Indonesian aircraft missing off Jakarta
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A bit surprised this hasn't been posted previously - it's from the Monday Seattle Times:
https://www.seattletimes.com/busines...ion-air-crash/
Quote: He said it was done primarily because the much bigger engines on the MAX changed the aerodynamics of the jet and shifted the conditions under which a stall could happen. That required further stall protection be implemented to certify the jet as safe.
https://www.seattletimes.com/busines...ion-air-crash/
Quote: He said it was done primarily because the much bigger engines on the MAX changed the aerodynamics of the jet and shifted the conditions under which a stall could happen. That required further stall protection be implemented to certify the jet as safe.
To alleviate this nose up pitch, you need to either:.. a. reduce power, which is a counter-intuitive action during an incipient stall; or b. trim forward. The trim operates on the stab, not the elevator, and can give you a lot more control authority to oppose the engine pitch-up moment.
But here is the problem - the 737-max has much bigger engines, and a larger thrust-pitching moment. So at slow speed approaching the stall, giving too much recovery power may simply flip the aircraft over backwards. Which would really spoil your day. To (partially) mitigate this, you want an auto-forward stab-trim, to give the pilot more nose-down control authority at slow speeds.
Enter the new MCAS system.
However, MCAS can generate its own litany of problems and failure modes. If it gives full nose-down pitch during a stall, you still have that forward trim applied during the dive-recovery, which will hamper the recovery. (Which is why a stick-pusher combined with a thrust limiter might be a better option.). Likewise, if MCAS gets false information and operates at 250 kts, it is possible it will overpower the elevator and pitch you down. And if the auto-trim was not noticed, that trim will still be applied while you are trying to recover from a dive, hampering any recovery.
So in attempting to alleviate one problem, you have generated several others. MCAS looks like design firefighting, rather than a clean-sheet holistic design philosophy. And in making this fix, nobody went through all the possible failure scenarios - including a failure giving full forward trim at 250 kt.
I think MCAS needed to be much more sophisticated, than a simple stall alleviation device. In fact, it is likely that the 737 needs a complete rethink and revision of its control system, which was designed in 1963 for the 727. There is only so much design-firefighting you can do, with a 60 year old design.
Silver
Last edited by silverstrata; 15th Nov 2018 at 03:06.
Thing is, if you look up the "power and pitch" settings, for UAS, they tell you to set figures that are almost exactly what you need to continue 'as you are'.
So, if you are at FL350, doing .78 and set the pitch and power in the table, you will still be going along much as before.
Now, while we can all see the value in this, I'd suggest you need a power setting that is safe, and start on down straightaway if practible.
I'd set a medium power setting, one that I know would prevent stall and overspeed. This would work nicely at low altitude.
If it happened upstairs, my choice would be the same, and descend.
If for some reason, you couldn't descend, then the numbers in the table are what you need.
So, if you are at FL350, doing .78 and set the pitch and power in the table, you will still be going along much as before.
Now, while we can all see the value in this, I'd suggest you need a power setting that is safe, and start on down straightaway if practible.
I'd set a medium power setting, one that I know would prevent stall and overspeed. This would work nicely at low altitude.
If it happened upstairs, my choice would be the same, and descend.
If for some reason, you couldn't descend, then the numbers in the table are what you need.
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Thing is, if you look up the "power and pitch" settings, for UAS, they tell you to set figures that are almost exactly what you need to continue 'as you are'. So, if you are at FL350, doing .78 and set the pitch and power in the table, you will still be going along much as before.
This incident is likely nothing to do with UAS.
Please read some back-pages.
Silver
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What Boeing were trying to guard against with MCAS, is a repetition of the Sept 2007 pitch up event at Bournemouth, where a 737 pitched to 44 degrees nose up during a stall event. This happened because as the aircraft approached the stall the pilot gave full thrust to alleviate the stall, and because the engine pitch moment is greater than the elevators (when flying at slow speeds), the engines can overcome elevator authority and pitch the aircraft up uncontrollably.
To alleviate this nose up pitch, you need to either:.. a. reduce power, which is a counter-intuitive action during an incipient stall; or b. trim forward. The trim operates on the stab, not the elevator, and can give you a lot more control authority to oppose the engine pitch-up moment.
But here is the problem - the 737-max has much bigger engines, and a larger thrust-pitching moment. So at slow speed approaching the stall, giving too much recovery power may simply flip the aircraft over backwards. Which would really spoil your day. To (partially) mitigate this, you want an auto-forward stab-trim, to give the pilot more nose-down control authority at slow speeds.
Enter the new MCAS system.
However, MCAS can generate its own litany of problems and failure modes. If it gives full nose-down pitch during a stall, you still have that forward trim applied during the dive-recovery, which will hamper the recovery. (Which is why a stick-pusher combined with a thrust limiter might be a better option.). Likewise, if MCAS gets false information and operates at 250 kts, it is possible it will overpower the elevator and pitch you down. And if the auto-trim was not noticed, that trim will still be applied while you are trying to recover from a dive, hampering any recovery.
So in attempting to alleviate one problem, you have generated several others. MCAS looks like design firefighting, rather than a clean-sheet holistic design philosophy. And in making this fix, nobody went through all the possible failure scenarios - including a failure giving full forward trim at 250 kt.
I think MCAS needed to be much more sophisticated, than a simple stall alleviation device. In fact, it is likely that the 737 needs a complete rethink and revision of its control system, which was designed in 1963 for the 727. There is only so much design-firefighting you can do, with a 60 year old design.
Silver
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A question was posted a couple of pages ago, asking who defined an "average pilot", taking issue with the "tone" of the Boeing statement about not giving too much info to said "average pilot" and implying the airframer was somehow in favour of the idea of a "lowest common denominator" pilot.
Nothing, from my perspective, could be further from the truth than that last item. But to address the points raised:
The concept of the "average pilot", or terms very similar to that, is found in many places in part 25. Picking a regulation at not-quite-random, 14CFR25.143 (b) states (my emphasis)
You'll find similar wording throughout the regulations where the ability of the pilot to do a task will influence whether you pass or fail the requirement. The purpose of these words is to prevent the Chuck Yeager of any specific company going out, flying some ridiculously perfect and intricate manoeuvre, and saying "it's fine, I can fly it, must be ok". Test pilots are, for the purpose of assessing the piloting demands of the aircraft, not allowed to use their innate flying skills (they can use them for the "academic data gathering manoeuvres, where pilot workload and human factors are not a criteria) - instead they have to use their TEST PILOT skills - and specifically the one which says "I can determine, from my knowledge of how pilots in general interact with aeroplanes, and what they can and cannot be expected to manage, that this is/is not acceptable". The term frequently used for this is "average pilot".
Who decides exactly what the average pilot can do - collectively, the Test Pilot community; it is ultimately a judgement, but one made by a group, including not just the company's own pilots but also the Test Pilots from the various regulatory agencies. And they will indeed base that judgement on an understanding of the target pilot for the machine in question - so a private pilot won't be assumed able to cope with as much as an airline one. And as awareness builds of shifts in the pilot population and their skillset - as in, for example, many fewer with a military/fighter background, the skill level assumed may be degraded or amended, OR it may be identified that remedial action in training etc is required to bridge the gap. Usually it's all of these things.
And why do I say the OEM has no interest in downgrading the average pilot? Look at the first point - the smarter the "average pilot", the easier it is to pass the cert tests, even with a really clunky and horrible human interface or terrible flying characteristics. We'd love to assume every pilot was the hero who saves the day - every failure would be classed as benign, and certification would be a breeze. But that's not realistic, nor frankly fair to the people we hand the machine over to when the cert is done.
Nothing, from my perspective, could be further from the truth than that last item. But to address the points raised:
The concept of the "average pilot", or terms very similar to that, is found in many places in part 25. Picking a regulation at not-quite-random, 14CFR25.143 (b) states (my emphasis)
(b) It must be possible to make a smooth transition from one flight condition to any other flight condition without exceptional piloting skill, alertness, or strength, and without danger of exceeding the airplane limit-load factor under any probable operating conditions, including
Who decides exactly what the average pilot can do - collectively, the Test Pilot community; it is ultimately a judgement, but one made by a group, including not just the company's own pilots but also the Test Pilots from the various regulatory agencies. And they will indeed base that judgement on an understanding of the target pilot for the machine in question - so a private pilot won't be assumed able to cope with as much as an airline one. And as awareness builds of shifts in the pilot population and their skillset - as in, for example, many fewer with a military/fighter background, the skill level assumed may be degraded or amended, OR it may be identified that remedial action in training etc is required to bridge the gap. Usually it's all of these things.
And why do I say the OEM has no interest in downgrading the average pilot? Look at the first point - the smarter the "average pilot", the easier it is to pass the cert tests, even with a really clunky and horrible human interface or terrible flying characteristics. We'd love to assume every pilot was the hero who saves the day - every failure would be classed as benign, and certification would be a breeze. But that's not realistic, nor frankly fair to the people we hand the machine over to when the cert is done.
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If MCAS only activates with no flap deployed, and activates more aggressively at slower speed and therefore likely lower altitude, then I think that my first action on intermittent runaway stab trim close to the ground might be to pop out flaps 1 and see if that stops it immediately. Just a thought.
Of course the pilot would have to actually know of the existence of the system and its limitations first in order to be able to do that. The Lion Air crew didn't
Of course the pilot would have to actually know of the existence of the system and its limitations first in order to be able to do that. The Lion Air crew didn't
Last edited by rmac2; 15th Nov 2018 at 05:58. Reason: clarification
What Boeing were trying to guard against with MCAS, is a repetition of the Sept 2007 pitch up event at Bournemouth, where a 737 pitched to 44 degrees nose up during a stall event. This happened because as the aircraft approached the stall the pilot gave full thrust to alleviate the stall, and because the engine pitch moment is greater than the elevators (when flying at slow speeds), the engines can overcome elevator authority and pitch the aircraft up uncontrollably.
<snip>
I think MCAS needed to be much more sophisticated, than a simple stall alleviation device. In fact, it is likely that the 737 needs a complete rethink and revision of its control system, which was designed in 1963 for the 727. There is only so much design-firefighting you can do, with a 60 year old design.
Silver
Sadly, this wouldn't be the first time that the 'fix' for an accident cause was a main contribution in another accident. I rather doubt it'll be the last...
Cranbrook 737 crashed because - as a safety enhancement - Boeing disabled the thrust reverser in-flight by closing the T/R hydraulic isolation valve, which allowed the T/R to re-deploy when they rejected the landing to miss the snowplow - https://aviation-safety.net/database...?id=19780211-0
Lauda 767 crashed because a reverser deployed in-flight when a mis-rigged sensor caused the auto-restow system - implemented to prevent a repeat of Cranbrook - allowed the isolation valve to open in flight. (Lauda is a bit personal and a very bad memory for me - auto-restow wasn't my system, but I was deeply involved in the investigation - I was half-sick for weeks - made worse because the effective gag order in effect during an accident investigation meant I couldn't talk to anyone about what was going on.)
I'm reasonably sure MCAS was properly certified, with the associated FMEA (Failure Modes and Effects Analysis), and perhaps a SSA (System Safety Assessment) - at least in the Propulsion world FMEAs are not probabilistic (basically shows no single failure is unsafe) and the SSA covers the probabilities for multiple or combinations of failures.
Again, I'm not involved and have no direct knowledge of what happened, but I can guess: Someone did an FMEA of MCAS - determined that the worse case failure was no worse than a stab trim runaway, which has a procedure - and decided it was acceptable. The people that reviewed it (including, in all likelihood, an FAA specialist) didn't dispute that - not recognizing how bad it might be if an overloaded crew didn't figure out what was happening.
I'm also reasonably sure there are some 737 flight control types who are pretty sick about it right now. I've never had an accident or serious incident attributed to a system that I was responsible for (and I pray that remains the case). But I know how the Lauda investigation affected me and trust me, it wasn't pretty. I can only imagine how much worse it would be if it was my system...
When it's all said and done, and a fix is certified and implemented, I have little doubt there will be some retirements and/or resignations among the 737 flight controls ranks - perhaps worse.
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If MCAS only activates with no flap deployed, and activates most positively at slower speed and therefore likely lower altitude, then I think that my first action on intermittent runaway Stab trim close to the ground might be to pop out flaps 1 and see if that stops it. Just a thought.
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Quote: He said it was done primarily because the much bigger engines on the MAX changed the aerodynamics of the jet and shifted the conditions under which a stall could happen. That required further stall protection be implemented to certify the jet as safe.
Pushed the 737 too far? Max or maxxed out?
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NTSC (KNKT) Chairman Soerjanto Tjahjono gave interviews on Monday, saying that the current working assumption is that the pinger was damaged or detached in the impact, since it can no longer be heard. On Sunday, KNKT and Basarnas announced that they would extend the CVR search for another two weeks at least. KNKT and Basarnas also acquired an ROV with "side-scan, multibeam" sonar from a regional partner to assist in the search. They say it can identify objects a few inches below the mud.
It's worth noting - especially in the sea of speculation above! - that, so far as I am aware, there's no specific news on the FDR data either. Nothing that Boeing or the FAA have said so far is explicitly linked to the FDR for this airframe. The FAA directive could be part-based on the FDR data; but it could equally - imho - be based on what is known about the flight from ATC data and communications, plus paperwork and witness statements about the prior difficulties that this particular a/c had recently encountered. So, previous flight crews could all have said that, having gone to manual flight following sensor disagree alerts, they then encountered strong and unexpected nose-down stab trim, uncommanded by them.
Last edited by Gary Brown; 15th Nov 2018 at 07:01. Reason: My fingers don't know my stabs from my elevators......
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This is one of the reasons 737 could not be equipped with P&W GTF engine. The base design (wing-ground clearance) is from the 60's. It worked fine for turbojets, needed squashed nacelles for larger turbofan but GTF is just step too far. You can only stretch the design so much...
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In my humble opinion much too much is made out of the MCAS issue. All the fleet transition and training documents available from the manufacturer have included it. Anyhow, as Boeing have mentioned themselves, a malfunction with it is already covered by present procedures. It's operation is transparent to the crew, so in case of false MCAS activation due to unreliable AOA data we basically have a runaway stabilizer - and flipping the cutout switches mitigates the problem.
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I can't get past similarities of this crash and FZ981.
In case of FZ981, does FDR show that the pilot was really holding trim button pressed, or does it only show that trim was moving, and the reason as to why is a speculation?
In case it's a speculation, could there be an alternative explanation that "an undocumented / unknown system trimmed the aircraft pitch down"? Can we with certainty say that there's no MCAS or similar software on NG? In case there was, would its actions be recorded on the FDR or just the effects?
In case of FZ981, does FDR show that the pilot was really holding trim button pressed, or does it only show that trim was moving, and the reason as to why is a speculation?
In case it's a speculation, could there be an alternative explanation that "an undocumented / unknown system trimmed the aircraft pitch down"? Can we with certainty say that there's no MCAS or similar software on NG? In case there was, would its actions be recorded on the FDR or just the effects?
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This is one of the reasons 737 could not be equipped with P&W GTF engine. The base design (wing-ground clearance) is from the 60's. It worked fine for turbojets, needed squashed nacelles for larger turbofan but GTF is just step too far. You can only stretch the design so much...
Having had a long relationship with CFM International, and having always being a single-source engine aircraft, it seemed straightforward to stay with CFM and use only a single engine type.
The A320 has always been offered with engines from different manufacturers, and they continued that for the neo.
By the way, even the first versions of the 737 had turbofan engines, albeit low-bypass turbofans (P&W JT8D).
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Can I just clarify something? If MCAS decides to activate inadvertantly and you either pull against it or trim via the stick trimmer, will that prevent MCAS from adjusting the trim? If I were to hear the wheel of fortune winding away but found it was trimming in the other direction...Yikes!
The US NTSB (not FAA), representing the state of manufacture will have the status of an accredited representative to the investigation, and can choose in turn to consult other parties (Boeing, GE, FAA, etc), but none of those parties will release data independently.
