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Indonesian aircraft missing off Jakarta

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Indonesian aircraft missing off Jakarta

Old 7th Nov 2018, 16:11
  #741 (permalink)  
 
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Originally Posted by Volume View Post
Not groundspeed, but indicated airspeed....

In a nutshell: total pressure is constant wherever you have airflow, hence you can place a pitot tube at any location where you have attached airflow, if the probe roughly points into the airflow, it will work satisfactory.
Measuring static pressure only works in places, where the airflow has exactly the same speed as the aircraft. If you would measure static pressure on the upper wing surface, you would actually measure the low pressure which keeps you in the air. Also on the nose fuselage, you have slightly increased airflow speed, an your static pressure is too low. Towards the end of the fuselage your airflow may be more the same as the general airspeed, but the turbulent boundary layer of the fuselage would create some high frequency pressure variations. So it is very, very hard to find a proper place for the static probe.

If you place the static probe in a position where the additional speed due to the airflow around the fuselage is well known, you may correct the measured pressure electronically. This obviously happen here, static pressure measurement is AoA sensitive, so you measure AoA to correct your static pressure.
This way, an AoA failure results in unreliable static pressure, hence UAS and unreliable altitude. Additionally your AoA driven stall warning goes havoc...

Not so nice if you are sitting in row 0.
The link between AoA and indicated airspeed is not so obvious. As part of my profession I get to see a lot of the internals of modern Air Data / Inertial Navigation systems. And without going into detail, the correction from AoA for the static pressure is minimal (other corrections like ground effect, reverse thrust etc are more pronounced and still small). If AoA is way off, it seems more likely a miscompare is triggered between AoA1 and AoA2. This could have all kinds of consequences and warnings which on a 737 probably need to be recognized by a pilot an actively worked (i.e., manually switch Air Data source of affected side to standby or opposite).
Agree with the stall warning, the amber band etc will be all over the place.

edit: link to Boeing Aero magazine regarding AoA indication on Boeing aircraft.

Last edited by Flutter speed; 7th Nov 2018 at 16:22. Reason: Added a link
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Old 7th Nov 2018, 16:15
  #742 (permalink)  
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Originally Posted by B738bbjsim View Post
Now that we have AOA in the spotlight, and reports of indicators issue on other Lion 737 Maxx, what could be the cause?
My understanding is that the reports of “issues” are not on other Lion aircraft - but multiple flights of the same aircraft.
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Old 7th Nov 2018, 16:26
  #743 (permalink)  
 
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So inhibiting the anti-stall trim system on AOA or other air data disagree would seem like a sensible thing to do?

Can Boeing even do that now without being sued?
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Old 7th Nov 2018, 16:36
  #744 (permalink)  
 
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Originally Posted by wiedehopf View Post
So inhibiting the anti-stall trim system on AOA or other air data disagree would seem like a sensible thing to do?

Can Boeing even do that now without being sued?
Let’s look at precedent. What did Airbus do as a result of AutoTrim full up to and through Stalll?
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Old 7th Nov 2018, 16:42
  #745 (permalink)  
 
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Originally Posted by Volume View Post
Originally Posted by A Squared
How does AoA data affect Groundspeed, and why?

Not groundspeed, but indicated airspeed....

In a nutshell: ...blah, blah,blah, blah, blah,blah, blah, blah,blah ....
Not so nice if you are sitting in row 0.
Look my friend, if you had bothered to read the post I was responding to, you would have seen that the poster had stated that AoA would affect airspeeed but surprisingly, in addition to affecting airspeed (which I understand just fine, thank you ) it *also* affects groundspeed. I'll quote for you (again) the post I was responding to, so that you might have another chance to read it, perhaps with better comprehension this time.

Originally Posted by Denti View Post
. It is a basic correction factor into the ADIRU that does effect all resulting air data related information and surprisingly the non air data related information of ground speed as well. .
So, once again, for Denti, yes it's surprising that AoA affects groundspeed and I'd be interested to have that explained.
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Old 7th Nov 2018, 16:50
  #746 (permalink)  
 
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Boeing has already said it can be dealt with with ‘existing procedure’.
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Old 7th Nov 2018, 16:52
  #747 (permalink)  
 
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Originally Posted by Volume View Post
Not groundspeed, but indicated airspeed....
Except that the author of the quoted post took pains to stress that both airspeed and groundspeed values are affected by unreliable AoA, in fact GS was mentioned twice in that context:

Originally Posted by Denti View Post
From my own experience on the NG a wrong AoA input will result in UAS, unreliable altitude, vertical speed, wind information and ground speed display. It is a basic correction factor into the ADIRU that does effect all resulting air data related information and surprisingly the non air data related information of ground speed as well
I look forward to learning how so.
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Old 7th Nov 2018, 16:55
  #748 (permalink)  
 
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The nose down stabilizer trim movement can be stopped and reversed with the use of the electric stabilizer trim switches but may restart 5 seconds after the electric stabilizer trim switches are released.
Jesus. So while you are fighting the other symptoms of UAS, the plane is quietly trying to kill you.
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Old 7th Nov 2018, 17:09
  #749 (permalink)  
 
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Originally Posted by ImbracableCrunk View Post
I've often wondered this, and maybe it could help: Why not go in to CWS when faced with UAS? Set the pitch (4deg or 10deg) via CWS and set the N1 (75% or 80%)?
It might work, but if the airplane is actively fighting your efforts to keep it in stable flight via uncontrollable erratic trim inputs, you might be reluctant to believe that engaging the A/P in any mode would help, and you might be right.
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Old 7th Nov 2018, 17:14
  #750 (permalink)  
 
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Originally Posted by Water pilot View Post
Jesus. So while you are fighting the other symptoms of UAS, the plane is quietly trying to kill you.
Airbus. same. Only reversing trim is done manually, and wheel must be held to retain new position. If one lets go the wheel, the aircraft reloads, and tries to kill again.

Last edited by Concours77; 7th Nov 2018 at 17:33.
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Old 7th Nov 2018, 17:30
  #751 (permalink)  
 
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Boing should fit three alpha vanes as they do pitot tubes.

Last edited by mross; 7th Nov 2018 at 18:15. Reason: typo on 'pitot'
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Old 7th Nov 2018, 17:44
  #752 (permalink)  
 
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Originally Posted by A Squared View Post
It might work, but if the airplane is actively fighting your efforts to keep it in stable flight via uncontrollable erratic trim inputs, you might be reluctant to believe that engaging the A/P in any mode would help, and you might be right.
I thought the issue with the trim was related to manual flight.
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Old 7th Nov 2018, 17:49
  #753 (permalink)  
 
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Originally Posted by Concours77 View Post
Originally Posted by Water pilot Jesus. So while you are fighting the other symptoms of UAS, the plane is quietly trying to kill you.
Airbus. same. Only reversing trim is done manually, and wheel must be held to retain new position. If one lets go the wheel, the aircraft reloads, and tries to kill again.
I thought this had come up earlier in the thread, but now can't find it...

Is the auto-trim-in-manual-flight issue dealt with in the memory / QRHs for assorted UAS events? Or is it a separate memory / QRH procedure?
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Old 7th Nov 2018, 17:51
  #754 (permalink)  
 
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The big question for me is why did they crash when they or others managed to recover from previous cases. Had this crew been on the previous problem flights ?.
This suggests that the problem was more difficult with either extra symptoms or maybe less time/altitude available to recover.
The devil is in the detail. Could a crew with normal skills and experience have been expected to cope in this case, I have my doubts.
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Old 7th Nov 2018, 17:57
  #755 (permalink)  
 
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Originally Posted by ImbracableCrunk View Post
I thought the issue with the trim was related to manual flight.
I'm not certain. my understanding of how the STS and related magic all works is imperfect at best. If it all switches off completely as soon as the A/P is engaged and the airplane knows it is no longer being flown manually, then yeah, that might help. On the other hand, AFAIK, the autopilot must have still have some means to trim the stabilizer, so if the erratic AoA is causing the pitch trim problems through those circuits/systems, then engaging the A/P might not make the problem go away.
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Old 7th Nov 2018, 18:03
  #756 (permalink)  
 
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Boeing Bulletin

On Nov 7th 2018 Boeing issued an Operations Manual Bulletin (OMB) to all Boeing 737 MAX Operators stating that the investigation into the crash of PK-LQP found one of the Angle of Attack Sensors had provided incorrect readings, which could cause the aircraft's trim system to uncommandedly trim nose down in order to avoid a stall during manual flight. The OMB directs "operators to existing flight crew procedures to address circumstances where there is erroneous input from an AOA sensor." The OMB reiterates the Stabilizer Runaway non-normal checklist.

The flight Crew Operations Manual Bulletin TBC-19 reads:

The Indonesian National Transportation Safety Committee has indicated that Lion Air flight 610 experienced erroneous AOA data. Boeing would like to call attention to an AOA failure condition that can occur during manual flight only.

This bulletin directs flight crews to existing procedures to address this condition. In the event of erroneous AOA data, the pitch trim system can trim the stabilizer nose down in increments lasting up to 10 seconds. The nose down stabilizer trim movement can be stopped and reversed with the use of the electric stabilizer trim switches but may restart 5 seconds after the electric stabilizer trim switches are released. Repetitive cycles of uncommanded nose down stabilizer continue to occur unless the stabilizer trim system is deactivated through use of both STAB TRIM CUTOUT switches in accordance with the existing procedures in the Runaway Stabilizer NNC. It is possible for the stabilizer to reach the nose down limit unless the system inputs are counteracted completely by pilot trim inputs and both STAB TRIM CUTOUT switches are moved to CUTOUT.

Additionally, pilots are reminded that an erroneous AOA can cause some or all of the following indications and effects:

- Continuous or intermittent stick shaker on the affected side only.
- Minimum speed bar (red and black) on the affected side only.
- Increasing nose down control forces.
- Inability to engage autopilot.
- Automatic disengagement of autopilot.
- IAS DISAGREE alert.
- ALT DISAGREE alert.
- AOA DISAGREE alert (if the AOA indicator option is installed)
- FEEL DIFF PRESS light.

In the event an uncommanded nose down stabilizer trim is experienced on the 737-8 /-9, in conjunction with one or more of the above indications or effects, do the Runaway Stabilizer NNC ensuring that the STAB TRIM CUTOUT switches are set to CUTOUT and stay in the CUTOUT position for the remainder of the flight.
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Old 7th Nov 2018, 18:06
  #757 (permalink)  
 
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Boeing would like to call attention to an AOA failure condition that can occur during manual flight only.
So ... you cannot use manual flight when a loss of control occurs ?
Wow, nice recommendation.
How much is the train again ?
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Old 7th Nov 2018, 18:12
  #758 (permalink)  
 
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Originally Posted by DaveReidUK View Post
Except that the author of the quoted post took pains to stress that both airspeed and groundspeed values are affected by unreliable AoA, in fact GS was mentioned twice in that context:

I look forward to learning how so.
Quite honestly, i would have loved to do so as well. Back then neither the safety department, nor the technical pilot, nor the internal investigation (it never went to outside investigation) nor the boeing technical pilots that had a look themselves could explain it coherently to me. Since then i have moved to another type and the company in question has vanished, after they got rid of their boeing fleet to begin with. So i don't see that it will be ever really explained, but it happened to quite a few of our planes after a boeing advised AoA retrofit with a different type that was then prone to freeze in a position.
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Old 7th Nov 2018, 18:12
  #759 (permalink)  
 
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Originally Posted by WillFlyForCheese View Post
Originally Posted by B738bbjsim View Post
Now that we have AOA in the spotlight, and reports of indicators issue on other Lion 737 Maxx, what could be the cause?


My understanding is that the reports of “issues” are not on other Lion aircraft - but multiple flights of the same aircraft.

It was earlier reported that Lion had identified potentially similar defects on other 737 max aircraft in their fleet. in addition to the other occurrences of UAS problems on the accident airplane.
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Old 7th Nov 2018, 18:17
  #760 (permalink)  
 
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Is the auto-trim-in-manual-flight issue dealt with in the memory / QRHs for assorted UAS events? Or is it a separate memory / QRH procedure?
I don’t have a 737 QRH in front of me but I suspect that one would be something like “Runaway Stabiliser” and the other “Airspeed Unreliable”.

I am having increasing sympathy for the situation the crew found themselves in: symptoms of UAS (biased by AML entries from previous flights) but with an effective trim runaway thrown in for good measure and all at low level before there was a chance to get a “feel” for the aircraft. I also suspect that there might have been the odd (false) warning or two going off, just to add to the confusion.
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