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Indonesian aircraft missing off Jakarta

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Indonesian aircraft missing off Jakarta

Old 14th Nov 2018, 17:14
  #1201 (permalink)  
 
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Originally Posted by silverstrata View Post
One imagines that the stick-shaker might also operate (there is no stall alert-tone or warning-lamp). Depends how close to the stall this stall-trimmer system starts to operate. Other than that, I presume the new system is like the STS, and all you get is the clunking and clanking of the trim-wheel.
Apart from the discussion if a ND trim automatism on a detected impeding stall (false or not) is a good choice, I think that any autonomous, severe intervention into the flight controls should always be clearly announced to the pilots. The stick shaker cannot replace that as it is only reporting on a condition of the AC, not on actions taken by it's (autonomous) systems.

Just my 2 cents as a systems engineer in other fields than aviation.
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Old 14th Nov 2018, 17:16
  #1202 (permalink)  
 
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Originally Posted by silverstrata View Post


That seems a lot of trim to me. Try flying at 5,000 and 250 kt, and then mis-trimming forward by 2.5 degrees. You will be struggling.

Strikes me that Boeing designed this stall-trim system assuming that the aircraft would be slow and approaching the stall, when 2.5 degrees of trim would be acceptable. But at 250 kt in a normal climb, 2.5 degrees of trim becomes a liability. And if it gives you another 2.5 degrees of trim, you are uncontrollable.

Sounds like Boeing made this system on the cheap. The reason aircraft have stick pushers, is because the push is instantaneous, on the stick, and its withdrawal is instantaneous too - so you can pull out of the dive. But if you ‘push’ via a trim input, you cannot easily undo that trim - and so you cannot easily recover from the dive. And as a basic flying principle, we always fly with the stick, and trim out the stick pressure. You never fly with the trimmer, which is what this system is trying to do.

Furthermore, an erroneous stall-trim input at 250 kt (due faulty AoA indication) will give huge stick forces that the system did not intend - and perhaps the designer and test pilots never considered. A faulty stick push can be easily overcome with stick pressure - it was designed that way, and haveing already got the T-shirt on that one, the aircraft is easily flyable. But if a system gives you 2.5 of trim, and then another 2.5 of trim, you are going down, because the stab-trim is much more powerful than the elevator. Did anyone ever test this...?

Silver

Absolutely agree
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Old 14th Nov 2018, 17:22
  #1203 (permalink)  
 
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Originally Posted by KenV View Post
This has been a basic design philosophy at both Boeing and Douglas since forever. And because that is the way it is designed, it is tested to ensure it behaves as designed and is part of the type certification. Indeed, when Boeing provides the flight training, that training includes flying the aircraft (in the sim) with a hardover trim condition. No it's not easy and may require both pilots. But yes it is very much possible.

Remember the 737 flight control system was designed in the 60s and is fully mechanical. The 737 has a measure of SCAS (Stabililty Control Augmentation System) with STS and MCAS, but those systems act entirely thru the mechanical flight control system (more specifically, the trim portion of the mechanical flight control system). Further, the exact action (though not the reason why) of these systems is easily discernible by looking at the trim wheel, the action is over ridden/cancelled with the push of the trim switch on either pilot's yoke, can be cut out completely with a cutout switch, and if all else fails, can be overpowered by applying high stick forces. The Boeing and Douglas design philosophy is to always give the pilots the ultimate say and the ability to overpower whatever an electronic system wants to do. Indeed that is why these systems only operate through the trim system, because the base flight control system can overpower trim. Yes, this means that unlike many Airbus aircraft, the pilots of Boeing/Douglas aircraft can do stupid things and endanger the aircraft. It's not like the A320 where if you have the wrong flight control mode selected the pilots cannot simply overpower the system by pushing on the stick and/or the throttle to over ride envelope protection, and must instead deselect the current mode and select another. I'm not saying or even suggesting one design approach is "better" or "safer" than the other, I'm saying the approaches are fundamentally and significantly different.
Not the only one saying this already, but I think you are wrong on how easy it would be to overpower the stabilizer. There is a thread about a Ryanair 737 losing control in the Go Around in Eindhoven a few years ago, I think they managed to get to 42 degrees ANU at a speed of 80 KIAS while pushing all the way forward, before they started trimming AND and were able to regain control (I think since added to the checklist is something about using trim in unusual attitudes). Also the 737 in Rostov-on-Don was likely trimmed full AND and the pilots were unable to pull out at 300+ KIAS, so very different sides of the envelope, but similar problem.
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Old 14th Nov 2018, 17:25
  #1204 (permalink)  
 
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the STS and the MCAS appear to be different. This 737 hit the water going very fast. The STS should have been trimming back. The MCAS, if the AoA was erroneously high, would have been trimming forward. The MCAS is only fitted to the Max.
No I dont concur...the plane was going fast but HAL (ie FCC A) thought it was near to a stall so....it trimmed down...hence the previous commanders observation of " STS trimming the wrong way"..AMM NG data implies that providing the controlling FCC is being fed corrupt data(CAS/AoA etc) and the crew dont disable AP stab trim(or ALL trim as per Boeing AD),then it will happen in a NG as well.....Boeing admit that it is MCAS that caused the unwanted trim in 610...ergo,MCAS is STS with some differences(software tweaks) perhaps...it cant be too different due type certificate commonality concerns as discussed.Maybe the exact sequence of 610 cant happen in a NG we need more data.

Design?As stated you cant design perfection so you have to design the best available considering the fallibilities of pitot-static system....somebody pointed out that Airbus have 3 AoA vanes and use a voting system but this was proven fallible at Perpignan...Bernd said its better to have a false stall warning than no stall warning...agreed. Airbus PRIM on Qantas 72 provided an elevator nose down input...here with 610 Boeing provides a nose down trim.Silver has argued quite well that perhaps Airbus is right in this matter.Fokker put the circuit breakers for these tactile/aural warnings close to the pilots,others bury them on the panels behind the pilots.Whatever the nuances and differences are....no design is perfect and the crew remain the last and best(hopefully) line of defense.
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Old 14th Nov 2018, 17:33
  #1205 (permalink)  
 
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Found this
, what is the alarm at 2:57? And are the switches to turn off the autotrim still there in the newer 737s, and would they disable the MCAS?
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Old 14th Nov 2018, 17:48
  #1206 (permalink)  
 
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I'm only a lowly SLF and yet, from reading the thread, it's quite clear to me that you can turn off the autotrim on the MAX, which is what they do in response to the alarm @ 2:57! Then you can plainly see them trimming by hand.

Last edited by Organfreak; 14th Nov 2018 at 17:54. Reason: added info
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Old 14th Nov 2018, 17:52
  #1207 (permalink)  
 
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Originally posted by A0283: "As a designer/manufacturer, WHATEVER you run into, you don't want to 'break the Type Certificate'. Breaking the TC is not only very costly but will also cause substantial delay. "

Gysbrecht's post: I would think that virtually ANY change to an approved type design requires an amendment to the Type Certificate.

DaveReidUK's post:Yes, of course. But the A0283's point was that having to amend an existing TC is greatly preferable to the FAA/EASA saying that the manufacturer has changed so much that it's essentially a new type and has to be certificated from scratch.
In general terms there are many different types and levels of 'changes'. Some - listed in decreasing level of certification impact - would require a new type certificate (breaking the TC), some a type certificate amendment, others neither.

Generally, the lower its certification impact, the easier it is to get a change implemented. In quite a few cases certification impact is estimated by the manufacturer and can be enough reason to select a design option with less certification impact. You would of course go to EASA/FAA when you to be sure of an estimate or get a process going.

On the other hand. You could say that having a large number of changes that are blocked on account certification impact are part of the reason for manufacturers to move up to a new model or even new type.
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Old 14th Nov 2018, 18:07
  #1208 (permalink)  
 
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Something to try in the sim which was used on the 737-300 in the 1980’s. In the event of a full nose down stabiliser jam. Try getting the pnf to put his lap strap around the control column. ( pnf will have to move seat fwd & possibly extend the belt) Pnf then moves their seat back to take the “weight” from the column. PF should find the aircraft will fly virtually hands free. PF will find he has some fwd & aft movement of the control column to enable normal flight, but a little bit more adjustment can be made by the pnf by moving the belt either up or down the control column. Once all QRH drills etc have been done at about 500ft on finals PF prepares to take the weight & gets PNF to release the belt & strap themselves in for landing.
(It works & takes the effort out of the excercise some trainers dislike it for various reasons. Imho make use of anything that can help the situation)
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Old 14th Nov 2018, 18:10
  #1209 (permalink)  
 
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Originally Posted by Organfreak View Post
I'm only a lowly SLF and yet, from reading the thread, it's quite clear to me that you can turn off the autotrim on the MAX, which is what they do in response to the alarm @ 2:57! Then you can plainly see them trimming by hand.

I'm not sure it's quite as clear as you think it is. The guidance in the Emergency Airworthiness Directive states:

Disengage autopilot and control airplane pitch attitude with control column and main electric trim as required. If relaxing the column causes the trim to move, set stabilizer trim switches to CUTOUT. If runaway continues, hold the stabilizer trim wheel against rotation and trim the airplane manually.
To me, that last sentence says "You can turn it off, but it might still keep running anyway" I'm not a 737 systems expert by any means, but I don't know how else you can reasonably interpret that sentence.
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Old 14th Nov 2018, 18:10
  #1210 (permalink)  
 
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Originally Posted by LEOCh View Post
Now that the new MCAS is firmly implicated, I wonder why it was implicated on the MAX. One possible reason was that it was considered to be required by transport airworthiness certification via section 25. But looking through section 25 it is hard to find which exact requirement. The full regs can be easily googled (can't post as am probationary)

The previous requirement for STS is fairly easy to understand, as section 25.173 calls for "(c) The average gradient of the stable slope of the stick force versus speed curve may not be less than 1 pound for each 6 knots". Effectively the certification standard demands the aircraft emulate (some!) characteristics of a theoretical longitudinally stable aircraft which would have:
a) COG significantly ahead of the Neutral point
b) generally have the trimmed horiz stab flying at significantly lower AoA than the wing (probably to the point of downward lift at the tail)

However for modern air transports, which are desired to have optimal efficiency, it should be best to operate at near neutral longitudinal stability (i.e. just stable). In this configuration the tail is generating very little lift in either direction and hence very little drag. "Manual flight" is really "Simulated longitudinally stable manual flight necessary to conform to 25.173", which is confusing because as other posters have noted, this is really a less-automated mode instead of a manual mode. It's not so relevant to this thread anymore, but the STS seems more regulatory than useful really...changing the stab trim position via STS does not really create a more longitudinally stable aircraft and the pilot is likely to respond by removing the trim input anyway to avoid unwanted altitude excursions. This leaves him back with a slightly stable aircraft which would probably cause fatigue if pilots were expected to fly whole sectors in "manual mode", but of course they are not.

So why MCAS? It is not a speed stability system, it appears to be a beefed up stall prevention system. If the MAX COG has crept further back, it's stability margin has decreased but I don't see why an antistall stab trimmer becomes a good idea or a regulatory requirement. Looking through the stall/stall warning sections (25.201, 25.203, 25.207) there is a preference for aggressive pitch down at stall which may have something to do with it:The airplane is considered stalled when the behavior of the airplane gives the pilot a clear and distinctive indication of an acceptable nature that the airplane is stalled. Acceptable indications of a stall, occurring either individually or in combination, are—
(1) A nose-down pitch that cannot be readily arrested;
(2) Buffeting, of a magnitude and severity that is a strong and effective deterrent to further speed reduction; or
(3) The pitch control reaches the aft stop and no further increase in pitch attitude occurs when the control is held full aft for a short time before recovery is initiated. If anyone has a better understanding of how MCAS is dictated by section 25 airworthiness would be very interesting to hear.
I'm guessing but: So, level flight autopilot on, but speed decays because of whatever, auto pilot trims stab up while pilots don't notice? When approaching stall, because of stab trim position, pilots don't have enough down elevator control to offset current stabilizer trim so Boeing trims down for them? Is this one of the things that Boeing added the MCAS for?
I also heard some reference to steep turns. If you trim during steep turns (my mantra learned during pilot training was "trim, trim, trim", but who knows now with the current aircraft) but let airspeed decay would the elevator not be able to unload the aircraft and Boeing wanted to help again.
Or is there some other reason for adding the MCAS other than the unlikely situation of the pilots trimming while slowing to that angle of attack that would trigger the MCAS. I could see something like that happening at altitude though but it could happen on earlier models also. Did the stretch change things where the stab might not allow elevator control? Of course, the older 737s ( I think) and the 727 could be put in a rare situation where you couldn't even trim electrically if you had opposite elevator input until you relaxed the elevator input and air load.
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Old 14th Nov 2018, 18:15
  #1211 (permalink)  
 
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Rananim;

MCAS pitch control authority might be different than STS, especially if MCAS false activate at high speed using high trim rate.

Existing runaway trim procedure may or may not be considered enough to mitigate hazard from MCAS false activate failure
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Old 14th Nov 2018, 18:34
  #1212 (permalink)  
 
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Originally Posted by A Squared View Post
I'm not sure it's quite as clear as you think it is. The guidance in the Emergency Airworthiness Directive states: [snip]

To me, that last sentence says "You can turn it off, but it might still keep running anyway" I'm not a 737 systems expert by any means, but I don't know how else you can reasonably interpret that sentence.
Thanks, A Squared. I was responding mostly to what I saw in the video, which wasn't even a MAX. That'll teach me to be a know-nothing know-it-all! [maybe]
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Old 14th Nov 2018, 19:44
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A Squared.....”......To me, that last sentence says "You can turn it off, but it might still keep running anyway" I'm not a 737 systems expert by any means, but I don't know how else you can reasonably interpret that sentence...”

If it is installed to provide Stall recovery, would it be reasonable to allow it to be disabled? I’m thinking there is more to Boeing’s Logic re non disclosure...

Last edited by Concours77; 14th Nov 2018 at 19:56.
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Old 14th Nov 2018, 19:48
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Who guards the guardians? “Self Certification?”
I've been staying out of this one for the most part - I'm not a pilot, and my expertise is Propulsion, not Flight Controls.
But "Self Certification" is a serious miss-representation of the process. Boeing is "Delegated", not self certifying. The FAA is intimately involved in the a new Amended Type Certificate (ATC) (which is the case for the MAX). I was a DER or the delegated equivalent AR for nearly 30 years and was quite involved in the certification process.

A new ATC is a long, complicated process, with countless meetings between Boeing and the FAA. There would be a high level 'Tech Fam' where Boeing described all the changes planned for the MAX relative to the NG, then a series of specialist meetings where the certification basis is negotiated (this somewhat unique for an ATC (rather than a new TC) and gets into something called the Change Product Rule or CPR - CPR basically says any system that's changed as part of the new ATC has to step up to the latest regulations and be re-certified, but there are exceptions and these are what get negotiated). Then there would be another set of specialists meetings where the planned changes are discussed in detail, and the FAA would determine which items will be FAA retained and what will be delegated. In the Propulsion world, precious few changes got delegated - although many of the documenting deliverables were delegated (in my case, the FAA would routinely delegated the flight test reports, since the FAA would nearly always participate in the flight test and the 'report' was simply my documentation of what they'd already witnessed).
I don't know what happened with MCAS - I'd be somewhat surprised if MCAS was fully delegated but it's certainly possible - but to say the FAA was unaware or uninvolved in the certification of MCAS doesn't reflect the way Boeing and the FAA work.

Last edited by tdracer; 14th Nov 2018 at 23:39. Reason: I used STC (Supplemental) instead of ATC (Amended) - edited to correct. Thanks DR
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Old 14th Nov 2018, 19:55
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Originally Posted by Concours77 View Post
A Squared.....”......To me, that last sentence says "You can turn it off, but it might still keep running anyway" I'm not a 737 systems expert by any means, but I don't know how else you can reasonably interpret that sentence...”

If it is installed to provide Stall recovery, would it be reasonable to allow it to be disabled? I’m thinking there is more to Boeing’s Logic re non disclosure...
I don't know. I'm just saying that Boeing's words in the AD seems to indicate that moving the switches to "CUTOUT" is not a 100% guaranteed clean kill on stabilizer trimming. Why that is, or how it happens, I can only speculate. I have a hard time imagining designing a system that moves your flight control surfaces, and you can't turn off completely. Perhaps Boeing is alluding to the possibility that the contacts in a STAB TRIM switch become welded and don't open the circuit. I'm grasping at straws here.
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Old 14th Nov 2018, 19:58
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Originally Posted by tdracer View Post
I've been staying out of this one for the most part -
I was just thinking the other day ..."Hmmmm Haven't seen much of tdracer lately ... " Not that I blame you.
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Old 14th Nov 2018, 20:02
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Originally Posted by A Squared View Post
I don't know. I'm just saying that Boeing's words in the AD seems to indicate that moving the switches to "CUTOUT" is not a 100% guaranteed clean kill on stabilizer trimming. Why that is, or how it happens, I can only speculate. I have a hard time imagining designing a system that moves your flight control surfaces, and you can't turn off completely. Perhaps Boeing is alluding to the possibility that the contacts in a STAB TRIM switch become welded and don't open the circuit. I'm grasping at straws here.
Then it must be an informal, quasi optional Stall Recovery tool. Pilot’s discretion. (I kid).

rgds.
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Old 14th Nov 2018, 20:07
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I am just an electronics engineer (non-aero) _and_ I may well have missed some important information from this discussion.
But earlier, I think it was mentioned that either one or the other of two possible sources of AoA information is selected "automatically" and remains throughout the flight, then switch to the other one for the next flight. Is this more or less correct? What is the rationale behind this? To me it appears as "we did not quite know what to do with two AoA sensors here". Presumably this also went through some review by the FAA ... who were happy with the explanation?
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Old 14th Nov 2018, 20:16
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Originally Posted by tdracer View Post
A new STC is a long, complicated process, with countless meetings between Boeing and the FAA. There would be a high level 'Tech Fam' where Boeing described all the changes planned for the MAX relative to the NG, then a series of specialist meetings where the certification basis is negotiated (this somewhat unique for an STC (rather than a new TC) and gets into something called the Change Product Rule or CPR - CPR basically says any system that's changed as part of the new STC has to step up to the latest regulations and be re-certified, but there are exceptions and these are what get negotiated).
The 737 MAX 8 was certificated (as the "737-8") in Revision 58 of the Boeing 737 Type Certificate (A16WE) on 8th March 2017.

While there are indeed a handful of STCs (16 in fact) that are applicable to the MAX 8, they do not form part of the TC.

Last edited by DaveReidUK; 15th Nov 2018 at 07:27. Reason: TC no corrected
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Old 14th Nov 2018, 20:27
  #1220 (permalink)  
 
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Question rom a "light" pilot.

I am having a hard time with the procedure for runaway trim in the matter under discussion.

The AD and such indicate that manual trim using the switches on the yoke will work, but then the system runs the trim again unless the whole trim sytem is turned off. So if the stab is fully trimmed and the crew tuens off the trim, then all they have is basic elevator, right?

Jez asking...
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