Boeing 737 Max Recertification Testing - Finally.
derjodel, # 196
The weakness of your point is the assumption that the nacelles change AoA; they do not. The change is in the relationship between stick force and speed (AoA) - the longitudinal stability.
At constant speed, AoA is increased by moving the stick and pitching the aircraft. Thus with the first high AoA 'event' - trim down, the stick force will increase, providing a force-feel feedback to the pilot. This change encourages the pilot to move the stick forward to reduce stick force and thus reduce AoA ( normal flying technique for speed / AoA change ).
There is a risk that the pilot will accept or even increase stick force - manoeuvring the aircraft to a higher AoA, this is like any other aircraft. MCAS was intended to achieve a similar stick force-to-speed (AoA) relationship the NG; i.e. the Max will feel the same as the NG and still meet the certification requirements even with the larger engines.
The MAX can be stalled, as the NG. One MCAS input might be judged as sufficient to cue pilot awareness of a deceleration / manoeuvre, but never intended to prevent a stall.
The aircraft stall characteristics would be the same with / without MCAS, although without MCAS the potentially lower stick force during recovery could encourage re stalling.
Big Pistons,
The FCCs will now be cross monitored to limit trim failures within a range where manual control is acceptable (13.3).
Non FCC related failures, e.g. stab trim runaway, are not protected - as in the NG, but wiring changes in the MAX reduce the likelihood of failure, the corrective action for which still relies on timely pilot intervention (13.6).
The weakness of your point is the assumption that the nacelles change AoA; they do not. The change is in the relationship between stick force and speed (AoA) - the longitudinal stability.
At constant speed, AoA is increased by moving the stick and pitching the aircraft. Thus with the first high AoA 'event' - trim down, the stick force will increase, providing a force-feel feedback to the pilot. This change encourages the pilot to move the stick forward to reduce stick force and thus reduce AoA ( normal flying technique for speed / AoA change ).
There is a risk that the pilot will accept or even increase stick force - manoeuvring the aircraft to a higher AoA, this is like any other aircraft. MCAS was intended to achieve a similar stick force-to-speed (AoA) relationship the NG; i.e. the Max will feel the same as the NG and still meet the certification requirements even with the larger engines.
The MAX can be stalled, as the NG. One MCAS input might be judged as sufficient to cue pilot awareness of a deceleration / manoeuvre, but never intended to prevent a stall.
The aircraft stall characteristics would be the same with / without MCAS, although without MCAS the potentially lower stick force during recovery could encourage re stalling.
Big Pistons,
The FCCs will now be cross monitored to limit trim failures within a range where manual control is acceptable (13.3).
Non FCC related failures, e.g. stab trim runaway, are not protected - as in the NG, but wiring changes in the MAX reduce the likelihood of failure, the corrective action for which still relies on timely pilot intervention (13.6).
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The EFS module increases hydraulic system A pressure to the elevator feel and
centering unit during a stall. This increases forward control column force to
approximately two times normal feel pressure. (...)
centering unit during a stall. This increases forward control column force to
approximately two times normal feel pressure. (...)
'… why didn't Boeing just change the parameters of Elevator Feel Computer (Elev Feel Shift Module) as it was already available on 737NG?'
I don't know, nor have access to Boeing's thoughts at the time.
There may be clues in the references to the integrated MCAS within the exsisting STS (NG). In terms of speed, adapting the STS would be logical, and when the 'Manoeuvring' requirement was identified - need for AoA and Mach, then the existing 'digital' computation from DADC (air data and AoA input) and FCS / FCC with stab trim output, was more practical for test and development than modifying the 'heavy engineering' at the back of the aircraft.
I don't know, nor have access to Boeing's thoughts at the time.
There may be clues in the references to the integrated MCAS within the exsisting STS (NG). In terms of speed, adapting the STS would be logical, and when the 'Manoeuvring' requirement was identified - need for AoA and Mach, then the existing 'digital' computation from DADC (air data and AoA input) and FCS / FCC with stab trim output, was more practical for test and development than modifying the 'heavy engineering' at the back of the aircraft.
Last edited by safetypee; 5th Aug 2020 at 16:19.
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Stall identification and control is enhanced by the yaw damper, the Elevator Feel
Shift (EFS) module and the speed trim system. These three systems work
together to help the pilot identify and prevent further movement into a stall
condition.
(...)
As airspeed decreases towards stall speed, the speed trim system trims the
stabilizer nose down and enables trim above stickshaker AOA. With this trim
schedule the pilot must pull more aft column to stall the aircraft. With the
column aft, the amount of column force increases with the onset of EFS.
Shift (EFS) module and the speed trim system. These three systems work
together to help the pilot identify and prevent further movement into a stall
condition.
(...)
As airspeed decreases towards stall speed, the speed trim system trims the
stabilizer nose down and enables trim above stickshaker AOA. With this trim
schedule the pilot must pull more aft column to stall the aircraft. With the
column aft, the amount of column force increases with the onset of EFS.
Conditions for speed trim operation are:
...
5 seconds following release of TRIM switches.
...
Autopilot not engaged.
...
5 seconds following release of TRIM switches.
...
Autopilot not engaged.
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SLF attorney reaching into aviation-related law - plus a custom disclaimer of anything like full understanding of the flight control dynamics being discussed.
But as to what Boeing engineering and management cadres might have known, it's true that a federal criminal inquiry still is open and underway, isn't it? And in pre-trial discovery in the direct liability cases as well as a number of related cases, there is a very large amount of factual information yet to be uncovered. The pertinent Committee of the House of Representatives released a not insignificant batch of documents some months ago, but it is hardly the complete picture. So whether Boeing just never thought about a particular flight control situation, or instead and more troubling, thought about it but tried to hide the ball, still is an unknown.
As for the lack of training initiatives by Ethiopia after the Lion Air accident . . . while the factual predicate of that point might be correct, did any of the other 737 MAX customers initiate such training after Lion Air? Again I'm not asserting support for either content of the prelim report or for any specific iteration of how those pilots responded or did not respond - but there's something quite starkly "hindsight, purely" about noting a lack of new training initiated, let alone a grounding by the operater
But as to what Boeing engineering and management cadres might have known, it's true that a federal criminal inquiry still is open and underway, isn't it? And in pre-trial discovery in the direct liability cases as well as a number of related cases, there is a very large amount of factual information yet to be uncovered. The pertinent Committee of the House of Representatives released a not insignificant batch of documents some months ago, but it is hardly the complete picture. So whether Boeing just never thought about a particular flight control situation, or instead and more troubling, thought about it but tried to hide the ball, still is an unknown.
As for the lack of training initiatives by Ethiopia after the Lion Air accident . . . while the factual predicate of that point might be correct, did any of the other 737 MAX customers initiate such training after Lion Air? Again I'm not asserting support for either content of the prelim report or for any specific iteration of how those pilots responded or did not respond - but there's something quite starkly "hindsight, purely" about noting a lack of new training initiated, let alone a grounding by the operater
de minimus non curat lex
Would I be right is saying that the defective MCAS system had activated erroneously on more than one occasion in flight in the US prior to the fatal accidents?
The crews dealt successfully with the situation, and reporting action to the FAA took place?
The crews dealt successfully with the situation, and reporting action to the FAA took place?
Keeping Danny in Sandwiches
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Someone in the FAA needs to dig out "Handling the Big Jets" by DP Davis, test pilot with the British Air Registration Board in the 1950's and read page 137 and the diagrams on 138 and 9.
The BAC 1-11 ended up with 4 AOA indicators, 2 on each side that operated a stick shaker and stick push to keep it out of a deep stall. Why is the Max restricted to 2 AOA's?
The BAC 1-11 ended up with 4 AOA indicators, 2 on each side that operated a stick shaker and stick push to keep it out of a deep stall. Why is the Max restricted to 2 AOA's?
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Bend alot
It is a widely accepted fact that the MCAS software has NEVER activated in in hundreds of thousand of hours in revenue service EXCEPT IN ERROR. Perhaps that is why at one point the Canadian authorities (perhaps tongue in cheek?) suggested doing away with the software altogether and simply training for the difference in handling, if significant, from previous editions of the B737. .
It is a widely accepted fact that the MCAS software has NEVER activated in in hundreds of thousand of hours in revenue service EXCEPT IN ERROR. Perhaps that is why at one point the Canadian authorities (perhaps tongue in cheek?) suggested doing away with the software altogether and simply training for the difference in handling, if significant, from previous editions of the B737. .
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It doesn't chime, doesn't deploy the RAT, doesn't electrocute the captain. We shall never know ... unless Boeing hid a MCAS activation counter somewhere. Which they have not disclosed.
I don't think it activated often though. But to claim it didnt' activate EXECPT those three incidents is a bit far fetched.
There are certainly no documented instances of MCAS deployment in regular service operations.
But while it's true that absence of evidence isn't evidence of absence, given that the stated aim of MCAS is/was "to provide consistent handling qualities in unusual flight conditions", I think it's reasonable to expect that we'd have heard by now of any instance where a MAX on a revenue service had entered the flight regime where MCAS would have deployed by design.
But while it's true that absence of evidence isn't evidence of absence, given that the stated aim of MCAS is/was "to provide consistent handling qualities in unusual flight conditions", I think it's reasonable to expect that we'd have heard by now of any instance where a MAX on a revenue service had entered the flight regime where MCAS would have deployed by design.
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No you would be incorrect.
Only 3 MCAS events are known to have happened, none in the US.
Only 3 MCAS events are known to have happened, none in the US.
Pilots were not aware of MCAS...therefore, they did not have any idea of the parameters associated with MCAS, or its initiation, or the aircraft response when MCAS initiated...
so when a report was filed, it was simply a FMS issue. They did not know what to call it, the issue was lost in the filing procedure.
In further review, the circumstances the pilots described were MCAS...
There were instances, especially AFTER MCAS was identified, where pilots encountered issues, that reports were being filed...., but again, as an FMS issue as you all are aware, there is no box on the form stating MCAS.
Well how many of those reports were for stabiliser trim runaway? Because a incorrectly activated MCAS could only be handled via the stab trim runaway checklist. An MCAS working as designed could only do so in a dangerously low speed event which itself is reportable.
turbidus
None of the reports to the FAA could have been a MCAS event - many had the Auto Pilot on at the time (so not possible), others not a character of MCAS.
All possibles were linked in a early MAX thread.
None of the reports to the FAA could have been a MCAS event - many had the Auto Pilot on at the time (so not possible), others not a character of MCAS.
All possibles were linked in a early MAX thread.
This whole thing feels like a fudge.
If you can safely disable MCAS after one actuation, why do you need it in the first place?
If it does not meet the certification standard without MCAS, why is it acceptable to turn it off when the certification standard essentially is looking for a benign and predictable control response across the flight envelope?
Have EASA updated their position on the Max?
There was a suggestion they were going to insist on a retrofit to upgrade to the equivalent of three sensors for AOA from two. Which would mean you would not need to turn it off if a sensor failed, since you could vote 2 oo 3 sensors.
If you can safely disable MCAS after one actuation, why do you need it in the first place?
If it does not meet the certification standard without MCAS, why is it acceptable to turn it off when the certification standard essentially is looking for a benign and predictable control response across the flight envelope?
Have EASA updated their position on the Max?
There was a suggestion they were going to insist on a retrofit to upgrade to the equivalent of three sensors for AOA from two. Which would mean you would not need to turn it off if a sensor failed, since you could vote 2 oo 3 sensors.
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Can you clarify something for a SLF with a fallible memory.
I can remember they had to enable MCAS -- with increased authority -- in a different region of the flight envelope. I can't remember them increasing MCAS authority in the original area of the flight envelope.
I can remember they had to enable MCAS -- with increased authority -- in a different region of the flight envelope. I can't remember them increasing MCAS authority in the original area of the flight envelope.
The BAC 1-11 ended up with 4 AOA indicators, 2 on each side that operated a stick shaker and stick push to keep it out of a deep stall.
Aircraft with underwing engines weren't nearly as likely to deep stall and the tailplane was less shielded by the wing at high pitch attitudes so control was easier to regain. Obviously any stall was something to be avoided no matter what type the aircraft was.
The regulations of the day took account of the consequences and likely hood of the event occurring and provided for extra protection.
One man's fudge is another's compromise
SLF3, '… safely disable MCAS after one actuation, why do you need it in the first place?'
In part see Boeing 737 Max Recertification Testing - Finally. and Boeing 737 Max Recertification Testing - Finally.
'… why is it acceptable to turn it off when the certification standard '
There are many safety and certification arguments in regulatory documents; essentially the difference is between normal and abnormal (emergency) operations. MCAS can be compared with requiring two engines, but if one fails then the aircraft must be able to fly and land at a suitable airport.
EASA position; wait and see, particularly if they comment publicly or not on the NPRM.
Reading between the lines; the modifications explained in the NPRM suggest that a compromise (equivalent level of safety) has already been agreed for detecting AoA failures and MCAS inhibition, particularly reducing the pilot's contribution. More info required for ADIRU AoA 'valid' cross monitor and in some circumstances the choice of which AoA to use (valid erroneous data ??).
Similarly, a compromise for manual trim range, but not necessarily all failures for '737' stab trim runaway - but arguments have been made for previous variants and accepted.
One unknown is the view about multiple alerts stemming from a single AoA failure. The NPRM discusses the provision of 'AoA Disagree' to aid diagnosis, but until the new checklist is published the diagnosis is unknown. Also the 'disagree' alert appears to be dual purpose ((MCAS diagnostic at 5 deg, and AoA EFIS indicator 10 deg), which could be misleading.
There is no discussion about continuous stick shake - stall warning, whether to inhibit or not. The weakness is that a single AoA failure can simultaneously disable stall warning and a stability enhancement (MCAS) for increasing stick force high AoA; in this the three AoA argument is valid, but wait for the detail on alternative mechanisms for computing AoA validity (ADIRU). Also note MCAS only required flaps up, and greater focus on speed (which could have errors), but there is a standby system for that - workload argument.
In part see Boeing 737 Max Recertification Testing - Finally. and Boeing 737 Max Recertification Testing - Finally.
'… why is it acceptable to turn it off when the certification standard '
There are many safety and certification arguments in regulatory documents; essentially the difference is between normal and abnormal (emergency) operations. MCAS can be compared with requiring two engines, but if one fails then the aircraft must be able to fly and land at a suitable airport.
EASA position; wait and see, particularly if they comment publicly or not on the NPRM.
Reading between the lines; the modifications explained in the NPRM suggest that a compromise (equivalent level of safety) has already been agreed for detecting AoA failures and MCAS inhibition, particularly reducing the pilot's contribution. More info required for ADIRU AoA 'valid' cross monitor and in some circumstances the choice of which AoA to use (valid erroneous data ??).
Similarly, a compromise for manual trim range, but not necessarily all failures for '737' stab trim runaway - but arguments have been made for previous variants and accepted.
One unknown is the view about multiple alerts stemming from a single AoA failure. The NPRM discusses the provision of 'AoA Disagree' to aid diagnosis, but until the new checklist is published the diagnosis is unknown. Also the 'disagree' alert appears to be dual purpose ((MCAS diagnostic at 5 deg, and AoA EFIS indicator 10 deg), which could be misleading.
There is no discussion about continuous stick shake - stall warning, whether to inhibit or not. The weakness is that a single AoA failure can simultaneously disable stall warning and a stability enhancement (MCAS) for increasing stick force high AoA; in this the three AoA argument is valid, but wait for the detail on alternative mechanisms for computing AoA validity (ADIRU). Also note MCAS only required flaps up, and greater focus on speed (which could have errors), but there is a standby system for that - workload argument.
Last edited by safetypee; 7th Aug 2020 at 13:17.