacturer decline to tell them and that the airline appears to have wanted the aircraft back in service is scandalous. It is easy with hindsight to point at the stabiliser trim.
Confronted with a cacophony of noise mostly erroneous, aural cautions and warnings and an aircraft doing things that got increasingly harder to correct leaves one with a lot of empathy for the individuals concerned.
Actually, we do not know if the Regulator allowed the manufacturer to tell them. In fact the Brazilian OER for the differences does reference the MCAS. Pages 18, 19 of this report reference MCAS: http://www.anac.gov.br/assuntos/seto...evoriginal.pdf.

Pages 20, 21 of the FAA FSB report does not reference MCAS:
FSIMS Document Viewer

There are also some differences in the answers to the FLT CHAR effect of the various items in the charts in the two reports. The Brazilian Report included the Boeing Chief Technical Pilot on the team.

So we don't know what info the Indonesian Regulators had in whatever report addressed the differences.

Brazilian Report


FAA Report
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Old 4th Dec 2018, 23:39
  #1974 (permalink)  
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Loose Rivets - please review last couple of pages of comments and explain your latest. How does fuel flow have anything to do with this event???
I've read every post, including of course the bulk of the ones modded out.

What I'm uncertain about, and in part, that was the point of posting, was the validity of the graph. However, the other parameters seem to tally.

As for what they have to do with the event is fairly straightforward. The taps were opened to a high fuel flow at the time of the vigorous climb in the last moments. I wondered just how great the effect on pitch may have been. I also wondered if such a huge change in power may have been to counter a ND event. (although I suspect it would have shown up as an actual descent.) If it was an intentional attempt to augment rotation, it would infer that there may have been another factor, and they were far more ahead of the game than we have given them credit for.

Indonesian aircraft missing off Jakarta
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Old 4th Dec 2018, 23:44
  #1975 (permalink)  
 
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Salute FC!

The previous comment had to do with the large increase in fuel flow and then the control wheel force disconnect and then the dive.

Looks like crew jammed throttles forward, for a few seconds, maybe to counter the nose down trim. Who knows? Then we saw the death dive.

Something happened at T - 20 or 30 seconds before the dive, but it was all set up by an undocumented feature of the plane"s flight control architecture and logic that every pilot should have known and seen in a sim.

From my grave, I would come back and haunt every engineer and company executive that let this new "feature' be implemented without letting this lowly line pillot know about the doofer and what it does and under what aero conditions and.....

Gums rants.....
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Old 5th Dec 2018, 00:57
  #1976 (permalink)  
 
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Originally Posted by Mad (Flt) Scientist View Post
Must be able to do so, otherwise the whole basis of the stall certification would have changed, as it would make the aircraft "envelope protected" if MCAS were able to prevent a stall. AFAIK 737MAX has no such radical change in the certification approach. It seems to be intended to enhance/amend/adjust stall characteristics, not to make a more fundamental design change.
That is actually what I was thinking. Had a series of questions planned to eventually get to what you have written.
From day one I concluded the MCAS was a response to a longitudinal static stability deficiency approaching the stall.
I have difficulty with the proposal that it does other than that concerning stall handling characteristics.
Also I am now thinking that the stabilising force applied by the elevator feel unit approaching the stall is just another longitudinal stability fix, given some smart Boeing name.
Could be that all the 737 series had low stability in this area and it was addressed by the elevator feel unit. Unfortunately I have no access to EFU tech description. Perhaps the EFU authority was insufficient for this one configuration on the MAX.

Last edited by zzuf; 5th Dec 2018 at 06:15.
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Old 5th Dec 2018, 03:53
  #1977 (permalink)  
 
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as can occur if COG is behind the neutral point)
That must never occur. You cannot get civil airplanes certified otherwise.
It must never occur within the flight envelope.
If you get close to stall, the neutral point normally shifts aft a lot, but under certain circumstances (e.g. highly swept wing) it may also move forward, leaving you with the CG behind the neutral point.
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Old 5th Dec 2018, 06:04
  #1978 (permalink)  
 
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Originally Posted by Mad (Flt) Scientist View Post
It seems to be intended to enhance/amend/adjust stall characteristics, not to make a more fundamental design change.
I simply cannot see this.
If the stab was required to assist the elevator for controllability on this flight phase I would be looking closely at all longitudinal controllability tests required by the FAR.
I can't see a situation where an aircraft had insufficient nose down elevator power but a bit of slow response stab trim is an acceptable fix.
Elevator on the forward stop during a stall, so just slowly wind in a bit of stab as fix. That would not come near being compliant with stall handling requirements.

Last edited by zzuf; 5th Dec 2018 at 06:17.
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Old 5th Dec 2018, 06:30
  #1979 (permalink)  
 
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Mad (Flt) Scientist is absolutely right when he says the MCAS is unable to prevent the stall. I have no idea exactly how or when the MCAS operates – I bow to FCeng84’s knowledge of the system and accept his description. Equally, I was never a 737 pilot, but I do know a little about stalling large transport aircraft, hence my attempt to try to tease out the differences/dividing line between stall protection devices and the enhancement of handling qualities before, during and after the stall.

MCAS does not prevent a stall, what it is intended to do is it redress the degraded longitudinal stability prior to and at the stall. It is all about stability and handling qualities. I first came across these issues on the VC10. Since it was a T-tail aircraft with the potential to become locked into a deep stall, it was fitted with a stick pusher which pushed after the natural aerodynamic buffet started and after the stick pusher activated but before a pre-determined AoA – i.e. before the ‘natural stall’ itself. Thus it prevented any tendency to pitch up into a deep stall and defined the stall point for certification purposes. You could say it was the ultimate anti-stall device.

On the other hand, the 747's stall qualities were immaculate except in the clean configuration when the stick force reduced to zero shortly before the stall and it would have gently progressed all the way into the stall on its own. Because of this, the UK CAA (D.P.Davies – see first sticky in Tech Log) required a stick nudger to activate at the same time as the shaker. It gave a small push (only about 15lbs) but just enough to increase the stick force and thus restore the required longitudinal stability.

The point I was trying to make in my previous posts is where do improvements to the longitudinal stability end and where do stall detection and protection systems start. Are subtle changes to the wing profile anti-stall devices or are they there to achieve certifiable handling qualities pre-stall? Is some kind of stability augmentation device an anti-stall device or is it there to provide a small force into the elevator circuit to impose positive stick free stability and thus remove the otherwise self-stalling tendency?

It seems therefore as zzuf has stated, that the MCAS was a response to a longitudinal static stability deficiency approaching the stall. And Boeing decided to do so by increasing the stick force by moving the stabiliser.

So, I leave you, the reader, to decide whether MCAS is an anti-stall device or, as safetypee says, a ‘crutch’ to address more specific nose up issues when approaching or at the stall (25.203) and, when turning, where the trim would be more nose up, and where there may be a greater pitching moment associated with the new engines on the 737 MAX.
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Old 5th Dec 2018, 10:44
  #1980 (permalink)  
 
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Originally Posted by silverstrata View Post
And if the high angle of attack persisted, the system would give another 2.5 degrees of forward trim.
Originally Posted by MickG0105 View Post
Well, that's what I thought but that is not the case according to FCeng84:
Originally Posted by FCeng84 View Post
MCAS will not command more than one increment of nose down trim without either having taken out the first one (via the same amount of nose up trim) or having sensed manual pilot electric pitch trim.
Hi Mick. I would suggest the difference in view is due to a difference in how you interpret the second last sentance, in the description of MCAS's operation. Here is the relevant text again:

The MCAS function becomes active when the airplane Angle of Attack exceeds a threshold based on airspeed and altitude. Stabilizer incremental commands are limited to 2.5 degrees and are provided at a rate of 0.27 degrees per second. The magnitude of the stabilizer input is lower at high Mach number and greater at low Mach numbers. The function is reset once angle of attack falls below the Angle of Attack threshold or if manual stabilizer commands are provided by the flight crew. If the original elevated AOA condition persists, the MCAS function commands another incremental stabilizer nose down command according to current aircraft Mach number at actuation.

It appears that FCeng84 is assuming the sentance describing how it is reset also applies to the second last sentance, as if it was written as one whole sentance like this: The function is reset once .... or if ... and if the original elevated AOA condition persists, the MCAS function commands another incremental stabilizer nose down command .."

In comparison, your interpreation see the two sentances as completely seperate, and as if the last sentance was before the second last sentance: If the original elevated AOA condition persists, the MCAS function commands another incremental stabilizer nose down command according to current aircraft Mach number at actuation. The function is reset once ...

Personaly, I think your view is quite valid, and is how I interpret the paragraph. If the AoA is high enough that MACS believes it should lower the nose, and it tries, but it does not succeed, would it really continue to do nothing, as the AoA would still be what it consders to be too high. But I think the way the statement is worded is quite ambiguous, and either interpretation has some merit. By the way, you seem pretty sharp to me..

Cheers
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