|
Angle-of-attack sensor replaced before 737 Max crash
Angle-of-attack sensor replaced before 737 Max crash
Indonesia's National Transportation Safety Committee says an angle-of-attack sensor on the ill-fated Lion Air Boeing 737 Max 8 had been replaced a day before it crashed. The committee says the sensor was replaced in Denpasar on 28 October, after pilots reported issues with the airspeed indicator.It adds that this was related to the faulty airspeed indication, which was first raised at a 5 November press conference by NTSC chief Soerjanto Tjahjono. After the replacement, however, pilots that flew a Denpasar-Jakarta flight still found a 20° difference on the left-hand angle-of-attack sensor. During this flight, the pilots implemented "a number of procedures" to rectify the issues, and the jet subsequently landed in Jakarta safely. The sensor that was removed in Denpasar has since been sent to NTSC offices in Jakarta, before being transferred to Boeing's headquarters in Chicago for further investigation. Investigators plan to reconstruct the flight and study the faults related to the sensor using Boeing's engineering simulator in Seattle. Interviews have also been conducted with the pilots and cabin crews that operated PK-LQP prior to the crash, as well as technicians that maintained the jet in Denpasar, Jakarta, and Manado. The NTSC adds that, based on issues faced by pilots on the Denpasar-Jakarta flight, it has recommended that Boeing notify 737 Max operators of the potential issues they could face with the sensors. NTSC investigators and officials from Boeing and General Electric have identified some of the wreckage recovered from the seabed. These include the left CFM International Leap-1B engine, the right-hand main landing-gear, a tail section, aircraft sections 43, 44, 46 and 48, a cockpit oxygen bottle, a left-hand passenger door, and a wing-tip. Boeing issued a operations manual bulletin on 6 November, directing operators to “existing flight crew procedures" to address circumstances involving erroneous angle-of-attack sensor information. Indonesia's national search and rescue agency Basarnas has extended its mission to 10 November |
So, the plot thickens???
The original Airspeed Issue that then [eventually] led to a AOA sensor change that in fact was obscurating the original Airspeed Issue that was still there but in replacing the AOA sensor they introduced ANOTHER fault into the aircrafts indication and flight control system[s]? None of which was reproducible on the ground....??? What a CF. |
Originally Posted by glad rag
(Post 10305416)
So, the plot thickens.
The original Airspeed Issue that then [eventually] led to a AOA sensor change that in fact was obscurating the original Airspeed Issue that was still there but in replacing the AOA sensor they introduced ANOTHER fault into the aircrafts indication and flight control system[s]? |
If data from one of two AOA sensors are bad there shall be no correcting nose up or down trim as the system cannot know which is the faulty sensor. If this behaviour was certified it is time to review the certification documents.
As it appears there is no guarantee that the pilots can timely recover from the upset when at low altitude this aircraft type should be grounded. The emergency AD is a joke. |
Assuming we have nose down trim
Can the FDR show if the ND trim was driven by STS vs pilots manual input? If Rostov is to be believed I guess No, hence the fantastic notion that a pilot trimmed Down for 12 seconds on GA at 3,000 ft. Anyway seems perhaps Boeings really can kill pilots. Maxes at least. |
Originally Posted by A Squared
(Post 10305424)
I'm not sure that is correct. The Airplane was written up for "IAS or ALT disagree shown after takeoff" and "Feel Diff Press LT ILL" (Feel Differential Pressure Light Illuminated, I think) If I've followed the technical discussion correctly (and I may not have) , both of those may have been caused by bad AoA data, and there may never have been an actual problem with the Airspeed.
Okay. The actual circumstances beforehand will, most likely, be detailed in the report when it appears. Tragic turn of events. |
Originally Posted by hans brinker
(Post 10305018)
Probably unrelated to this accident but deserves a rebuttal....As far as I know the B737 manual states that during stall recovery with high engine power full nose down elevator might not be enough and reminds the crew to use aircraft nose down trim.
|
Originally Posted by Longtimer
(Post 10305401)
The sensor that was removed in Denpasar has since been sent to NTSC offices in Jakarta, before being transferred to Boeing's headquarters in Chicago for further investigation. Investigators plan to reconstruct the flight and study the faults related to the sensor using Boeing's engineering simulator in Seattle.
Originally Posted by A Squared
(Post 10305424)
I'm not sure that is correct. The Airplane was written up for "IAS or ALT disagree shown after takeoff" and "Feel Diff Press LT ILL" (Feel Differential Pressure Light Illuminated, I think) If I've followed the technical discussion correctly (and I may not have) , both of those may have been caused by bad AoA data, and there may never have been an actual problem with the Airspeed.
|
A trim runaway condition should be quite identifiable on a 737, these clunky trim wheels leave no misunderstanding of the direction and speed of trim at least. However, if the AOA disagree warning was triggered, and this also triggered a IAS disagree warning, it might have been difficult for the pilots to see the causal relation. If they would think that airspeed was unreliable and trusted AOA somehow more (despite the inevitable message), and the pilots would think that indeed they were approaching a high alfa condition, it would explain why they didn't intervene in the nose down trim by selecting the cutout switches.
Really hope the voice recorder will be found. |
Originally Posted by threemiles
(Post 10305428)
If data from one of two AOA sensors are bad there shall be no correcting nose up or down trim as the system cannot know which is the faulty sensor. If this behaviour was certified it is time to review the certification documents.
As it appears there is no guarantee that the pilots can timely recover from the upset when at low altitude this aircraft type should be grounded. The emergency AD is a joke. The AD addresses this and the expectation is that it will be employed and the fleets (of many other manufacturers) will benefit by this "lesson learned" |
Originally Posted by DaveReidUK
(Post 10305472)
I would be very surprised if any fault is found with the removed sensor, given that its replacement clearly didn't rectify the issue.
|
I just cannot believe that one faulty AOA sensor can make the aircraft trim nose down. There has to be more protection in the system design for this not to happen.
In the B777 which I am familiar with, each of the two ADIRUs (Air Data Inertial reference unit) receive both AOA inputs (There are two AOA sensors on most aircraft, same config on B737 also). This is compared with 'Calculated AOA' and a mid value is used. This is the redundancy built in the system on B777. Also each of the AOA sensor has two outputs, feed into two different computational channels. See the redundancy. There are actually 4 signals from two AOA sensors. The full text from the B777 AMM is as below. AOA Redundancy Management The AOA redundancy management logic uses a modified midvalue selection. The modified mid-value selection chooses the mid-value of these three AOA values: * Left corrected AOA * Right corrected AOA * Calculated AOA. The AOA redundancy management logic receives inputs from the inertial and air data systems to calculate the calculated AOA. Has any one in this forum have access to B737 MAX AMM (Pages from AMM Chap 34-20-00) and if you can post the same system info for B737 MAX redudancy management of AOA signals. I am just curious, and hope it does not bore other users. |
Why does a faulty AoA sensor trigger a 'Feel Diff Press'? It does not seem to be an informative message, especially when there is no hydraulic system failure.
And why does an aircraft have three pitots (so that one can be 'voted out') but only two alpha vanes? |
Salute! Good poop, Hi Tech. And I agree. I just cannot believe that one faulty AOA sensor can make the aircraft trim nose down. There has to be more protection in the system design for this not to happen. My FBW experience involved a quad computer design, but used only three processors until one failed. Great debate over "middle value" versus "average", but finally the decision was "most benign" value. So that was the data used for gains and total movement and AoA/gee protection. Only problem with that was failure detection, because a subtle failure could result a static data value, which is not a good deal for some maneuvers or changing speed/pitch/roll etc. Gums sends... |
Originally Posted by mross
(Post 10305506)
Why does a faulty AoA sensor trigger a 'Feel Diff Press'? It does not seem to be an informative message, especially when there is no hydraulic system failure.
And why does an aircraft have three pitots (so that one can be 'voted out') but only two alpha vanes? Hope that makes sense? |
Originally Posted by Flutter speed
(Post 10305379)
Just ran some numbers, in very general terms, depending on sensor position and factors like Mach number, in extreme cases... expect up to 5kts difference. Probably enough to trigger an IAS and ALT disagree on a modern airliner. For a plane flying at moderate speed the difference will be more towards 2kts.
As a mere PPL-I, I cannot fathom this. What am I missing? As a daytime teacher of software engineers, I am beginning to suspect that this design decision, if confirmed true, will ever so sadly be useful in my teaching, together with the Apollo 1 fire and the Ariane 501 explosion... :{ |
I've got a fairly basic question, my understanding is that a fundamental concept in aircraft design is that a control surface should never be able to be neutralized let alone over ridden by it's trim tab(s). Yet from some of what I've read on this thread it seems that that it's actually possible with the 737 for the force of the trim tab to exceed the capability of the main control surface?? That makes no sense to me at all...
|
Originally Posted by SLFinAZ
(Post 10305545)
I've got a fairly basic question, my understanding is that a fundamental concept in aircraft design is that a control surface should never be able to be neutralized let alone over ridden by it's trim tab(s). Yet from some of what I've read on this thread it seems that that it's actually possible with the 737 for the force of the trim tab to exceed the capability of the main control surface?? That makes no sense to me at all...
|
The technical, design, and certification aspects emerging from this accident are similar to other high profile accidents. A ‘first-pass’ evaluation avoiding blinding thoughts of error, identifies that the ‘initiating’ factors were technical malfunctions (also AF447, Swedish CRJ, 737 AMS). Subsequently, ‘enabling’ factors which determine if the event was recoverable or not, depend on the assumptions relating to awareness, knowledge, and human intervention. It would be interesting to understand how the design Failure Mode Effect Analysis (FMEA) dealt with an erroneous AoA input. What was assumed about the overall system integrity, failure frequency, and obviously the effect. And for the latter, what human contribution was assumed in mitigation. From the AD, it appears that there is now concern about awareness and meaning of the multiple alerts and range of instruments / systems affected; the difficulty for the crew to make sense of the overall situation. If there is a specific checklist, is it dependent on situation, context, or that crew performance would alway be adequate? In certification, what was assumed about input systems, normal or failed, not requiring ‘significant pilot strength, awareness, or attention’ (or similar judgemental text more often divorced from operational context)? In airworthiness, how many previous sensor failures, consequential events; what analysis was made / tracked. What was assumed in this process, assembling isolated features requiring in-depth system knowledge to foresee and circumvent an accident? In mitigation, will the AD information and check list be enough to counter the adverse effects as above; is this also an assumption that crews will now manage all future encounters, in all foreseeable circumstances, in every context? Or is this still a judgement, a short-term gamble ? |
Originally Posted by DaveReidUK
(Post 10305576)
Trimmable Horizontal Stabiliser
I'm not allowed to post URLs, but I hope a friendly moderator will review it and do the necessary magic. hotel tango tango papa sierra ://www.skybrary.aero/bookshelf/books/2627.pdf |
| All times are GMT. The time now is 17:11. |
Copyright © 2024 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.