Indonesian aircraft missing off Jakarta
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PF is flying manually in vmc with all non-pressure instruments serviceable and has things set up and while going through memory items for UAS the aircraft trims nose down. This is unwanted as it creates quite a pull force so the pilot retrims with the standard electric trim back to trimmed flight which unbeknown to the pilot inhibits MCAS for the next 10 seconds. So after 10 seconds of trimmed flight MCAS starts again it creates quite a pull force so the pilot retrims back to trimmed flight with the standard electric trim .......- surely at the third automatic stab trim down the pilot would cut off the stab trim - which just so happens to be what the memory items suggest anyway? It's not like the aircraft was IMC or at night, and all non-pressure instruments were functional. Not only that but the MCAS is not ratcheting down continually as the PF has trimmed back to level flight each time. This sequence of events would describe the porpoising that was observed from the FR24 near the beginning of the thread. This is what would be expected before the stab trim cut off.
It does not explain the apparent bunt into a power dive - something else happened at that point..
That is the issue that I do not understand.
PF is flying manually in vmc with all non-pressure instruments serviceable and has things set up and while going through memory items for UAS the aircraft trims nose down. This is unwanted as it creates quite a pull force so the pilot retrims with the standard electric trim back to trimmed flight which unbeknown to the pilot inhibits MCAS for the next 10 seconds. So after 10 seconds of trimmed flight MCAS starts again it creates quite a pull force so the pilot retrims back to trimmed flight with the standard electric trim .......- surely at the third automatic stab trim down the pilot would cut off the stab trim - which just so happens to be what the memory items suggest anyway? It's not like the aircraft was IMC or at night, and all non-pressure instruments were functional. Not only that but the MCAS is not ratcheting down continually as the PF has trimmed back to level flight each time. This sequence of events would describe the porpoising that was observed from the FR24 near the beginning of the thread. This is what would be expected before the stab trim cut off.
It does not explain the apparent bunt into a power dive - something else happened at that point..
PF is flying manually in vmc with all non-pressure instruments serviceable and has things set up and while going through memory items for UAS the aircraft trims nose down. This is unwanted as it creates quite a pull force so the pilot retrims with the standard electric trim back to trimmed flight which unbeknown to the pilot inhibits MCAS for the next 10 seconds. So after 10 seconds of trimmed flight MCAS starts again it creates quite a pull force so the pilot retrims back to trimmed flight with the standard electric trim .......- surely at the third automatic stab trim down the pilot would cut off the stab trim - which just so happens to be what the memory items suggest anyway? It's not like the aircraft was IMC or at night, and all non-pressure instruments were functional. Not only that but the MCAS is not ratcheting down continually as the PF has trimmed back to level flight each time. This sequence of events would describe the porpoising that was observed from the FR24 near the beginning of the thread. This is what would be expected before the stab trim cut off.
It does not explain the apparent bunt into a power dive - something else happened at that point..
It is the strange “periodic” cessation of Trim that distracted PF from the possibility of runaway, and allowed the HS to incrementally steal the airplane from him....he wasn’t getting “level flight” with re-trim. All Nose Down Trim is not too terribly much. After two rounds of MCAS, you’re there. He experienced only one interruption of Auto TRIM to get to AND. Adding velocity makes the HS a large gorilla.
There was no third auto TRIM Down. It was at the stop before it began the third round.
Last edited by Concours77; 21st Nov 2018 at 13:29.
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That is the issue that I do not understand.
PF is flying manually in vmc with all non-pressure instruments serviceable and has things set up and while going through memory items for UAS the aircraft trims nose down. This is unwanted as it creates quite a pull force so the pilot retrims with the standard electric trim back to trimmed flight which unbeknown to the pilot inhibits MCAS for the next 10 seconds. So after 10 seconds of trimmed flight MCAS starts again it creates quite a pull force so the pilot retrims back to trimmed flight with the standard electric trim .......- surely at the third automatic stab trim down the pilot would cut off the stab trim - which just so happens to be what the memory items suggest anyway? It's not like the aircraft was IMC or at night, and all non-pressure instruments were functional. Not only that but the MCAS is not ratcheting down continually as the PF has trimmed back to level flight each time. This sequence of events would describe the porpoising that was observed from the FR24 near the beginning of the thread. This is what would be expected before the stab trim cut off.
It does not explain the apparent bunt into a power dive - something else happened at that point..
PF is flying manually in vmc with all non-pressure instruments serviceable and has things set up and while going through memory items for UAS the aircraft trims nose down. This is unwanted as it creates quite a pull force so the pilot retrims with the standard electric trim back to trimmed flight which unbeknown to the pilot inhibits MCAS for the next 10 seconds. So after 10 seconds of trimmed flight MCAS starts again it creates quite a pull force so the pilot retrims back to trimmed flight with the standard electric trim .......- surely at the third automatic stab trim down the pilot would cut off the stab trim - which just so happens to be what the memory items suggest anyway? It's not like the aircraft was IMC or at night, and all non-pressure instruments were functional. Not only that but the MCAS is not ratcheting down continually as the PF has trimmed back to level flight each time. This sequence of events would describe the porpoising that was observed from the FR24 near the beginning of the thread. This is what would be expected before the stab trim cut off.
It does not explain the apparent bunt into a power dive - something else happened at that point..
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Wiedehopf, #1466, gbnf, et al. Your discussion focuses on a single aspect; consider the overall situation.
An AoA probe malfunction may give indications, individually or collectively, of stickshaker (on one side), erroneous low speed display, nose down trim, IAS disagree, ALT disagree (AOA disagree if display option fitted), FEEL DIF PRESS alert.
A crews’ perception of these aspects is likely to differ; only PF feels trim change (note FEEL alert), but more likely the stick shaker will be prioritised, particularly as the direction of change is nose down, which is the required corrective action.
Both crew should be alerted by central warnings; the display order might suggest priority; IAS disagree as the first line of thought, consider a drill for UAS (has IAS failed and the aircraft stalled). Other alerts add confusion, distract / limit understanding the overall situation.
The critical issue is when the crew appreciate that control is the dominant problem, and thereafter that trim is contributing.
A significant aspect is the airspeed at which the malfunction occurs. If at high speed (climb), then the amount of trim movement, more likely optimised for low speed situations (9sec), could result in considerable nose down stick force requiring great effort for recovery (not forgetting that this might contradict thoughts about stickshake - cognitive dissidence). Manual trim would cut out MCAS, but would a crew keep on trimming nose up - debatable - as we shouldn’t fly with trim, non SOP (more dissonance, breaking rules)
It could be concluded that the crew would be totally focussed on the control issue - #1 ‘fly the aircraft’, and only later deduce a trim problem from the intermittent MCAS command, - this takes time. No doubt there are as many different interpretations as there are posts, but we don’t know which one the crew had.
Other posts have questioned the use of single source AoA. In some aircraft (737 config no known), a single AoA malfunction could result in a stickshake in isolation. The certification safety case probably concluded that this was not a hazardous misleading indication, particularly as there was no change in flight path, nor speed display.
The discussion re MCAS is if that safety case was made on the same assumption (not hazardous), if so this accident (and other events) indicate otherwise, or if not (AoA malfunction could result in a hazardous situation), then the system’s integrity appears insufficient for the AoA failure.
For those who like to consider design judgement, there is an example in OODA Learning Activity
With respect to the Double Loop Learning diagram, the critical ‘grey box’ is overcoming Defensive Reasoning; perhaps something now a problem at corporate level.
An AoA probe malfunction may give indications, individually or collectively, of stickshaker (on one side), erroneous low speed display, nose down trim, IAS disagree, ALT disagree (AOA disagree if display option fitted), FEEL DIF PRESS alert.
A crews’ perception of these aspects is likely to differ; only PF feels trim change (note FEEL alert), but more likely the stick shaker will be prioritised, particularly as the direction of change is nose down, which is the required corrective action.
Both crew should be alerted by central warnings; the display order might suggest priority; IAS disagree as the first line of thought, consider a drill for UAS (has IAS failed and the aircraft stalled). Other alerts add confusion, distract / limit understanding the overall situation.
The critical issue is when the crew appreciate that control is the dominant problem, and thereafter that trim is contributing.
A significant aspect is the airspeed at which the malfunction occurs. If at high speed (climb), then the amount of trim movement, more likely optimised for low speed situations (9sec), could result in considerable nose down stick force requiring great effort for recovery (not forgetting that this might contradict thoughts about stickshake - cognitive dissidence). Manual trim would cut out MCAS, but would a crew keep on trimming nose up - debatable - as we shouldn’t fly with trim, non SOP (more dissonance, breaking rules)
It could be concluded that the crew would be totally focussed on the control issue - #1 ‘fly the aircraft’, and only later deduce a trim problem from the intermittent MCAS command, - this takes time. No doubt there are as many different interpretations as there are posts, but we don’t know which one the crew had.
Other posts have questioned the use of single source AoA. In some aircraft (737 config no known), a single AoA malfunction could result in a stickshake in isolation. The certification safety case probably concluded that this was not a hazardous misleading indication, particularly as there was no change in flight path, nor speed display.
The discussion re MCAS is if that safety case was made on the same assumption (not hazardous), if so this accident (and other events) indicate otherwise, or if not (AoA malfunction could result in a hazardous situation), then the system’s integrity appears insufficient for the AoA failure.
For those who like to consider design judgement, there is an example in OODA Learning Activity
With respect to the Double Loop Learning diagram, the critical ‘grey box’ is overcoming Defensive Reasoning; perhaps something now a problem at corporate level.
But as I have read on here one has to first switch off the 2 Stab trim cut off's first otherwise the MACS system will override and continue to keep trimming the a/c nose down despite whatever the pilots do either with the Trim button or manually when in manual flight - and THAT task is what it seems Boeing forgot to tell anyone in the MAX training differences - Am I correct?
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But as I have read on here one has to first switch off the 2 Stab trim cut off's first otherwise the MACS system will override and continue to keep trimming the a/c nose down despite whatever the pilots do either with the Trim button or manually when in manual flight - and THAT task is what it seems Boeing forgot to tell anyone in the MAX training differences - Am I correct?
But as I have read on here one has to first switch off the 2 Stab trim cut off's first otherwise the MACS system will override and continue to keep trimming the a/c nose down despite whatever the pilots do either with the Trim button or manually when in manual flight - and THAT task is what it seems Boeing forgot to tell anyone in the MAX training differences - Am I correct?
Runaway Stabilizer
1 CONTROL COLUMN - HOLD FIRMLY
2 AUTOPILOT (if engaged) - DISENGAGE
Do not re-engage the autopilot. Control airplane pitch attitude manually with control column and main electric trim as needed
If the Runaway Continues
3 STAB TRIM CUTOUT SWITCHES (both) - CUTOUT
If the Runaway Continues
4 STABILIZER TRIM WHEEL - GRASP and HOLD
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The Case for an FCC Software Update
https://www.satcom.guru/2018/11/737-...n-command.html
...FCC Pitch Axis augmentation (Mach Trim, Speed Trim, and MCAS) commands may be based on a single sensor input. These commands should be checked against a calculation based on a second sensor set before becoming valid. A software update to the FCC may provide support for a dual channel mandate.
And, at the risk of simply going round in circles here, there's the oddity of the final 2 lines in the "reminder" Runaway Stab memory item list that Boeing later pointed to:
Runaway Stabilizer
1 CONTROL COLUMN - HOLD FIRMLY
2 AUTOPILOT (if engaged) - DISENGAGE
Do not re-engage the autopilot. Control airplane pitch attitude manually with control column and main electric trim as needed
If the Runaway Continues
3 STAB TRIM CUTOUT SWITCHES (both) - CUTOUT
If the Runaway Continues
4 STABILIZER TRIM WHEEL - GRASP and HOLD
Runaway Stabilizer
1 CONTROL COLUMN - HOLD FIRMLY
2 AUTOPILOT (if engaged) - DISENGAGE
Do not re-engage the autopilot. Control airplane pitch attitude manually with control column and main electric trim as needed
If the Runaway Continues
3 STAB TRIM CUTOUT SWITCHES (both) - CUTOUT
If the Runaway Continues
4 STABILIZER TRIM WHEEL - GRASP and HOLD
Number two item above demonstrates this list does not apply to MCAS.
Auto Pilot is OFF already, MCAS is only active in manual.
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I'm not necessarily taking the Boeing's side here, but you're going to have to come up with something a little better than that to counter the claim that the Runaway Trim procedure covers erroneous MCAS activation.
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Even if it didn't say that checklists are not magically not applicable because you can't perform one of the items because it's already done.
Do you even logic?
As much as i agree that the system has bad design you are making it sound like a pilot couldn't possibly cope with this condition.
Also the electric trim will simply reach it's stop, not jam.
Originally Posted by Concours77
My inclination would be that the bunt happened when the elevator lost all (what remained of) its authority...
But, perhaps more to the point, is that what I quoted is the existing Runaway Trim procedure for the more recent 737s, and it is to that existing procedure and wording that Boeing and the FAA urgently pointed a couple of weeks ago. That kind-of suggests - but doesn't quite exactly say - that very wording is to be found in the Max QRF (rather than some other wording specific to MCAS scenarios).....
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We wont know for sure now until they release more data but we can assume they were fighting MCAS trim with opposite trim and stick input.The flight lasted 13 minutes.Stick input doesnt override MCAS(!!) only electrical main trim(stick input does override STS).Stabilizer trim operation like anything else mechanical has limitations.(13 minutes off for every 2 minutes operation) or possibly continual engagement of opposite direction trim by the crew burnt out the motor anyway.
The accident Captain surely would have read the previous commanders write up mentioning STS trimming wrong way.He knows nothing about MCAS and he may well remember from his training that the way to disengage STS is to deselect the AP STAB trim motor which apparently doesnt exist on the MAX but is now called B/U.If he disengages B/U (the one on the right) and not the PRI(the one on the left) he may think he has successfully disengaged the trim but the culprit isnt STS but MCAS and you have to disengage both switches(RUNAWAY NNC).But he doesnt know this.
What the accident investigation will have to decide is were the crew negligent in not killing ALL trim once their opposing trim/stick input proved unsuccessful.The previous crew(s) presumably did just that.Comes under fly the plane.They will definitely crucify Boeing for this but MCAS will only be a contributing factor in the final report I am guessing.The Pilots must fly the plane.
Other possibility is they finally figured it out and disengaged both trim switches but the THS was by this time at its limits and elevator authority wasnt sufficient nor was there time to trim up manually.
The accident Captain surely would have read the previous commanders write up mentioning STS trimming wrong way.He knows nothing about MCAS and he may well remember from his training that the way to disengage STS is to deselect the AP STAB trim motor which apparently doesnt exist on the MAX but is now called B/U.If he disengages B/U (the one on the right) and not the PRI(the one on the left) he may think he has successfully disengaged the trim but the culprit isnt STS but MCAS and you have to disengage both switches(RUNAWAY NNC).But he doesnt know this.
What the accident investigation will have to decide is were the crew negligent in not killing ALL trim once their opposing trim/stick input proved unsuccessful.The previous crew(s) presumably did just that.Comes under fly the plane.They will definitely crucify Boeing for this but MCAS will only be a contributing factor in the final report I am guessing.The Pilots must fly the plane.
Other possibility is they finally figured it out and disengaged both trim switches but the THS was by this time at its limits and elevator authority wasnt sufficient nor was there time to trim up manually.
Last edited by Rananim; 21st Nov 2018 at 17:10.
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@ATCWATCHER; Remembering sim sessions regarding this in other types(havn't flown a 73) , with full nose down stab trim(runaway stab trim situation) we were able to fly and land the ac using elevator only. I remember planting my feet on the panel in order to lock the yoke with my knees in order to relieve my arms continuous pulling on the yoke. Co and I alternated doing this. AC was flyable this way at high and low speeds, it became a bit tricky when configuring for landing but fully controllable. This was on several Boeing ac and the MD11.
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While there seems to be a lot of ' he/they/pilots/ "Shoulda . .." some simple math sort of bothers this SLF. For example if one ASSUMES altitude of 5000 feet, and a Air spped of say 300 mph, AND a dive at 45 degrees resulting from ????, then from onset of dive to impact takes about 1/3 minute or 20 seconds. To have survived, via a high g pullout , the correct action ? ( pull up ? ) would have had to be instituted PROBABLY in about 10 seconds or less. ( I'm sure others can/will argue that 10 seconds remaining is NOT enough to level out above sea re elevator movement, control effectiveness, etc - 737 are not considered high G qualified aerobatic aircraft ) . Add in a startle factor of a few seconds " what now ?? " and the results of the 'event' starting at 5000 ft and 300 mph was unfortunately preordained.
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Historically, planes used to be introduced/launched by major airlines with large engineering departments. The MAX is an example of LCC's as the early adopters. Would be interesting to get insight from the reporting on the certification process later if this has had any influence on design and operational choices.
Tx...PJ2
