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Indonesian aircraft missing off Jakarta

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Indonesian aircraft missing off Jakarta

Old 14th Nov 2018, 10:26
  #1161 (permalink)  
 
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Originally Posted by gearlever View Post
- degrees
- units
- % MAC

Where does a 737 jock read "degrees" of the STAB ?
That is a 737 colloquialism, using ’degrees’ in the generic sense - eveyone says ‘degrees’ but we mean ‘units’. As you say, they are not real degrees - you would have to look on the trim-sheet to see the conversion between MAC and trim, as it is not linear.

And regards rhe STS, people here saying it is controlled by speed. It is a bit more complex than that, as the inputs to the STS are:

Stab position.
Thrust lever position.
Airspeed.
Vertical speed.
Time after take-off (more than 10 seconds)
Time after manual trim (more than 5 seconds)
N1 more than 60%.
No autopilot.
Elevator trim requirements

Silver.





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Old 14th Nov 2018, 10:32
  #1162 (permalink)  
 
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Originally Posted by silverstrata View Post


That is a 737 colloquialism, using ’degrees’ in the generic sense - eveyone says ‘degrees’ but we mean ‘units’. As you say, they are not real degrees - you would have to look on the trim-sheet to see the conversion between MAC and trim, as it is not linear.


I would have to assume that when Boeing publishes a technical description of a control system in a bulletin to one of their manuals whcih says "...2.5 degrees ..... 0.27 degrees per second ...." That the word "degrees" means precisely that: Degrees. And that if they had really meant "Units of Aircraft Nose Up" (or whatever the term is in the FCOM) they would have said exactly that.

Last edited by A Squared; 14th Nov 2018 at 11:27.
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Old 14th Nov 2018, 10:39
  #1163 (permalink)  
 
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Originally Posted by A Squared
As nearly as I can tell, you theory is that Boeing just changed the name from STS to MCAS and that whatever happened to the Lion airplane could happen to other 737 model prior to the MAX . There's a lot of evidence which suggests that this theory is fanciful.
I agree ("fanciful" is a bit harsh though ). Rananim, 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.
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Old 14th Nov 2018, 10:40
  #1164 (permalink)  
 
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Originally Posted by silverstrata View Post
...
And regards rhe STS, people here saying it is controlled by speed. It is a bit more complex than that, as the inputs to the STS are:

Stab position.
Thrust lever position.
Airspeed.
Vertical speed.
Time after take-off (more than 10 seconds)
Time after manual trim (more than 5 seconds)
N1 more than 60%.
No autopilot.
Elevator trim requirements

Silver.
Can I ask where you got the 'N1 more than 60%' condition for the STS from please?
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Old 14th Nov 2018, 11:22
  #1165 (permalink)  
 
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Originally Posted by Capn Bloggs View Post
I agree ("fanciful" is a bit harsh though ). Rananim, 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.
I agree with your comments about the MCAS, I disagree that "fanciful" is unnecessarily harsh. Boeing knows what caused the crash and how their automated stabilizer trim systems contributed to that. That's why they published the Bulletin, which spawned the Emergency Airworthiness Directive. Boeing has delivered 200-ish MAX's They have delivered 7,000-ish NG's. Given that the Bulletin and Emergency AD are very clearly specific to the MAX's and not applicable to the NG's, only 2 possibilities exist:

1.) Boeing knows that the malfunction that caused the Lion Air Crash is something that could occur in all 7200 737 NGs and MAXs, but for reasons known only to themselves, limited the applicability of the Emergency Airworthiness Directive to the 200 MAXs, and not to the other 7,000 NGs whcih could also experience the same malfunction.

2.) Boeing, the designer and manufacturers of both the NGs and MAXs, has very good reasons for believing that the malfunction will only happen in the 200 MAXs and not in the 7,000 NGs in service.

I would say "fanciful" is a fairly restrained characterization of thinking which considers 1 to be more plausible than 2.
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Old 14th Nov 2018, 11:23
  #1166 (permalink)  
 
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Originally Posted by MickG0105 View Post
Can I ask where you got the 'N1 more than 60%' condition for the STS from please?
From the FCOM (737 classic). OMB ll 9.20.9. STS requires 60% N1 before operation.
I presume that limits STS to the climb.

Silver
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Old 14th Nov 2018, 11:38
  #1167 (permalink)  
 
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Originally Posted by A Squared View Post
I would have to assume that when Boeing publishes a technical description of a control system in a bulletin to one of their manuals whcih says "...2.5 degrees ..... 0.27 degrees per second ...." That the word "degrees" means precisely that: Degrees. And that if they had really meant "Units of Aircraft Nose Up" (or whatever the term is in the FCOM) they would have said exactly that.
Good point.

The trim sheet has: Indexes, MACs, and Units. The FCOM has Units. Nowhere that I can see mentions trim degrees. If Boeing is quoting real degrees here, and not trim units, then that is a whole different ballgame. What is the conversion between degrees and units?

However, if the aircraft was slow, nearing the stall, you would indeed need 2.5 units (or degrees?) of forward trim. The problem being that you definitely don’t want 2.5 units (degrees?) when doing 250 kt...

Silver
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Old 14th Nov 2018, 11:46
  #1168 (permalink)  
 
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Originally Posted by A Squared View Post
I would have to assume that when Boeing publishes a technical description of a control system in a bulletin to one of their manuals whcih says "...2.5 degrees ..... 0.27 degrees per second ...." That the word "degrees" means precisely that: Degrees. And that if they had really meant "Units of Aircraft Nose Up" (or whatever the term is in the FCOM) they would have said exactly that.
Yep.
That's how I feel as well, but may be wrong.
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Old 14th Nov 2018, 11:46
  #1169 (permalink)  
 
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Originally Posted by silverstrata View Post


Good point.

The trim sheet has: Indexes, MACs, and Units. The FCOM has Units. Nowhere that I can see mentions trim degrees.

If Boeing is quoting real degrees here, and not trim units, then that is a whole different ballgame. What is the conversion between degrees and units? Besides, if the aircraft was slow, nearing the stall, you would indeed need 2.5 units (or degrees?) of forward trim. The problem being that you definitely don’t want 2.5 units (degrees?) when doing 250 kt...

Silver
You're probably correct that "degrees" doesn't appear in *operational* publications, But I would speculate that you would find "degrees" in design/engineering/maintenance publications, and that's probably where the numbers in the AD come from; drawn directly from the design specs for the MCAS.

Originally Posted by silverstrata View Post
The problem being that you definitely don’t want 2.5 units (degrees?) when doing 250 kt...


..or *another* 2.5 degrees/units 5 seconds later ...

Last edited by A Squared; 14th Nov 2018 at 12:28.
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Old 14th Nov 2018, 12:08
  #1170 (permalink)  
 
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Originally Posted by A Squared View Post
As nearly as I can tell, you theory is that Boeing just changed the name from STS to MCAS and that whatever happened to the Lion airplane could happen to other 737 model prior to the MAX . [...] Point being, Boeing probably had pretty sound reasons for saying that the problem was exclusive to the 737 MAX -8 and -9, and not including the NG and prior models.
Boeing probably had pretty sound (to Boeing) reasons for not telling anyone about MCAS, those reasons probably look like "WTF" to everyone else, but that's with the benefit of hindsight.

I would caution that the AD is quite specific that it addresses an issue caused by erroneously high AOA input, if AOA is not an input to STS on previous models then this issue can't happen on them. That doesn't mean that STS on NG doesn't have the capability to dump you into the ocean, just that it cannot do it because of one dud AOA input, and the runaway stab trim procedure is there for the other cases. It is also possible that STS runaway on NG and earlier would require multiple sensor or ADIRU failures making it a lot less likely - I still don't get how only one sensor failure can upset MCAS, I can't (or don't want to) believe they took a direct output from one sensor into the stab trim.

My personal theory on STS/MCAS naming is that MCAS was invented and named in engineering/design to meet Part 25, and the existing STS was the cheapest/simplest implementation option (possibly just software change and an additional input on the same unit). The MCAS acronym (definition only) sneaked out into some versions of the documentation but then someone decided that if it was effectively just an existing system, STS, why document it separately - it's all STS which is already covered. Further down the line someone decided that if the additional STS documentation was about an area of the envelope where only test pilots would go, why worry anyone else with it. So it became undocumented.

Add to the above a very slight suspicion that treating it as a separate new control system just might have opened a certification can of worms that Boeing wanted to avoid (as someone else suggested earlier in thread).
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Old 14th Nov 2018, 12:12
  #1171 (permalink)  
 
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I think many of those on PPRuNe are not only interested in the WHAT but also the WHY. Why did Boeing design it this way and communicate it this way.

A real life option of what might have happened in the WHY field is that during design and test they stumbled on some issues in the envelope with respect to stall in certain regimes. 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. (I change the sequence of the items compared to what A Squared posted earlier in a reply to Silverstrata - changed it because it is meaningful). So they generated a number of possible solutions. They selected the 'MCAS' solution for implementation which did not require breaking the TC. What we don't know, next to knowing surprising little about MCAS at this stage, is how it was tested and under what assumptions and rules. And if anyone rang a bell (would surprise me if not, but ..).

A proper solution requires proper communication to all stakeholders, so not only to pilots, trainers but also to maintainers. It is hard to understand why that communication was not up to the expected level. Especially as a number of consequences of the MCAS design decision will appear counter-intuitive to quite a few people.

In another 737 case I was surprised that 737's did not have a certain well proven system. The published reason for that was documented as being to stop short from breaking the TC. Will try to dig up that information.

When there are answers on the issues above, it would not surprise me if there will be other questions that will zoom in on the history of the envelope issue. Was it there before the MAX, and how much margin was there for earlier solutions.

Last edited by A0283; 14th Nov 2018 at 12:25.
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Old 14th Nov 2018, 12:17
  #1172 (permalink)  
 
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Originally Posted by infrequentflyer789 View Post
That doesn't mean that STS on NG doesn't have the capability to dump you into the ocean, just that it cannot do it because of one dud AOA input,
No disagreement. Any time you're changing the angle of a stabilizer, the potential exists for loss of control of the aircraft if you lose control of the system adjusting the stabilizer. I doubt anyone disagrees on that point. The context of the discussion at hand is the specific malfunction whcih (apparently) led to loss of control of Lion's 737MAX as the result of bad data from a single AoA sensor.
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Old 14th Nov 2018, 12:25
  #1173 (permalink)  
 
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Originally Posted by infrequentflyer789 View Post
The MCAS acronym (definition only) sneaked out into some versions of the documentation but then someone decided that if it was effectively just an existing system, STS, why document it separately - it's all STS which is already covered.
Not to say that you're wrong, that may well have been the thinking. But the objections to that rationale is that the MCAS does something pretty different than the STS (almost opposite) for different reasons and in a very different flight regime ,so saying that "it's just the same "system"" is a weak reason for not disclosing it.
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Old 14th Nov 2018, 12:33
  #1174 (permalink)  
 
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MCAS is different, and was developed to accomodate the new powerplants, and their re orientation on the airframe.

Further forward, lifted, and contributing to 14% improvement in SFC.

Some would claim this was a challenge to all sorts of previously certified characteristics (sic). Handling changed, controls response changed, and required a new protection against NU getting too aggressive.

Data was kept from pilots Boeing considered “average”. Too sophisticated to clue in the operator? Interesting? Because in Boeing’s response, it was claimed “new research” uncovered the problem, that is ridiculous.

FAA reserved the right to “augment” the AD at a later date. I will repeat my comment re the CofA. If handling is markedly different sufficient to add a new trimming feature, should the certification be amended? Boeing obviously thought it was subtle enough to not mention in the FCOM. If the handling is different, to what degree should it remain ignored? (Purposely ignored, and not acknowledged to the operators.)

Last edited by Concours77; 16th Nov 2018 at 02:27.
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Old 14th Nov 2018, 12:36
  #1175 (permalink)  
 
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Originally Posted by Concours77 View Post
If handling is markedly different sufficient to add a new trimming feature, should the certification be amended? Boeing obviously thought it was subtle enough to not mention in the FCOM. If the handling is different to what degree should it remain ignored?
Or to put it differently: When is a 737 no longer a 737?
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Old 14th Nov 2018, 12:37
  #1176 (permalink)  
 
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Originally Posted by silverstrata View Post


From the FCOM (737 classic). OMB ll 9.20.9. STS requires 60% N1 before operation.
I presume that limits STS to the climb.

Silver
Thanks for that, that is interesting, that engine parameter is not listed in the conditions for speed trim for the NG.

Conditions for speed trim operation are listed below:
• STS Mach gain is fully enabled between 100 KIAS and Mach 0.60 with a fadeout to zero by Mach 0.68
• 10 seconds after takeoff
• 5 seconds following release of trim switches
• Autopilot not engaged
• Sensing of trim requirement
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Old 14th Nov 2018, 12:42
  #1177 (permalink)  
 
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Originally Posted by A Squared View Post
You're probably correct that "degrees" doesn't appear in *operational* publications, But I would speculate that you would find "degrees" in design/engineering/maintenance publications, and that's probably where the numbers in the AD come from; drawn directly from the design specs for the MCAS.
If it's any help, typical values for older 737s (may vary for the NG/Max, but unlikely to be much different) are:

Max downwards stab (i.e. a/c nose up) angle: -12.9°
Max upwards stab (i.e. a/c nose down) angle: +4.2°

A stab angle of 0° corresponds to 4 units of nose-up (ANU) trim at the lever. Presumably the purpose of ANU units is simply to avoid confusion between positive and negative values, so it may well be that each ANU unit increment = 1°.
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Old 14th Nov 2018, 12:42
  #1178 (permalink)  
 
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Originally posted by AO283: "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. "

I would think that virtually ANY change to an approved type design requires an amendment to the Type Certificate. See the B737 Type Certification Data Sheet (TCDS) on the FAA website:

TCDS A16WE Rev 64 Boeing Company, The
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Old 14th Nov 2018, 12:46
  #1179 (permalink)  
 
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Originally Posted by A Squared View Post
Or to put it differently: When is a 737 no longer a 737?
Affirm. But then that’s just an opinion. Once more for good measure: Disclosure is a mandatory feature of change. Without it, airplanes can start to become “What’s it doing now”. Ask Bernard.

Compared to the Classic, the MAX is “hot”. Airbus builds wildly different airframes under the same TC, ?No? Boeing afforded the same discretion? Sim only, no airtime to upgrade? Sisters?

Last edited by Concours77; 14th Nov 2018 at 13:00.
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Old 14th Nov 2018, 12:47
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Originally Posted by DaveReidUK View Post
If it's any help, typical values for older 737s (may vary for the NG/Max, but unlikely to be much different) are:

Max downwards stab (i.e. a/c nose up) angle: -12.9°
Max upwards stab (i.e. a/c nose down) angle: +4.2°

A stab angle of 0° corresponds to 4 units of nose-up (ANU) trim at the lever. Presumably the purpose of ANU units is simply to avoid confusion between positive and negative values, so it may well be that each ANU unit increment = 1°.
So, assuming the MAX is not terribly different, you may only be 2 operation cycles of the MCAS away from having your stabilizer at full stab-up (nose down) travel.
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