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Indonesian aircraft missing off Jakarta

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Indonesian aircraft missing off Jakarta

Old 6th Dec 2018, 09:47
  #2021 (permalink)  
 
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Originally Posted by MickG0105 View Post
Boeing fails to make it clear that a trigger for MCAS to continue trimming in more nose down trim is a countermand from the crew. Frankly, I think that is astounding. The system will ignore the persistence of the trigger condition after it makes its initial nose down trim adjustment but it will make a further nose down trim adjustment if the crew countermands it! Moreover, it will do that each and every time the crew countermands it. That is a very clear cut case of an automated system that is designed to override a specific crew command. To borrow an image from Gums, when the crew looks to wind off some of the automatically commanded nose down trim, HAL says, 'I'm afraid I can't allow that, Dave.
That was my view also. But, after reading FCeng84ís comments to this statement, I would explain it this way. Letís assume the system logic was the opposite. That is, if after MACS commands a pitch change, the pilot uses the electric trim switch or manual trim wheel, MACS says ďok, the pilot has deliberately changed the pitch, I will disable myselfĒ. For how long should MACS remain disabled? What happens if 20 min later, the AoA gets into dangerous territory? Clearly, a time period needs to exist at which MCAS becomes re-enabled. So, this reverse logic scenario is actually exactly how it does work. Itís just that the time period, according to FCeng84 who seems to know this system well, is 5 seconds. (Which given how quickly an aircraft could get out of control, is probably quite a long time?)
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Old 6th Dec 2018, 11:17
  #2022 (permalink)  
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Mr Cheese
yes, but limited to 2.5 AND each time - depending on speed and AoA. It appears as though everyone is assuming it trimmed at the max limit each time - and I’m curious where that information is found. How do we know, for example, MCAS didn’t trim 1.3 AND each time, if that’s what speed and AoA called for? Or 0.7 . . .or 2.1?
First, there is that AND word, which an earlier poster suggested, ' . . . AND, and etc., etc.'

Again, wording is so critical. But your main point: while the extent of the trim via the mini-corrections IS variable, (i.e. Mach), for the most part the repeated blips are very similar on the graphs available to us. Although the Mach modification is very small, it does get me wondering what it was doing during the fatal dive.
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Old 6th Dec 2018, 13:44
  #2023 (permalink)  
 
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Rounded estimate of the status of the identification process based on public information - and excluding empty seats:
About 50% of those in front rows have been identified.
About 25% of those in the back rows have been identified.
The cabin apparently had seatrows numbered 1-34 but probably excluded rows 13 and 14 (in spite of 1 passenger mentioned to be in 13).
Standout is seat column F which has the largest percentage missing.

From the data it is not possible to estimate where the 3+3 cabin crew were seated. A guess is that at least 1 in a front folding seat, and 2 at the back galley, with the 3 trainees in row 1 seats DEF.

It suggests a nose down impact and right hand roll (more than 90 degrees would not surprise me).
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Old 6th Dec 2018, 13:45
  #2024 (permalink)  
 
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MCAS was not introduced to make up for deficient elevator nose down control authority. MCAS is there to improve handling characteristics as proscribed by the FARs at elevated AOA. The pilot has plenty of control power to lower the nose via the column. That is not the issue
.
FCeng84, could this statement be also interpreted as fixing a non-linearity in control response at high AOA? Is that the type of improvement in handling characteristics being created?
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Old 6th Dec 2018, 14:25
  #2025 (permalink)  
 
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Originally Posted by Loose rivets View Post
Mr Cheese


First, there is that AND word, which an earlier poster suggested, ' . . . AND, and etc., etc.'

Again, wording is so critical. But your main point: while the extent of the trim via the mini-corrections IS variable, (i.e. Mach), for the most part the repeated blips are very similar on the graphs available to us. Although the Mach modification is very small, it does get me wondering what it was doing during the fatal dive.
The ratchet effect exists - and is very apparent from the post of the FDR permalink here to post





Regardless of wordings in spec, the pitch trim position can be seen to 'ratchet' down in 3 full steps without correction back by the PF who just blips the trim. From the scale on the left of the image the stab trim reached max nose down.
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Old 6th Dec 2018, 14:53
  #2026 (permalink)  
 
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Something I'd not noticed before on that FDR data - not sure it's been remarked on. At the VERY end of the trace, the stab is seen to apparently move a long way back towards to trim state. Given the compressed scale, it's not possible to see if that is a "final instant" artifact and not "real" or if there's an actual slope to the stab trace. If it's "real" it indicates that the stab remained under the potential for crew control throughout the flight to the very end, and that the ratcheting prior to that was not "uncorrectable".
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Old 6th Dec 2018, 15:27
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The graph in the report shows a number of the parameters end in vertical lines like that.
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Old 6th Dec 2018, 16:03
  #2028 (permalink)  
 
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Originally Posted by Machinbird View Post
.
FCeng84, could this statement be also interpreted as fixing a non-linearity in control response at high AOA? Is that the type of improvement in handling characteristics being created?
To address your question I think it is important to be very clear with regard to your phrase "non-linearity in control response". The FAR in question speaks to the progression of column pull force required to command increased AOA. Well behaved stability characteristics would involve the need for the pilot to apply ever increasing column pull to command ever increasing AOA. Not having that desired characteristic could be a result of either the pitching moment vs. AOA (Cm-alpha) characteristic of the overall airplane or the pitching moment vs. column (Cm-column or similarly Cm-elevator) generated via deflection of the column. The net effect that the FARs speak to is how these two sources of pitching moment (AOA vs column) balance one against the other.

The issue that MCAS addresses is non-linearity of Cm-alpha stability derivative. The pitching moment generated by the elevator (and thus the column) is much more linear. As I have mentioned in previous posts, it is not a matter of the pilot having insufficient ability to generate pitching moment via the column. The issue is that as AOA increases into a range higher than normal operation (near and beyond the AOA for stick shaker activation), the amount of nose down pitching moment generated by additional increase in AOA is not sufficient to require the FAR mandated increase in column pull to balance.
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Old 6th Dec 2018, 17:36
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Originally Posted by CurtainTwitcher View Post
While you are contemplating this, try putting up with what this crew had to endure while they were trying to figure out what was going on.
CT with all due respect... I don't see anyone disputing that Boeing's MCAS system ran the nose down and crashed the jet killing 189 people, but it is noteworthy that two switches may have prevented this tragedy.

What remains to be seen is how ambiguity in the QRH, distraction, lack of training, a flying spanner, CRM, or perhaps even crew competency factor in as possible contributing causes.

"We study and discuss accidents so we don't become the subject of a thread on PPRuNe."
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Old 6th Dec 2018, 19:30
  #2030 (permalink)  
 
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Originally Posted by climber314 View Post
but it is noteworthy that two switches may have prevented this tragedy
Or, come to that (with 20/20 hindsight), simply grabbing and holding the trim wheel.

N.B. No criticism of the crew intended.
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Old 6th Dec 2018, 19:56
  #2031 (permalink)  
 
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I am with gums on this one, in terms of "if something moves the flight control surfaces, the pilots need to be aware that it exists and how it helps them, and thus some training/education is required." Also needed for safe operations is "what turns it on or off" if it moves the flight control surfaces.
That quoted "executive" needs a talking to from the pilot's union.
Anything that is built can break, anything with computer code or electricity involved can malfunction.

Question on the MEL: is one AoA sensor INOP a downing discrepancy? I'd assume so, but I've been surprised before.
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Old 6th Dec 2018, 20:15
  #2032 (permalink)  
 
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Originally Posted by climber314 View Post
CT with all due respect... I don't see anyone disputing that Boeing's MCAS system ran the nose down and crashed the jet killing 189 people, but it is noteworthy that two switches may have prevented this tragedy.

What remains to be seen is how ambiguity in the QRH, distraction, lack of training, a flying spanner, CRM, or perhaps even crew competency factor in as possible contributing causes.

"We study and discuss accidents so we don't become the subject of a thread on PPRuNe."
Regardless of the fault or cause, one does wonder after all those trim corrections, or whatever fault that caused the trim to run counter to where they wanted it , at what stage would the crew have used those switches.
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Old 6th Dec 2018, 20:21
  #2033 (permalink)  
 
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Originally Posted by Lonewolf_50 View Post
Question on the MEL: is one AoA sensor INOP a downing discrepancy? I'd assume so, but I've been surprised before.
B-737-8/9 MMEL on FAA website (rev 1, Jan 2018) only mentions AOA heater as related MMEL option; AOA sensor itself does not appear on MMEL. (Based on a few search terms)
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Old 6th Dec 2018, 22:41
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from this SLF- when ONE faulty sensor input allows " hal " to override manual control AFTER correction by the pilots without warning or deliberate reset, without notice, or mention, the " chain of command" which allowed/approved/designed such a process to be incorporated needs to be held responsible.
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Old 6th Dec 2018, 23:03
  #2035 (permalink)  
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Well yes, but the majority of the thread is leaning towards the stabilizer not being disabled. Something is nagging away in the back of my mind. Stabilizer runaway memory actions are so fundamental that the thread's detective logic changed quickly to looking at the ~5 second delay adding to the confusion of an already chaotic flight-deck - causing a radical diversion from old established SOPs.

The relatively calm ATC and the prolonged period of approximate hight holding, coupled with the huge handful of power in the last moments, is leaving me with a mind that's wide open.
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Old 7th Dec 2018, 00:15
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Originally Posted by Loose rivets View Post
Stabilizer runaway memory actions are so fundamental that the thread's detective logic changed quickly to looking at the ~5 second delay adding to the confusion of an already chaotic flight-deck - causing a radical diversion from old established SOPs.
There seems to be a significant amount of hindsight bias regarding the specific term "runaway". A lot of the discussion is now anchored to what has been revealed about MCAS and the behavior of this aircraft. If you survey every single poster in this thread before this incident and asked them to describe what "runaway stabilizer" looks like, I imagine the answers would be quite different than what you would get in a poll with hindsight of this incident. For an example of a 2016 reference guide description of runaway stabilizer, google: The Ultimate B737 NG Technical Handbook STABILISER TRIM SYSTEM AND RUNAWAY STABILISER

It's not even clear that the previous crew diagnosed a "runaway" and followed the NNC. They may have gone directly to the cutout switches as a way to isolate an unknown electrical trim problem- they later reported that they believed it was STS operating in reverse due to IAS issues. This may even have been a conclusion/guess they reached later in the flight rather than a contemporaneous diagnosis.

EDIT: climber314 pointed out that the preliminary report says they followed the Runaway Stabilizer NNC. Hopefully we get more specifics on this later.

Last edited by Roger_Murdock; 7th Dec 2018 at 01:14.
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Old 7th Dec 2018, 00:26
  #2037 (permalink)  
 
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Why didnít they get this ?
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Old 7th Dec 2018, 00:38
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Originally Posted by Roger_Murdock View Post
It's not even clear that the previous crew diagnosed a "runaway" and followed the NNC.
The JT43 crew ran the R/S NNC and cut out Stab Trim. I posted this shortly after the report was released.
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Old 7th Dec 2018, 01:08
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Originally Posted by FCeng84 View Post

While it does not present itself as continuously running run-away automatic stabilizer motion, with the airplane flying at a relatively steady flight condition repeated instances of the system moving the airplane away from the trim condition that the pilot has established manually should be recognized as improper automatic stabilizer operation and disabled via the cutout switches.
I think it's a fair distinction to raise but feel this presentation is disengenous.

As far as we can tell, MCAS is intended to provide a more consistent and predictable handling response in incipient stall conditions.

How does an intermittent and unadvertised control input, of complex algorithmic intensity, every 5 seconds, provide that?
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Old 7th Dec 2018, 01:11
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Originally Posted by climber314 View Post
The JT43 crew ran the R/S NNC and cut out Stab Trim. I posted this shortly after the report was released.
Sure enough. Is that statement based on interview with the crew?
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