MAX’s Return Delayed by FAA Reevaluation of 737 Safety Procedures
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Stall at low altitude and airspeed not a good thing. Can this help explain why the second part of MCAS, for lower airspeeds, was added and made stronger than original MCAS? Are there data on this issue from wind tunnel tests or flight tests – does anyone know where I can find them?
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One question if anyone could answer, is there a flight regime where the electric trim motors/their gear train would not have had the power to further manipulate the stab? Seeing how great the forces were, and even though the electric trim is electrically driven and aided I imagine by a reduction gear set, it would not seem to me to be multiple magnitudes more powerful than the handle+wheel-cable/pulley driven manual system (which itself is a lever and force reduction aided system (handle+wheel plus cable movement per turn vs pulley size on the jack screw drive side) with certain mechanical advantage).Could it be postulated that at a certain speed and AOA, the electric trim may have stalled out itself, unable to bring the stab nose up even if commanded, seeing that the manual system was practically seized?
A stalled stabilise drive can occur on some types where it is possible. with a very high elevator hinge moment. to apply a load on the tailplane so high that the drive mechanism is completely defeated and fails to produce any movement. It is unlikely that this will ever occur with the aeroplane in trim since a pilot is most unlikely ever to require a manoeuvre involving such large elevator angles and high stick forces. In an upset of some kind, however, where the stabiliser has achieved a gross out-of-trim condition, this position can arise. In turbulence, for example, a pilot might have run the stabiliser rather a long way away from the trimmed condition, a large and rapid change in speed could produce a very high stick force or the autopilot height lock in a long draught could have run the stabiliser a long way. All these could result in a grossly out-of-trim stabiliser setting with the immediate need of a very high stick force to keep control of the flight path.
While it is obviously disturbing to find that the trim will not run when signalled to relieve a high stick force, very recent tests by the author have shown that recovery from this condition is comparatively simple. Just sitting there and pulling a very high load, while it is the instinctive reaction in order to produce the required flight path, only compounds the difficulty. The stabiliser will not run until the hinge moment is relieved. So, keeping the trim button engaged, slowly ease ofl‘ the stick force. The aeroplane will not react very strongly because you are not doing much trade with all that force anyway. As the force falls through the critical value (actually, about 120 lb. pull on the type tested — although you won't know this, of course) the stabiliser will run and the aeroplane will come back under control.
Some aeroplanes with powered stabilisers and manual elevators have been cleared against a full aircraft nose down runaway stabiliser condition from a simulated jet upset manoeuvre, but only after the maximum nose down stabiliser range has been restricted. Tests in a particular ease showed that. provided the proper drill was followed, the aeroplane could be recovered although not, of course, within its normal speed limitations. The dive having been entered and speed brakes pulled, both pilots had to hold maximum up-elevator forces. The aeroplane stabilised at 0.93 true Mach number in the dive and held this condition for a comparatively long period of time. As the EAS increased at constant Mach number with decreasing altitude the stability of the aeroplane returned very slowly, the elevator began to take effect and very slowly the Mach number started to fall. Once this occurred it could be seen that the recovery would be made. As the Mach number decreased the full elevator effect returned. pitch attitude decreased and the speed began to fall. The recovery rapidly improved in quality thereafter, although very high stick forces had to be maintained until level flight was established at a much lower altitude.
Power was not reduced from the cruise setting during the whole of this manoeuvre because containment of the initial dive angle was so marginal that nothing, but nothing, was allowed to add any nose down pitching moment —which is just what reducing power would have done. It should be pointed out that this manoeuvre was flown on a type where it had been proved that the stabiliser drive could not be stalled in the presence of very high stick forces.
How this rather hairy manoeuvre will be regarded by an airline pilot is not known, but it must be of some comfort to know that there is a drill which will provide a recovery, although at the expense of at large height loss. Remember that this is for a stabiliser stuck at full nose down. a most unlikely event. For the more likely [but still improbable, of course] case of a stalled drive just remember the drill described earlier: keep the trim switch selected and ease off the high load on the stick. When the stabiliser starts running again ease off the stick force progressively and stop trimming when the stick force is down to a low value.
Again. in order to be fair to the designers it must be pointed out that originally there were no requirements relating to stabiliser drive stalling, although, in some cases, an arbitrary figure of about 100 lb. stick force was used. In the light of more recent experience, requirements in this area are being applied. This is always difficult to handle retrospectively: the easiest way out has been simply to limit the maximum amount of aircraft nose down stabiliser range under power.
While it is obviously disturbing to find that the trim will not run when signalled to relieve a high stick force, very recent tests by the author have shown that recovery from this condition is comparatively simple. Just sitting there and pulling a very high load, while it is the instinctive reaction in order to produce the required flight path, only compounds the difficulty. The stabiliser will not run until the hinge moment is relieved. So, keeping the trim button engaged, slowly ease ofl‘ the stick force. The aeroplane will not react very strongly because you are not doing much trade with all that force anyway. As the force falls through the critical value (actually, about 120 lb. pull on the type tested — although you won't know this, of course) the stabiliser will run and the aeroplane will come back under control.
Some aeroplanes with powered stabilisers and manual elevators have been cleared against a full aircraft nose down runaway stabiliser condition from a simulated jet upset manoeuvre, but only after the maximum nose down stabiliser range has been restricted. Tests in a particular ease showed that. provided the proper drill was followed, the aeroplane could be recovered although not, of course, within its normal speed limitations. The dive having been entered and speed brakes pulled, both pilots had to hold maximum up-elevator forces. The aeroplane stabilised at 0.93 true Mach number in the dive and held this condition for a comparatively long period of time. As the EAS increased at constant Mach number with decreasing altitude the stability of the aeroplane returned very slowly, the elevator began to take effect and very slowly the Mach number started to fall. Once this occurred it could be seen that the recovery would be made. As the Mach number decreased the full elevator effect returned. pitch attitude decreased and the speed began to fall. The recovery rapidly improved in quality thereafter, although very high stick forces had to be maintained until level flight was established at a much lower altitude.
Power was not reduced from the cruise setting during the whole of this manoeuvre because containment of the initial dive angle was so marginal that nothing, but nothing, was allowed to add any nose down pitching moment —which is just what reducing power would have done. It should be pointed out that this manoeuvre was flown on a type where it had been proved that the stabiliser drive could not be stalled in the presence of very high stick forces.
How this rather hairy manoeuvre will be regarded by an airline pilot is not known, but it must be of some comfort to know that there is a drill which will provide a recovery, although at the expense of at large height loss. Remember that this is for a stabiliser stuck at full nose down. a most unlikely event. For the more likely [but still improbable, of course] case of a stalled drive just remember the drill described earlier: keep the trim switch selected and ease off the high load on the stick. When the stabiliser starts running again ease off the stick force progressively and stop trimming when the stick force is down to a low value.
Again. in order to be fair to the designers it must be pointed out that originally there were no requirements relating to stabiliser drive stalling, although, in some cases, an arbitrary figure of about 100 lb. stick force was used. In the light of more recent experience, requirements in this area are being applied. This is always difficult to handle retrospectively: the easiest way out has been simply to limit the maximum amount of aircraft nose down stabiliser range under power.
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Following this Boeing 720 (a 707 variant) accident in 1963 the investigators determined that the stabilizer trim stalled for exactly that reason. In that case it was the crew who ran the trim well away from the neutral column force position in response to a strong updraft, at least according to what the investigators theorized from the limited data recorder info. My understanding is that the 737 stab control system design was based on the 707 system, but I don't know the system force differences between the models at extreme out of trim conditions.
https://lessonslearned.faa.gov/ll_ma...abID=1&LLID=66
https://lessonslearned.faa.gov/ll_ma...abID=1&LLID=66
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Regulators must only allow the aircraft to return to service when system redundancy will allow safe flight throughout the flight envelope. AOA sensors do fail regularly, which i understand will render MCAS unserviceable, and in that event the aircraft needs to still be stable throughout the flight envelope. Crew must be aware of the consequences of AOA and/or MCAS failures, and simulators must reflect true aircraft behaviour in mandatory training for such events. Manual trim authority at all speed and configurations must be discovered and shared with flight crew for all 737 variants following these crashes in addition.
MCAS has turned out to be a powerful deadly system when it goes wrong. After such events, training must fully and accurately reveal in depth true aircraft behaviour and be trained for throughout the extremes of speed and pitch regimes to include electric and manual trim limitations.
It's time for regulators and Boeing to do the absolute maximum and reveal all data to flight crew that need to regain faith in such systems. It is also time for all flight crew to demand nothing short of full disclosure and proper training into all such issues now revealed by these crash investigations.
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One thing that should have resulted after the first crash,and most certainly has,after the second..everyone now understands what the stab trim cutout switches do...Stated,"there but for the grace of god"
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Test Flight Nov 2 Looks Liked Repeated Stalls
https://flightaware.com/live/flight/BOE1
Edited to note : The test flights all show up on flightaware.com as BOE1. There is a new extensive flight test today, Nov 7, although my original post was about the Nov 2 flight. You can see the history of the test flights on this site.
Edited to note : The test flights all show up on flightaware.com as BOE1. There is a new extensive flight test today, Nov 7, although my original post was about the Nov 2 flight. You can see the history of the test flights on this site.
Last edited by Lake1952; 7th Nov 2019 at 20:11.
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In the news today
Boeing states that FAA(*) and EASA found documentation of 737 MAX software changes insufficient.
Source: CNBC article.
Chairman David Calhoun, about CEO Dennis Muilenburg voluntarily waiving his bonus and equity grants for 2019 and
Source: other CNBC article and audio (at 8:00)
(*) updated thanks to airsound
Source: CNBC article.
Chairman David Calhoun, about CEO Dennis Muilenburg voluntarily waiving his bonus and equity grants for 2019 and
...until the Max in its entirety is back in the air and flying safely. As you know, MAX in its entirety takes us thru all of the next calendar year, and probably until the beginning of 2021
(*) updated thanks to airsound
Last edited by fgrieu; 8th Nov 2019 at 10:29. Reason: Fix per comment
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Good to see that the spinmeisters are still on the job. At what point does this become securities fraud?
Boeing spokesman Gordon Johndroe said Wednesday the company “provided technical documentation to the regulators as part of the software validation process. The documentation was complete, and it was provided in a format consistent with past submissions. Regulators have requested that the information be conveyed in a different form, and the documentation is being revised accordingly."
One person briefed on the matter characterized the issue differently and said Boeing’s paperwork had gaps, was substandard and meant regulators could not complete the audit, a crucial step before the plane can be certified to return to service.
One person briefed on the matter characterized the issue differently and said Boeing’s paperwork had gaps, was substandard and meant regulators could not complete the audit, a crucial step before the plane can be certified to return to service.
fgrieu - my reading of the CNBC piece says it wasn't just EASA that objected - the FAA did as well
airsound
U.S. and European regulators have asked Boeing to revise documentation on its proposed 737 Max software fix, the planemaker confirmed Wednesday,
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All test flights are listed below the map as BOE1. My earlier post referred to the Nov 2 flight. There was another busy flight today, looks like a potential vomit comet.
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Looks like more of the go cheap or go home mantra by Boeing to one of its suppliers - Collins- who did the software involved. Extract from NYT
For years, the company had been “a technology leader, an innovator,” Clayton Jones, then the chief executive, told Fortune Magazine “Unfortunately, along the way they forgot to hone their financial skills.”
In 1998, Boeing executives summoned Mr. Jones to Seattle, he later recalled in a speech, and made clear that, to get more of Boeing’s business, Collins would have to cut prices dramatically. In response, Collins introduced what it called “lean electronics,” its take on a belt-tightening philosophy popularized by Toyota.
Collins reorganized business units and retrained managers, with an eye toward efficiency and speed. It pushed its suppliers to do the same, and established partnerships with companies such as HCL Technologies, which provides outsourced, lower-cost engineering services from India.
Along the way, Collins unseated Honeywell as the provider of flight control computers on a predecessor of the Max, the 737 NG, and supplied numerous systems for the Boeing 787, which went into operation in 2011.
When Collins secured the contract for the Max displays in 2012, it credited the belt-tightening. “There were a lot of cost-saving measures — a lot of tough decisions — that had to be made,” one manager said at the time in a company publication The display system is now at issue in both the congressional investigation and the private lawsuits.
In 1998, Boeing executives summoned Mr. Jones to Seattle, he later recalled in a speech, and made clear that, to get more of Boeing’s business, Collins would have to cut prices dramatically. In response, Collins introduced what it called “lean electronics,” its take on a belt-tightening philosophy popularized by Toyota.
Collins reorganized business units and retrained managers, with an eye toward efficiency and speed. It pushed its suppliers to do the same, and established partnerships with companies such as HCL Technologies, which provides outsourced, lower-cost engineering services from India.
Along the way, Collins unseated Honeywell as the provider of flight control computers on a predecessor of the Max, the 737 NG, and supplied numerous systems for the Boeing 787, which went into operation in 2011.
When Collins secured the contract for the Max displays in 2012, it credited the belt-tightening. “There were a lot of cost-saving measures — a lot of tough decisions — that had to be made,” one manager said at the time in a company publication The display system is now at issue in both the congressional investigation and the private lawsuits.
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Watching the return to BFI "live". Fascinating flight profile. If only we knew what cases they were running, especially that last low speed one.
Last edited by ktcanuck; 7th Nov 2019 at 22:48.
