Indonesian aircraft missing off Jakarta
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I'm not sure if I am totally barking up the wrong tree here, but there may be another possibility to consider as well:
What happens if MCAS and STS are both active? We have no information as to how they interact in that case, whether one system inhibits the other, whether outputs are summed or or-ed, whether the reactivation timers are identical and in-sync or not.
As I understand it, if speed (data) is increased STS will trim nose up, if AOA (data) is high MCAS will trim nose down, what if both? If the activation times are different the trim will be up then down, which might lead to a report of "sts trimming wrong way" in absence of knowledge of MCAS, no? So the pilot might be fighting an aircraft that is porpoising all on it's own. Note that this may not look like a classic "runaway trim" either, it isn't at this point running away in either direction.
Now if speed is reduced, will STS and MCAS (assuming indicated AOA remains high) both trim nose down, depending on the cyclic timing, possibly suddenly and at the same time (additive commands? - we don't know)?
But like I said, I may be totally barking up the wrong tree and have misunderstood STS/MCAS.
What happens if MCAS and STS are both active? We have no information as to how they interact in that case, whether one system inhibits the other, whether outputs are summed or or-ed, whether the reactivation timers are identical and in-sync or not.
As I understand it, if speed (data) is increased STS will trim nose up, if AOA (data) is high MCAS will trim nose down, what if both? If the activation times are different the trim will be up then down, which might lead to a report of "sts trimming wrong way" in absence of knowledge of MCAS, no? So the pilot might be fighting an aircraft that is porpoising all on it's own. Note that this may not look like a classic "runaway trim" either, it isn't at this point running away in either direction.
Now if speed is reduced, will STS and MCAS (assuming indicated AOA remains high) both trim nose down, depending on the cyclic timing, possibly suddenly and at the same time (additive commands? - we don't know)?
But like I said, I may be totally barking up the wrong tree and have misunderstood STS/MCAS.
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There is some more info in earlierWSJ article on it. Seems they replaced both sensors as part of trouble shooting a fault where autothrust refused to engage (also replaced two others from ramp damage incidents). It then gets a bit confusing because the sensors were apparently found to be not the cause of the problem, but one or both was then repaired 
- make of that what you will I guess.
- make of that what you will I guess.A rotable is removed from an aircraft by a mechanic/engineer and is placed on the Unserviceable shelf.
An actual engineer from lets says tech services inspects the part and have 2 options:
Relase it back to service or send it for repair. If he is not absolutely certain of its status he/she selects to send it for repair.
When it comes back from repair, and it migh just have been tested/inspected to tolerances by a certified company, it is taken into stock with a tag as repaired.
In this sample it just means it went via the authorized repairer (this could be the manufacturer) instead of having been directly released back into stock by the tech services engineer.
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Might just be a system figure of speech. Sample:
A rotable is removed from an aircraft by a mechanic/engineer and is placed on the Unserviceable shelf.
An actual engineer from lets says tech services inspects the part and have 2 options:
Relase it back to service or send it for repair. If he is not absolutely certain of its status he/she selects to send it for repair.
When it comes back from repair, and it migh just have been tested/inspected to tolerances by a certified company, it is taken into stock with a tag as repaired.
In this sample it just means it went via the authorized repairer (this could be the manufacturer) instead of having been directly released back into stock by the tech services engineer.
A rotable is removed from an aircraft by a mechanic/engineer and is placed on the Unserviceable shelf.
An actual engineer from lets says tech services inspects the part and have 2 options:
Relase it back to service or send it for repair. If he is not absolutely certain of its status he/she selects to send it for repair.
When it comes back from repair, and it migh just have been tested/inspected to tolerances by a certified company, it is taken into stock with a tag as repaired.
In this sample it just means it went via the authorized repairer (this could be the manufacturer) instead of having been directly released back into stock by the tech services engineer.
If SW changed a couple attempting to solve an autothrust fault wonder what the maintenance manuals say about AoA sensor false readings?
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FWIW in seattle times re world wide conference call ..Boeing to hold global conference call with airlines that fly 737 MAX model that was involved in crash
Originally published November 19, 2018 at 6:19 pm Updated November 19, 2018 at 7:39 pm The conference call Tuesday will allow Boeing to field queries all at once from the group instead of having multiple individual conversations on the same points, said one of the people who asked not to be identified since the call is private.
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MCAS apparently only operates with AP disengaged. The fact that the EAD refers to the runaway trim procedure seems to indicate that the FDR indicated runaway trim. As I've mentioned elsewhere, if MCAS is implicated it means the pilots were flying by hand and counteracting MCAS auto-trim activity reasonably successfully up to a point. What happened to change that situation and trigger the horrific descent is very much unknown.
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I'm not sure if I am totally barking up the wrong tree here, but there may be another possibility to consider as well:
What happens if MCAS and STS are both active?
What happens if MCAS and STS are both active?
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Before November 6th there was not much in the way of consensus as to a possible cause. Unreliable Air Speed (UAS) possibly due to the covers being left on the speed sensors was the talk of the day.
On November 6th Boeing issued a statement that changed the focus to the Angle Of Attack (AOA) sensor. Specifically that bad data from an AOA sensor could cause uncommanded nose down trim on the stabilizer (horizontal part of the tail).
Link to November 6th discussion: Indonesian aircraft missing off Jakarta
At some point Boeing disclosed the existence of a new feature named MCAS that was evidently not previously disclosed to pilots. MCAS is an anti-stall feature that was not required (or implemented) on older versions of the 737. When working properly with good AOA data MCAS trims the stabilizer to lower the plane's nose in an attempt to mitigate a stall. MCAS could have been a lifesaver under different circumstances, however...
MCAS along with faulty AOA data seems to be prevailing theory as to what might have been at least a contributing factor in the crash. The idea being that bad AOA data fed to MCAS caused the plane to be trimmed more and more nose down, ultimately leading to a crash.
Disclaimer: This is my interpretation of the speculation posted here by others. I could easily have goofed something up.
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I can find plenty of references that say MCAS only with flaps up, but looks like it is flaps up or down for STS (except on classics?)
-Flaps not up (737-300 & 500).
-Flaps up or down (737-400 & NG).
-Flaps up or down (737-400 & NG).
No definite confirmation of conditions for MAX, but I haven't seen anything that says it is different.
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It's clearly not beneath just 30m of water, but also under an indeterminate depth of sea bed.
If it near floats on the sea, it ought to take years for it to sink below the sea bed.
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Most likely the crew could not overcome the stab authority any more, in DC8 times known as "jackknifing".
If as a result the crooked negative camber of the stab/elev position, it might have stalled completely.
Similar to a plane that looses its complete stab, and we all know what happens then, don't we? With the CG in front of the Lift vector, the fuse will rotate DOWN. FAST.
A dramatic video about this phenomenon can be seen with the demise of some poor helpless chaps, note the wings indicating a dramatic "negative G"
If as a result the crooked negative camber of the stab/elev position, it might have stalled completely.
Similar to a plane that looses its complete stab, and we all know what happens then, don't we? With the CG in front of the Lift vector, the fuse will rotate DOWN. FAST.
A dramatic video about this phenomenon can be seen with the demise of some poor helpless chaps, note the wings indicating a dramatic "negative G"
Last edited by Double Back; 20th Nov 2018 at 11:50.
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I am not your knowledgeable authority figure but I have read the entire thread from the beginning and will be happy to summarize.
Before November 6th there was not much in the way of consensus as to a possible cause. Unreliable Air Speed (UAS) possibly due to the covers being left on the speed sensors was the talk of the day.
On November 6th Boeing issued a statement that changed the focus to the Angle Of Attack (AOA) sensor. Specifically that bad data from an AOA sensor could cause uncommanded nose down trim on the stabilizer (horizontal part of the tail).
Link to November 6th discussion: Indonesian aircraft missing off Jakarta
At some point Boeing disclosed the existence of a new feature named MCAS that was evidently not previously disclosed to pilots. MCAS is an anti-stall feature that was not required (or implemented) on older versions of the 737. When working properly with good AOA data MCAS trims the stabilizer to lower the plane's nose in an attempt to mitigate a stall. MCAS could have been a lifesaver under different circumstances, however...
MCAS along with faulty AOA data seems to be prevailing theory as to what might have been at least a contributing factor in the crash. The idea being that bad AOA data fed to MCAS caused the plane to be trimmed more and more nose down, ultimately leading to a crash.
Disclaimer: This is my interpretation of the speculation posted here by others. I could easily have goofed something up.
Before November 6th there was not much in the way of consensus as to a possible cause. Unreliable Air Speed (UAS) possibly due to the covers being left on the speed sensors was the talk of the day.
On November 6th Boeing issued a statement that changed the focus to the Angle Of Attack (AOA) sensor. Specifically that bad data from an AOA sensor could cause uncommanded nose down trim on the stabilizer (horizontal part of the tail).
Link to November 6th discussion: Indonesian aircraft missing off Jakarta
At some point Boeing disclosed the existence of a new feature named MCAS that was evidently not previously disclosed to pilots. MCAS is an anti-stall feature that was not required (or implemented) on older versions of the 737. When working properly with good AOA data MCAS trims the stabilizer to lower the plane's nose in an attempt to mitigate a stall. MCAS could have been a lifesaver under different circumstances, however...
MCAS along with faulty AOA data seems to be prevailing theory as to what might have been at least a contributing factor in the crash. The idea being that bad AOA data fed to MCAS caused the plane to be trimmed more and more nose down, ultimately leading to a crash.
Disclaimer: This is my interpretation of the speculation posted here by others. I could easily have goofed something up.
But the simple fact is that at this point, PPRuNe readers have no knowledge of what happened outside of FR24 data and what has been released in the media.
The only thing that has been released in the media is the Boeing AD regarding MCAS. It hasn’t been concluded that MCAS was the primary cause of the accident, however, given the publication of the AD, it has been the main subject of conjecture on this forum as a “contributing factor” (thank you lakedude).
Most accidents prove to have several “contributing factors”, and I’m sure this will be one of them.
I would suggest that the eventual analysis will include other contributing factors, including the quality of maintenance (given the evidence of the previous 3 sectors), and the quality of training (given that this event should have been survivable if indeed it was only an air data problem).
I don’t work for Boeing, but given the track record of this operator, I am really surprised at the outburst against Boeing on this thread. Boeing is not in charge of maintenance standards or training standards at customer airlines. If you want to buy a ticket to fly on a low cost airline, you carry a risk. Until the planes start crashing, low cost operators will continue to reduce standards. Eventually they will reach a point where the planes start crashing, and tragically, people will die. I will leave it to the community to decide when that point has been reached.
Last edited by Derfred; 20th Nov 2018 at 12:24.
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B737 NG FCOM
STS operates most frequently during takeoffs, climb and go-arounds. Conditions
for speed trim operation are listed below:
• Airspeed between 100 KIAS and
Mach 0.5
• 10 seconds after takeoff
• 5 seconds following release of
trim switches
• Autopilot not engaged
• Sensing of trim requirement
B737 -300 (non-EFIS) FCOM
Flaps not up
Air speed 100-300 KIAS
N1 above 60%
(Rest of the conditions same as NG above)
MAX unknown. (some one in this forum can help)
Only Tristar_Drvr has mentioned he is flying the MAX in this forum. So he should know.
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Originally Posted by EDLB 
After full three weeks they did not find the CVR in 30m deep water. Indonesia way of dealing with such desasters?
Nothing to do with Indonesia - it is the lackadaisical aviation industry way of dealing with disasters. There is no technical reason why CVR and for that matter DFDR data cannot be streamed into remote storage, the communication bandwidths available now are sufficient. Instead the aviation industry prefer to use barely 'survivable' recorders with cheap short range pingers with cheap short life batteries. Like the ELBs that never seem to work these recorders are antediluvian and need to be replaced. Just think if both DFDR and CVR recording had been retrievable from an online escrow system a lot of the current speculation would not have happened. And, either the 737 Max would be grounded or MCAS could have been completely exonerated within hours of the crash. MH370 would not be a mystery and AF447 would also have been properly investigated within a day or so rather than years. Billions of dollars could have been saved.