Ethiopian airliner down in Africa
I agree with you on the sensor problem, but entirely disagree with the perception that a failure of the system would have minimal risk to the aircraft.
In the scenario given, hundreds of feet above ground, stick shaker, the captain is in a pretty tough situation and is trouble shooting- does stab memory item, switches stab cut-out. Alarms off, and thanks to yoke and increased thrust returns to some form of climb. At this stage, despite your argument that the pilot should be able to return trim to normal, he has just followed the procedures that Boeing themselves have instructed. Checklist complete, some stability back and now focus on safely trying to get back to ground.
Now, throughout this next phase, with the stab having X° nose down, it may not present itself as a problem due to the additional engine power, because, at increased thrust, as you've explained, the Max set-up has a tendency toward a higher AoA, which is why MCAS is required in the first place! So, in a sense, the characteristics of the MAX set-up will be masking the trim. MCAS is required for certification to counteract the increasing rotation around the CG caused by the both the thrust moment and aerodynamic behaviour of the engine cowling of the Max engine. So, in this precise stage of the hypothetical flight the PIC may not actually notice the plane being out of trim, and if he does, will surely not realise just how much out of trim he actually is. Any other time in normal MAX operation the autotrim or the MCAS trim would be operating anyway. MCAS, as you've said, is a certification requirement for MAX aircraft because of it's specific aerodynamic characteristics. What I'm portraying in this scenario is that if the pilot functioned perfectly during the emergency at 190, but in doing so cut-off the trim at X° nose down. That trim hasn't changed and the checklists didnt require the crew to make any manual trim wheel changes. By the time he actually realises how out of trim he is, he's doing 350, and despite the stab still being at the same X° nose down since cut-out, the forces have multiplied. Now if there is any nose down attitude or reduction in power then there's absolutely no chance to recover.
Obviously all hypothetical, an airspeed disagree on takeoff could lead to circumstances similarly, or a multitude of other possible reasons, however, in the situation that I've hypothesized, it would almost certainly always end in a similar steep nose down attitude.
In the scenario given, hundreds of feet above ground, stick shaker, the captain is in a pretty tough situation and is trouble shooting- does stab memory item, switches stab cut-out. Alarms off, and thanks to yoke and increased thrust returns to some form of climb. At this stage, despite your argument that the pilot should be able to return trim to normal, he has just followed the procedures that Boeing themselves have instructed. Checklist complete, some stability back and now focus on safely trying to get back to ground.
Now, throughout this next phase, with the stab having X° nose down, it may not present itself as a problem due to the additional engine power, because, at increased thrust, as you've explained, the Max set-up has a tendency toward a higher AoA, which is why MCAS is required in the first place! So, in a sense, the characteristics of the MAX set-up will be masking the trim. MCAS is required for certification to counteract the increasing rotation around the CG caused by the both the thrust moment and aerodynamic behaviour of the engine cowling of the Max engine. So, in this precise stage of the hypothetical flight the PIC may not actually notice the plane being out of trim, and if he does, will surely not realise just how much out of trim he actually is. Any other time in normal MAX operation the autotrim or the MCAS trim would be operating anyway. MCAS, as you've said, is a certification requirement for MAX aircraft because of it's specific aerodynamic characteristics. What I'm portraying in this scenario is that if the pilot functioned perfectly during the emergency at 190, but in doing so cut-off the trim at X° nose down. That trim hasn't changed and the checklists didnt require the crew to make any manual trim wheel changes. By the time he actually realises how out of trim he is, he's doing 350, and despite the stab still being at the same X° nose down since cut-out, the forces have multiplied. Now if there is any nose down attitude or reduction in power then there's absolutely no chance to recover.
Obviously all hypothetical, an airspeed disagree on takeoff could lead to circumstances similarly, or a multitude of other possible reasons, however, in the situation that I've hypothesized, it would almost certainly always end in a similar steep nose down attitude.
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Automatic disabling of MCAS if pilot counters with manual trim
Assuming everyone well motivated, which fleet grounding should do nicely, here's a guess on timescales:
Redesign and recode: 2 weeks
Safety case, prototype, testing, certification: 6 months
Fleet fitment: 3 months
My consultancy fees: $2.4m
Another SLFie who may be stating the obvious, but surely the goal of any airframer is to produce an aircraft that even the minimum standard of pilot can fly and have a decent chance of successfully troubleshooting in the event of something going awry? Not prejudging anything BTW
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Adding a spring system would involve incorporating new hardware on the Max. That new hardware would come at an on cost. The attraction of the Boeing solution is that it uses the existing hardware plus some new software.
Boeing got the cheaper solution certified so what is there motivation to go to a more expensive solution.
The question is is should it have been certified.
The question is is should it have been certified.
Why adding a spring ?
There is already an Elevator Feel System (EFS) which is controlled by a calculator, the Elevator Feel Computer module, thus it is controlled by software.
The elevator feel computer receives pitot pressure, hydraulic pressure, and mechanical inputs. The elevator feel computer sends metered hydraulic pressure output to the dual feel actuator on the elevator feel and centering unit. Feel hydraulic pressure in the elevator feel computer varies between 180 psi (base feel) and 1400 psi (maximum). The actual feel pressure to the dual feel actuator is determined by the hydraulic pressure from system A and system B, pitot pressure, and stabilizer mechanic
(from www.satcom.guru)
So, if the issue at stake was just about restoring the linearity of the control pitch force versus AoA angle, this could have been done just by enhancing the EFS system algorithms.
Just one little issue: the FCC doesn't feed the Elevator Feel computer with the AoA information.
It bet that Boeing engineers have balanced this against creating a new software function called MCAS whose implementation only affects the FCC module.
Now that the implementation costs of MCAS appear to be vastly different from the initial estimates, Boeing might reconsider enhancing the EFS.
I mean, that route might be preferable to adding even more complexity to a MCAS module whose existence and purpose is debatable.
Last edited by Luc Lion; 14th Mar 2019 at 12:16.
Originally Posted by mfeldt 
So there was a captain unable to quickly grasp what was happening because it was only his/her second flight on the type, and an FO with less than month of experience who couldn't grasp what was wrong??? I'm never going to fly again!
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You may want to add "figured out how the aircraft was certified in the first place with input from a sole sensor".
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LL,
Thanks. That would work, while requiring differing software and inputs for Max and rest of the family, however.
Could be that a dedicated separate system would be simpler to calibrate and quicker to certify.
Thanks. That would work, while requiring differing software and inputs for Max and rest of the family, however.
Could be that a dedicated separate system would be simpler to calibrate and quicker to certify.
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Another SLFie who may be stating the obvious, but surely the goal of any airframer is to produce an aircraft that even the minimum standard of pilot can fly and have a decent chance of successfully troubleshooting in the event of something going awry? Not prejudging anything BTW
However the first time such a pilot now gets behind the yoke is after the aircraft has been certificated, sold to an airline, built, delivered... Maybe they should involve FO Joe Average a bit earlier in the lifecycle.
Gone are the days when airline chief pilots would schlepp off to Seattle and throw the thing around the sky for a while before telling the executives whether it was suitable to buy. Now if it promises a 1% operational cost saving it is bought straight off the CAD screen.
Back in the 1960s there was even a BOAC inspection and flight-test team at Everett!
Last edited by El Bunto; 14th Mar 2019 at 12:46.
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look at the diagram below ; the "control column stab trim cutout switches" only operate on the electric path of the electric trim buttons.
The STS is on the other electric path, the path of the autopilot system.
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But the question was what will Boing do. And the answer is, whatever gets the fleet flying the quickest.
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Originally Posted by Luc Lion
Why adding a spring ?
There is already an Elevator Feel System (EFS) which is controlled by a calculator, the Elevator Feel Computer module, thus it is controlled by software.
There is already an Elevator Feel System (EFS) which is controlled by a calculator, the Elevator Feel Computer module, thus it is controlled by software.
Dave.
Last edited by dgordon42; 14th Mar 2019 at 16:22.
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These were discussed before - they are not MCAS as they occurred with autopilot ON, A previous poster pointed out that this change of level happens on NG as well not only 'this aircraft' and is due to pressure setting mismatch.
The purpose of ASRS is to highlight issues that need attention without come back on crews that may have made errors. A common error (and pressure setting problems are VERY common around 2 a day reported) indicates that attention is needed in one or more areas. Unfortunately, journalists are not capable of reading these reports and understanding what is being reported indeed they do NOT want to understand they want things to be as inflammatory as possible so readers click on their headlines. This is not really useful to the industry.
The purpose of ASRS is to highlight issues that need attention without come back on crews that may have made errors. A common error (and pressure setting problems are VERY common around 2 a day reported) indicates that attention is needed in one or more areas. Unfortunately, journalists are not capable of reading these reports and understanding what is being reported indeed they do NOT want to understand they want things to be as inflammatory as possible so readers click on their headlines. This is not really useful to the industry.
A few quick points from memory
Pilots not being able to usual scan and see what they are looking for
Pilots not familiar with new displays
Pilots not Knowing what the "Maint" msg means and unable to find it in pilots notes
Pilots not knowing what a particular switch labelled SEL was for.
That is abysmal conversion training (classroom and sim) , abysmal documentation and DANGEROUS
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Choose : You just got airborne and cleaned up in IMC;
2 Warnings come up, screaming and flashing, ringing and tinkling.
- Overspeed
- STALL
Choose what you will do because : Some failing sensors, or some twisted code lines, are giving you and your co-pilot conflicting information and warnings.
Then, like in most modern airframes where ( willing or not) some parts of the flight control management system are ALWAYS ON, the airplane itself starts trimming itself.
But to what information IS the airplane trimming itself? ? ? ? ?
Do you know?
Does it know?
Food for thought.
2 Warnings come up, screaming and flashing, ringing and tinkling.
- Overspeed
- STALL
Choose what you will do because : Some failing sensors, or some twisted code lines, are giving you and your co-pilot conflicting information and warnings.
Then, like in most modern airframes where ( willing or not) some parts of the flight control management system are ALWAYS ON, the airplane itself starts trimming itself.
But to what information IS the airplane trimming itself? ? ? ? ?
Do you know?
Does it know?
Food for thought.
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I am going to assume you don’t fly for a living. There is no way you would not recognize a stab out of trim condition while hand flying the aircraft. In addition the aircraft would only accelerate like you portray if the pilot failed to adjust power to maintain the desired airspeed.
At 3,000 ft, the stick shaker starts going like mad, causing a hell of a racket, and you think you have an airspeed problem and might be stalling. Sure, the controls are getting heavier and heavier, and you trim a bit for that, but this is caused by the aircraft stalling - isn’t it? (You cannot hear the trimmer, over the din of the stick-shaker.) So you have to let the nose drop, untill you are sure you have enough airspeed.
Ok, you are getting a bit low now, time to pull back. Ahh, but the aircraft will not respond - pull as hard as you like, but the stick feels jammed (you need 60 kg of force to counter full stab-trim). You shout to the f/o to help pull, but the ground is coming up fast.... End of short story...
Silver
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I'm a 30-year flight test engineer and am currently helping to rewrite the handling qualities flight test manual for the US Navy Test Pilot School. We focus more on military than transport aircraft, but we do test and fly 737-derivative airplanes (P-8A Poseidon and the C-40 Clipper) and the test methods are universally applicable.
Windup turns are done with set thrust, yes. Power is set as required to maintain the specified airspeed. What is being compared is the response of the airframe to increasing AOA or g at a fixed airspeed, and throttles are fixed during the maneuver to avoid contaminating the results with another independent variable. Several things can be learned from WUTs, including control force or deflection as a function of load factor or AOA, buffet characteristics as a function of AOA, and structural characteristics as a function of g.
While each test point is conducted at a specific power setting, the tests are typically conducted at a range of power settings. This provides a chance to assess the effect of power setting on the various aforementioned characteristics.
MCAS is (at the core) merely a trim application system, designed to reduce the control forces which develop at higher AOA and g with the new-and-repositioned engines. As such, it is a handling qualities difference that is definitely related to thrust line changes. Those differences would typically be revealed by a series of WUT test points. In this case, a WUT at high thrust would be worse, because of the increased pitch-with-power tendencies.
Originally Posted by Brandon
MCAS is (at the core) merely a trim application system, designed to reduce the control forces which develop at higher AOA and g with the new-and-repositioned engines.
In any case, the mention of wind up turns in the context of pax 737s is a bit of a furphy, IMO. The main issue is high AOA and reducing stick force, however you arrive at the high AOA.
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I think that deserves some clarification. I don't fully agree.
I'm a 30-year flight test engineer and am currently helping to rewrite the handling qualities flight test manual for the US Navy Test Pilot School. We focus more on military than transport aircraft, but we do test and fly 737-derivative airplanes (P-8A Poseidon and the C-40 Clipper) and the test methods are universally applicable.
Windup turns are done with set thrust, yes. Power is set as required to maintain the specified airspeed. What is being compared is the response of the airframe to increasing AOA or g at a fixed airspeed, and throttles are fixed during the maneuver to avoid contaminating the results with another independent variable. Several things can be learned from WUTs, including control force or deflection as a function of load factor or AOA, buffet characteristics as a function of AOA, and structural characteristics as a function of g.
While each test point is conducted at a specific power setting, the tests are typically conducted at a range of power settings. This provides a chance to assess the effect of power setting on the various aforementioned characteristics.
MCAS is (at the core) merely a trim application system, designed to reduce the control forces which develop at higher AOA and g with the new-and-repositioned engines. As such, it is a handling qualities difference that is definitely related to thrust line changes. Those differences would typically be revealed by a series of WUT test points. In this case, a WUT at high thrust would be worse, because of the increased pitch-with-power tendencies.
I'm a 30-year flight test engineer and am currently helping to rewrite the handling qualities flight test manual for the US Navy Test Pilot School. We focus more on military than transport aircraft, but we do test and fly 737-derivative airplanes (P-8A Poseidon and the C-40 Clipper) and the test methods are universally applicable.
Windup turns are done with set thrust, yes. Power is set as required to maintain the specified airspeed. What is being compared is the response of the airframe to increasing AOA or g at a fixed airspeed, and throttles are fixed during the maneuver to avoid contaminating the results with another independent variable. Several things can be learned from WUTs, including control force or deflection as a function of load factor or AOA, buffet characteristics as a function of AOA, and structural characteristics as a function of g.
While each test point is conducted at a specific power setting, the tests are typically conducted at a range of power settings. This provides a chance to assess the effect of power setting on the various aforementioned characteristics.
MCAS is (at the core) merely a trim application system, designed to reduce the control forces which develop at higher AOA and g with the new-and-repositioned engines. As such, it is a handling qualities difference that is definitely related to thrust line changes. Those differences would typically be revealed by a series of WUT test points. In this case, a WUT at high thrust would be worse, because of the increased pitch-with-power tendencies.
AF447 => An iced over AOA probe gives false info, and it does not matter what happened the next minutes, they never figured out in time what the actual problem was.
The B-2 crash => See? With failing sensors there simply is NO TIME.
The X-31 crash => (Where pitot anti-ice was not connected) => The plane goes out of control SO FAST, the only way out is to eject.
When given false information from failing sensors, (and not knowing it) or the failure codes redraw so fast on screen that you simply can not keep track of all of them => It is game over.
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Sure about that? Someone new to the aircraft, captain average, and a bit tired.
At 3,000 ft, the stick shaker starts going like mad, causing a hell of a racket, and you think you have an airspeed problem and might be stalling. Sure, the controls are getting heavier and heavier, and you trim a bit for that, but this is caused by the aircraft stalling - isn’t it? (You cannot hear the trimmer, over the din of the stick-shaker.) So you have to let the nose drop, untill you are sure you have enough airspeed.
Ok, you are getting a bit low now, time to pull back. Ahh, but the aircraft will not respond - pull as hard as you like, but the stick feels jammed (you need 60 kg of force to counter full stab-trim). You shout to the f/o to help pull, but the ground is coming up fast.... End of short story...
At 3,000 ft, the stick shaker starts going like mad, causing a hell of a racket, and you think you have an airspeed problem and might be stalling. Sure, the controls are getting heavier and heavier, and you trim a bit for that, but this is caused by the aircraft stalling - isn’t it? (You cannot hear the trimmer, over the din of the stick-shaker.) So you have to let the nose drop, untill you are sure you have enough airspeed.
Ok, you are getting a bit low now, time to pull back. Ahh, but the aircraft will not respond - pull as hard as you like, but the stick feels jammed (you need 60 kg of force to counter full stab-trim). You shout to the f/o to help pull, but the ground is coming up fast.... End of short story...
This is one of the better comments in this discussion. As a long-time flight test engineer with personal experience flying jumpseat aboard Boeing commercial-class airplanes during many test flights, and many hundreds of hours in the related simulators both testing and flying the simulators, I have personally witnessed aircrew in simulated emergency situations with all four feet standing on the bottom of the forward display panel to gain enough leverage to pull with >100 lb on the yoke to control the airplane (and specifically in runaway stab trim and hyd-fail situations). Nobody had a hand free to fiddle with manual trim or deal with the stab trim cutout switch and its guard (at least on the aircraft types I've tested).
Yes, these things are addressed in pilot training. But I've also watched extremely-intelligent and supremely-trained flight test pilots miss very obvious things due to the confusion surrounding automation (I helped test a major glass-cockpit upgrade to a Boeing 707-derived military jet, which included adding modern-day Honeywell autopilot systems). I've personally sat behind them in the jump seat and watched them completely miss what was going on outside the window, as two pilots and a flight engineer got wrapped up trying to figure out some stupid aspect of VNAV operation, to the point where a safety "knock it off" call had to be made to get them to break their "target fixation" on the troubleshooting.
Watch the video of the recreation of the cockpit troubleshooting in the AeroPeru crash due to tape covering the Pitot-static ports... two well-trained pilots so confounded by an airplane acting differently than they expected, that some very basic hand-flying principles and alternate information sources were missed for nearly an hour.
It's easy to armchair-quarterback a situation after the fact - but I've seen it all happen enough times to recognize the reality of the man-machine interfaces.
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That is all good and well, and always works when all sensors are giving you and the systems the correct information.
AF447 => An iced over AOA probe gives false info, and it does not matter what happened the next minutes, they never figured out in time what the actual problem was.
The B-2 crash => See? With failing sensors there simply is NO TIME.
The X-31 crash => (Where pitot anti-ice was not connected) => The plane goes out of control SO FAST, the only way out is to eject.
When given false information from failing sensors, (and not knowing it) or the failure codes redraw so fast on screen that you simply can not keep track of all of them => It is game over.
AF447 => An iced over AOA probe gives false info, and it does not matter what happened the next minutes, they never figured out in time what the actual problem was.
The B-2 crash => See? With failing sensors there simply is NO TIME.
The X-31 crash => (Where pitot anti-ice was not connected) => The plane goes out of control SO FAST, the only way out is to eject.
When given false information from failing sensors, (and not knowing it) or the failure codes redraw so fast on screen that you simply can not keep track of all of them => It is game over.
https://www.dailymotion.com/video/x6m3uwb
