|
Originally Posted by Volume
(Post 10304534)
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. 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. |
Originally Posted by B738bbjsim
(Post 10304369)
Now that we have AOA in the spotlight, and reports of indicators issue on other Lion 737 Maxx, what could be the cause?
|
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? |
Originally Posted by wiedehopf
(Post 10304576)
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? |
Originally Posted by Volume
(Post 10304534)
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.
Originally Posted by Denti
(Post 10304439)
. 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. .
|
Boeing has already said it can be dealt with with ‘existing procedure’. |
Originally Posted by Volume
(Post 10304534)
Not groundspeed, but indicated airspeed....
Originally Posted by Denti
(Post 10304439)
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
|
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. |
Originally Posted by ImbracableCrunk
(Post 10304544)
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%)?
|
Originally Posted by Water pilot
(Post 10304599)
Jesus. So while you are fighting the other symptoms of UAS, the plane is quietly trying to kill you.
|
Boing should fit three alpha vanes as they do pitot tubes.
|
Originally Posted by A Squared
(Post 10304606)
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.
|
Originally Posted by Concours77
(Post 10304611)
Originally Posted by Water pilot https://www.pprune.org/images/buttons/viewpost.gifJesus. So while you are fighting the other symptoms of UAS, the plane is quietly trying to kill you. 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? |
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. |
Originally Posted by ImbracableCrunk
(Post 10304635)
I thought the issue with the trim was related to manual flight.
|
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. |
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 ? |
Originally Posted by DaveReidUK
(Post 10304595)
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. |
Originally Posted by WillFlyForCheese
(Post 10304568)
Originally Posted by B738bbjsim
(Post 10304369)
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. |
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 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. :confused: |
| All times are GMT. The time now is 16:01. |
Copyright © 2024 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.