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Indonesian aircraft missing off Jakarta

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Indonesian aircraft missing off Jakarta

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Old 6th Dec 2018, 02:50
  #2021 (permalink)  
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Mr Cheese.

I see lots of folks assuming MCAS trims 2.5ND each time - but that's not what I get from the description of MCAS, which provides: 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.

So - depending on speed and AoA, MCAS will trim up to 2.5ND - correct? Or - is there something else that confirms MCAS trimmed JT610 2.5ND repeatedly, and then continuously?
It's just that one letter that makes the difference. It suggests ". . . limited to 2.5 degrees each and are . . ." might be an improved wording.
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Old 6th Dec 2018, 03:42
  #2022 (permalink)  
 
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Originally Posted by rubik101 View Post
What was wrong with the old system?
The old system required you to roll into a 45 degree bank in the simulator and raise the nose the degree or two to maintain level flight, increasing the load factor, and either holding pressure or trimming it off while getting your instrument crosscheck warmed up. The new aerodynamics of the Max, if replicated in whatever simulator might have them, might allow you to roll into 45 degree bank and as you pulled , you might not have to pull anymore or pull less because the engines aero effect started pulling for you. What's wrong with that I don't know. If you have a good crosscheck you fly the attitude and performance instruments and do what ever it requires with the stick. If there is any other flight envelope where the old system would be deficient for the MAX and you needed the MCAS we don't know and Boeing isn't telling us but previous threads have hinted that a normal flight would never see it. This is all supposition by me wondering why steep (or step) turns were mentioned in the one of the Boeing blurbs.
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Old 6th Dec 2018, 03:49
  #2023 (permalink)  
 
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Why did Boeing not fit a stick nudger as was done for the 747 on the British register?
Because the elevator alone seems to be not powerful enough, no matter whether the pilot or an actuator pushes it. Obviously nose down trim is required to create enough nose down pitching moment. Which does not speak for the design, but is typical for the late 50s, when most elevators were so small that no hydraulic power was required to operate it.
If the 737 MAX would be a clean sheet design, the horizontal stabilizer and elevator would for sure look differently.

Trim has always been a killer item requiring close attention and it will always be, even if now a box of chips and a bunch of sensors pays the attention. Having new aircraft designs without a trim wheel is unbelieveable for me, but it becomes the standard. Trim becomes a computer assist item.
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Old 6th Dec 2018, 05:12
  #2024 (permalink)  
 
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During each pre-flight, admittedly in a different airframe, each pilot would test the stick shaker and I would expect from that, they would be quite used to the feel on their column and sound, is it the same on the B737 MAX ?
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Old 6th Dec 2018, 05:25
  #2025 (permalink)  
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Originally Posted by Loose rivets View Post
Mr Cheese.
It's just that one letter that makes the difference. It suggests ". . . limited to 2.5 degrees each and are . . ." might be an improved wording.
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?
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Old 6th Dec 2018, 06:08
  #2026 (permalink)  
 
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Originally Posted by MickG0105 View Post
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.'
MickG - careful now. This is not at all a matter of HAL saying "I'm afraid I can't allow that, Dave." What is happening here is that MCAS has inserted a nose down stabilizer motion increment in response to its understanding that the airplane is at high AOA. The pilot has then manually re-positioned the stabilizer. MCAS is in no way preventing the pilot from moving the stabilizer in either direction as much as the pilot wants. MCAS is further waiting five seconds to allow the pilot to make sure that the pitch trim situation is to the liking of the pilot. If the pilot is not satisfied with the pitch trim and makes another manual trim input, MCAS sits it out for another five seconds. Only after that does MCAS make a new assessment as to whether or not the conditions exist (flaps up, proper speed range, and high indicated AOA) to trigger MCAS to insert a limited increment of nose down stabilizer motion. The design assumption here is that when a pilot makes pitch trim inputs, those inputs will be in the direction to trim the airplane such that each time MCAS makes its assessment as to whether or not to add more nose down, it is starting from a relatively trimmed condition. The pilot maintains highest priority for stabilizer control throughout and MCAS only moves the stabilizer a limited increment each time that the pilot finishes trimming manually.

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.

Last edited by FCeng84; 6th Dec 2018 at 06:22. Reason: typo
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Old 6th Dec 2018, 06:40
  #2027 (permalink)  
 
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Volume - 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.

Last edited by FCeng84; 6th Dec 2018 at 11:24. Reason: typo
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Old 6th Dec 2018, 10:31
  #2028 (permalink)  
 
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Any of you who are interested in flight test and the required handling qualitiies of aircraft, particularly at the stall, will probably enjoy these podcasts and find them most informative. At 1hr 14mins into the second podcast he describes the 707 and in the third he talks about the 747 and later the T-tail aircraft. Much of what he has to say is relevant to this 737 accident.

D P Davies interviews on certificating aircraft

Last edited by Bergerie1; 6th Dec 2018 at 11:58.
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Old 6th Dec 2018, 10:47
  #2029 (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 FCeng84’s comments to this statement, I would explain it this way. Let’s 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 “ok, the pilot has deliberately changed the pitch, I will disable myself”. 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. It’s 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, 12:17
  #2030 (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, 14:44
  #2031 (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, 14:45
  #2032 (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, 15:25
  #2033 (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, 15:53
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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, 16:27
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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, 17:03
  #2036 (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, 18:36
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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.

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Old 6th Dec 2018, 20:30
  #2038 (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, 20:56
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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, 21:15
  #2040 (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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