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Ethiopian airliner down in Africa

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Ethiopian airliner down in Africa

Old 4th Apr 2019, 07:25
  #3021 (permalink)  
 
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Originally Posted by gmx
I will wait for the official report. MCAS is clearly an unintuitive and potentially lethal system when triggered by faulty AoA data. I'm not going to blame Boeing, the FAA, the pilots or airlines until we know the exact failure modes triggered in these two accidents.
I am not a pilot, and probably being adversarial, but the question that might be asked is: Will waiting for the grounding to be lifted be enough, or the FAA oversight review, or until either or both of the accident reports, which ever comes later?

The FDR readouts provide a lot of information, but the state of the AOA sensor, cutoff switches, and flight computers is impossible to determine after the crashes. We may never know the the full answers, given the complex fault tree. Could this saga take years, like the Comet metal fatigue investigation?
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Old 4th Apr 2019, 07:38
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It could take years, but it may be more like the rudder hardover problem. Two fatal accidents, keep the thing flying, quite a few hairy moments, then find and fix the problem years later.
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Old 4th Apr 2019, 07:39
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Originally Posted by GordonR_Cape
Could this saga take years, like the Comet metal fatigue investigation?
Very unlikely. The modern digital FDR used store lots of parameters and with the CVR synced there will be enough data to come up with a very good picture of what happened why. There are lots of grounded MAX too where you can look into and test fly them to check your assumptions about the events.
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Old 4th Apr 2019, 07:42
  #3024 (permalink)  

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Boeing's mistake was to use the stabiliser to actuate MCAS. If they had followed the example of the BAC 1-11 and the deep stall problem in the 1960's they would have had a stick shaker followed by a stick pusher on the elevators. At least the pilot was in the loop throughout the period of designed operation, even if as I suspect like MCAS the pusher was there to satisfy the certification regime and unlike the 737 probably never actuated in flight.

The only issue then would be the small size of the elevators on the 737 but that could have been resolved.

Last edited by sky9; 4th Apr 2019 at 09:38.
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Old 4th Apr 2019, 07:53
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Originally Posted by Water pilot

So that is a selonoid on the column cutout override switch, right? What happens if it sticks ON, perhaps because someone installed the wrong one? (I have seen this.). Since MCAS is never engaged normally such a fault could lie in wait a long time. What woukd be the consequence, there is not much detail in that schematic.
The system (stabilizer trim control column switching module) must be checked every 6000 FH according MPD.

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Old 4th Apr 2019, 07:58
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I have a simple question:
What happens if the trim cutoff switches are in on position (electrical trim possible), MCAS wan’t to trim the stabiliser nose down while the pilot holds his dual trim switches on the colom for nose up?

Who wins?

According to water_pilot the PF win’s but is it verified?

With the outcome of ET302 I have my doubts.
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Old 4th Apr 2019, 08:15
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Originally Posted by RatherBeFlying
It appears that MCAS can in ten seconds move the stab whenever it's in the mood, but the pilots need considerably longer to bring back the stab with dozens of cranks of manual trim - provided that airload allows them to move the trim.

Very much an unequal contest.

​​​​​​MCAS really shouldn't be putting in more trim than can be corrected by the crew in the interval before it reactivates.

But then, limiting MCAS authority might fail to achieve required stick force increase approaching stall. The software will have to get even fancier to satisfy 10E-9 reliability.

I'm with gums. Dump MCAS and fix the nacelle aerodynamics.
Me too, and I would be willing to bet quite a few Boeing engineers long before anyone else ever heard of it.
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Old 4th Apr 2019, 08:49
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Originally Posted by EDLB
I have a simple question:
What happens if the trim cutoff switches are in on position (electrical trim possible), MCAS wan’t to trim the stabiliser nose down while the pilot holds his dual trim switches on the colom for nose up?

Who wins?

According to water_pilot the PF win’s but is it verified?

With the outcome of ET302 I have my doubts.
PF wins, see FDR traces.

On a different topic, failure of control column deflection cutoff override to always closed would mean that if the trim thumb button is pressed nose down, or fails to that position, it will not be cut if the pilot pulls.

Very rare coincidence, but previous posters are right in that periodical check would be needed.

It is in my eyes as dangerous and as rare as the control column defection switches failing to always open, and they installed an override for that (before mcas)
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Old 4th Apr 2019, 09:05
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Apart from Boeing the FAA will be left holding the legal responsibility here.

Trying to get the Max back in the air and fully certified will not be easy.

This is not a quick fix and rubber stamp job.

Added to that the passenger viewpoint of an old design past its sell by date and clearly not safe and you have a whole bundle of problems for the manufacturer.
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Old 4th Apr 2019, 09:27
  #3030 (permalink)  
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I was talking this week end with a senior from one of the European airline affected, they had last week decided stop training on the Max, transfer all the pilots to their other types for upgrade and strike out the type from their summer schedule. Whether the airline will survive this financially is in the balance now as the Summer season is where the money is made... this will have rippling effect on the industry well beyond Boeing..
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Old 4th Apr 2019, 09:38
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So the prelim report, issued by the Ethiopian government says the crew "repeatedly followed procedures recommended by Boeing.."
https://www.bbc.co.uk/news/business-47812225
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Old 4th Apr 2019, 09:46
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Repeatedly?! How is that possible?
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Old 4th Apr 2019, 09:48
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Interesting. When it appears in print and is verified, I await all those that said that "sub standard training was the cause", and that "it would never have happened in the US" to say they were wrong.
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Old 4th Apr 2019, 09:49
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From the Ethiopian Airlines press release,
"The preliminary report clearly showed that the Ethiopian Airlines Pilots who were commanding FlightET 302/10 March have followed the Boeing recommended and FAA approved emergency procedures to handle the most difficult emergency situation created on the airplane. Despite their hard work and full compliance with the emergency procedures, it was very unfortunate that they could not recover the airplane from the persistence of nose diving"
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Old 4th Apr 2019, 10:01
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Wall Street Journal article

The Wall Street Journal, relying on sources who have seen the flight data recorder readout, reported that the pilots, upon experiencing uncommanded nose-down trim, used the 737’s stabilizer trim cutout switches. And while the 737 MAX 8 retains the manual trim wheels it has had from day one, it’s not known if they used these to re-trim the aircraft. The sources told the Journal that the pilots appeared to have reengaged the stabilizer trim cutout switches, which would have re-enabled the MCAS stall protection system.

The underline is mine. Why would a pilot reengage a failed system? If they wanted to control the stab trim after turning of the electric stab trim switches, why did they not use the manual trim wheel?
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Old 4th Apr 2019, 10:02
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No “foreign object damage” or “structural design problem” identified

That’s likely to be a significant finding
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Old 4th Apr 2019, 10:08
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Originally Posted by compressor stall
Interesting. When it appears in print and is verified, I await all those that said that "sub standard training was the cause", and that "it would never have happened in the US" to say they were wrong.
I think it would be wise to wait for the report. If the aircraft wasn't put in trim using the switches as per Boeing recommendation before placing them to cutout and it turns out that the switches were then turned back on, again contrary to guidance, the press release from Ethiopian is misleading to say the least.

​​​​​​
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Old 4th Apr 2019, 10:09
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Why would a pilot reengage a failed system? If they wanted to control the stab trim after turning of the electric stab trim switches, why did they not use the manual trim wheel?
Maybe because the control loadings at the speed they were doing made it difficult/impossible to manually trim, so they tried the electric trim again? If you’ve got both (or even four) hands on the control column trying to stop the aircraft pitching down, there’s not many hands left for the manual trim...
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Old 4th Apr 2019, 10:12
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Originally Posted by warbirdfinder
Why would a pilot reengage a failed system? If they wanted to control the stab trim after turning of the electric stab trim switches, why did they not use the manual trim wheel?
Speculatively with both pilots hauling back on the control column and no electric trim, the moment any of them lets go to give the wheel a try the nose would dip down again. Additionally, at nose low and high speed with stab overloaded the wheel might have been much too stiff to both move and do so enough times to make an impact.
I hate to speculate on this but it points to proper action by crew finding themselves unable to bring nose up with manual means and reactivating the cutouts to regain electric trim capability. (Which should come back and if used should stop MCAS either way, unless...)
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Old 4th Apr 2019, 10:24
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Originally Posted by Albino
I think it would be wise to wait for the report. If the aircraft wasn't put in trim using the switches as per Boeing recommendation before placing them to cutout and it turns out that the switches were then turned back on, again contrary to guidance, the press release from Ethiopian is misleading to say the least.

​​​​​​
The Boeing NNC regarding runaway trim, or MCAS, has never told us to trim to neutral before placing the switches to cut off. It tells us to stop the trim with the switches if the trim doesn’t stop after disconnecting the autopilot. Then use manual trim. Period.

The armchair experts have now moved from «They should just have placed the cut off switches to off and contained the problem» to «They should just have trimmed neutral and then used cut out switches and contained the problem».
Congratulations! It took only a few weeks to come to this conclusion.
The Lion Air and Ethiopian pilots only had a few minutes.
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