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Ethiopian airliner down in Africa

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Ethiopian airliner down in Africa

Old 4th Apr 2019, 09:41
  #3061 (permalink)  
 
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Originally Posted by ManaAdaSystem View Post


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.
And that highlights a big difference between runaway stab. and MCAS unwanted operation:

With runaway stab you can’t catch it by trimming - it is running away...

With MCAS unwanted (for want of a better description) operation you can theoretically catch (reverse) it by trimming.

Another difference is that in an AoA fault caused unwanted MCAS operation other factors also come into play - even before MCAS operation on flap retraction:

Autothrottle, Instrument anomaly, Stall warning etc. which can cause:

Unwanted rise in speed, initial pilot input nose down response, cockpit confusion.

Therefore the standard drill for stab runaway does not necessarily apply. Trimming to neutral and ATS disconnect might well have priority over stab trim cutoff operation.

Of course we didn’t know this, the poor crews didn’t know it and it is a scenario which Boeing will be looking at hard before submitting a suitable procedure for recertification, I am sure.

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Old 4th Apr 2019, 09:49
  #3062 (permalink)  
 
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[QUOTE=bill fly;10438384]
Originally Posted by ManaAdaSystem View Post


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.
[/QOTE]
And that highlights a big difference between runaway stab. and MCAS unwanted operation:

With runaway stab you can’t catch it by trimming - it is running away...

With MCAS unwanted (for want of a better description) operation you can theoretically catch (reverse) it by trimming.

Another difference is that in an AoA fault caused unwanted MCAS operation other factors also come into play - even before MCAS operation on flap retraction:

Autothrottle, Instrument anomaly, Stall warning etc. which can cause:

Unwanted rise in speed, initial pilot input nose down response, cockpit confusion.

Therefore the standard drill for stab runaway does not necessarily apply. Trimming to neutral and ATS disconnect might well have priority over stab trim cutoff operation.

Of course we didn’t know this, the poor crews didn’t know it and it is a scenario which Boeing will be looking at hard before submitting a suitable procedure for recertification, I am sure.
You are both right. But Boing introduced a very "clever" sentence in the AD:

"Electric estabilizer trim can be used to neutralize control column pitch forces before moving the stab trim cutout switches to cutout"

Can: means that if you do it and you crash, they would say they didn't tell you to do it, but if you don't do it and you crash they will say that they mentioned it for something.

Not that any amount of words is going to change the fact that they are responsible. If only they were as clever in the design as they are with legal-related documents.
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Old 4th Apr 2019, 09:49
  #3063 (permalink)  
 
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They should've kept airframe and hydraulics (99% of them, just a touch here and there)
DO you have any idea of how much of the 737 does not meet the modern day safety standards and is grandfathered? That's probably worthy of a thread of its own.

Nearly 25 years ago Airbus complained to JAA about Boeing's grandfather rights that were letting the 737 get away with nearly half a dozen more seats than a comparable newly certified airliner could, all being equal.
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Old 4th Apr 2019, 09:52
  #3064 (permalink)  
 
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Originally Posted by ManaAdaSystem View Post


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.
Have you read the Boeing bulletin?

​​​​​​Under operating instructions
'Electric stabilizer trim can be used to neutralise control column pitch forces before moving the stab trim switches to cutout'

Why wouldn't you?
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Old 4th Apr 2019, 09:59
  #3065 (permalink)  
 
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Originally Posted by FullWings View Post
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...
Agreed.. I think the FAA's AD acknowledges the challenge that pilot's might face. At the bottom of (h) it reads:
"Initially, higher control forces may be needed to overcome any stabilizer nose down trim already applied. Electric stabilizer trim can be used to neutralize control column pitch forces before moving the STAB TRIM CUTOUT switches to CUTOUT. Manual stabilizer trim can be used before and after the STAB TRIM CUTOUT switches are moved to CUTOUT."

The pilots runaway stabilizer checklist.... is clear... operate the CUTOUT. It doesn't say... try and trim first. For me the AD has never been clear. Are the pilots supposed to follow their trained checklists... and the very same AD, a few paragraphs above, that simply says operate the CUTOUT... Or are they required to make the judgment that they'll never have the strength to turn the trim manually, so they need to rely on the failing electrical trim first.... and then CUTOUT?
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Old 4th Apr 2019, 10:07
  #3066 (permalink)  
 
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Originally Posted by GarageYears View Post
Ok, I have to confess I’m confused.

All this talk of manual trim forces, etc.

MCAS applies trim in increments of 2.5 degrees over 10 seconds. Any pilot pickle switch trim ceases MCAS action for 5 seconds.

If MCAS runs again, again any pilot trim action defeats MCAS.

There is is no need to manually trim against any large nose down MCAS trim surely? Electrically trim the aircraft neutral AND THEN DISABLE ELECTRIC TRIM. From then on you are tweaking trim manually and no heroic fight against aero forces is required.

Am I wrong?

- GY
They were already trying to deal with a stickshaker event. Then they followed Boeings list and disabled electrical trim. They then found that they could not use manual trim to recover. As can be seen in videos, manual trim can be a lot harder for some people than others.
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Old 4th Apr 2019, 10:11
  #3067 (permalink)  
 
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Anyone got a link to the report (or a scanned copy of it)?

All I can find are press reports as to what it says.....



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Old 4th Apr 2019, 10:12
  #3068 (permalink)  
 
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Originally Posted by quentinc View Post
Agreed.. I think the FAA's AD acknowledges the challenge that pilot's might face. At the bottom of (h) it reads:
"Initially, higher control forces may be needed to overcome any stabilizer nose down trim already applied. Electric stabilizer trim can be used to neutralize control column pitch forces before moving the STAB TRIM CUTOUT switches to CUTOUT. Manual stabilizer trim can be used before and after the STAB TRIM CUTOUT switches are moved to CUTOUT."

The pilots runaway stabilizer checklist.... is clear... operate the CUTOUT. It doesn't say... try and trim first. For me the AD has never been clear. Are the pilots supposed to follow their trained checklists... and the very same AD, a few paragraphs above, that simply says operate the CUTOUT... Or are they required to make the judgment that they'll never have the strength to turn the trim manually, so they need to rely on the failing electrical trim first.... and then CUTOUT?

actually it does.......


“Control airplane pitch attitude manually with control column and main electric trim as needed”

its the point in the memory items right after disengagjng the autopilot......
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Old 4th Apr 2019, 10:14
  #3069 (permalink)  
 
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Originally Posted by quentinc View Post
Agreed.. I think the FAA's AD acknowledges the challenge that pilot's might face. At the bottom of (h) it reads:
"Initially, higher control forces may be needed to overcome any stabilizer nose down trim already applied. Electric stabilizer trim can be used to neutralize control column pitch forces before moving the STAB TRIM CUTOUT switches to CUTOUT. Manual stabilizer trim can be used before and after the STAB TRIM CUTOUT switches are moved to CUTOUT."

The pilots runaway stabilizer checklist.... is clear... operate the CUTOUT. It doesn't say... try and trim first. For me the AD has never been clear. Are the pilots supposed to follow their trained checklists... and the very same AD, a few paragraphs above, that simply says operate the CUTOUT... Or are they required to make the judgment that they'll never have the strength to turn the trim manually, so they need to rely on the failing electrical trim first.... and then CUTOUT?
Yes that’s the Point, There should be a runaway stab checklist and a different one for unwanted MCAS operation.

This should specify the possible symptoms.
It should then cover the AD items and go further to disconnecting ATS and flying pitch and thrust. It could well suggest reselecting flap and landing as soon as possible.
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Old 4th Apr 2019, 10:18
  #3070 (permalink)  
 
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Originally Posted by SLF3 View Post
Anyone got a link to the report (or a scanned copy of it)?

All I can find are press reports as to what it says.....
The report should have been issued four hours ago if you go by what the Ethiopian authorities had said. Its release seems to have been delayed, but we are getting bits of information from different sources (ie. Ethiopian Twitter).

Fairly poor handling of a serious investigation if you ask me.
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Old 4th Apr 2019, 10:19
  #3071 (permalink)  
gmx
 
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Originally Posted by quentinc View Post
Agreed.. I think the FAA's AD acknowledges the challenge that pilot's might face. At the bottom of (h) it reads:
"Initially, higher control forces may be needed to overcome any stabilizer nose down trim already applied. Electric stabilizer trim can be used to neutralize control column pitch forces before moving the STAB TRIM CUTOUT switches to CUTOUT. Manual stabilizer trim can be used before and after the STAB TRIM CUTOUT switches are moved to CUTOUT."

The pilots runaway stabilizer checklist.... is clear... operate the CUTOUT. It doesn't say... try and trim first. For me the AD has never been clear. Are the pilots supposed to follow their trained checklists... and the very same AD, a few paragraphs above, that simply says operate the CUTOUT... Or are they required to make the judgment that they'll never have the strength to turn the trim manually, so they need to rely on the failing electrical trim first.... and then CUTOUT?
It's a reasonable point. However, after the LionAir investigation, everyone knew the electric trim could be used to neutralize MCAS, even if temporarily (as the accident crew had done 20-odd times on the disaster flight). Having MCAS run the trim to its maximum nose down position, and being fully aware of the LionAir situation, wouldn't it make sense to re-trim the aircraft to neutral attitude *before* hitting the cutout switches? Having said that, the pilots have clearly encountered a terrifying circumstance. The only thing you might hope is that after the LionAir incident they were better equipped to deal with it than what seems to have been the case.
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Old 4th Apr 2019, 10:23
  #3072 (permalink)  
 
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Originally Posted by ManaAdaSystem View Post

The Boeing NNC regarding runaway trim, or MCAS, has never told us to trim to neutral before placing the switches to cut off.
(Not a pilot) This random internet grab from 2009 (authenticity unknown to me) does say to use electric trim and does not say to apply it again if runaway continues.

As these are terse instructions, if one might soon be grabbing a rotating wheel, and not long after that, hand cranking it, [would he] not want to do the best possible work with the electric trim before cutout?


Last edited by fotoguzzi; 4th Apr 2019 at 10:30. Reason: Complete sentence; cut down extra words
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Old 4th Apr 2019, 10:38
  #3073 (permalink)  
 
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Question

What is the direction of the pitching moment when extending the air brake in the 737 NG / Max, and separately (or as operationally used), when selecting flap from a clean configuration ?
How would slat extension affect these changes ?
Would the logical reverse be a reasonable assumption ?


GordonR, re PM,
and Ethiopian airliner down in Africa
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Old 4th Apr 2019, 10:39
  #3074 (permalink)  
 
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Clearly these incidents have raised many questions, especially for the trained professionals that operate the 737. The consequences of these accidents could maybe also demonstrate issues relating to trim on other 737 variants including the 737 NG. We must remain open to learning the lessons and exploring the wider implications.

If the Ethiopian crew did follow the Stabiliser Runaway memory actions as per QRH NNC, then clearly the very next action that would have followed would have been an attempt to use the manual trim handles. Should the excessive control loading have either blocked or restricted movement of that control due to control loading, the crew would find themselves in a perilous situation and dive. At that point out of desperation perhaps they tried to use the electric trim again and reactivated the electric trim cutout switches. I don't think that is too extraordinary to have attempted out of desperation if the trained procedures were failing were failing.




Manual control of trim on the 737 NG is usually straightforward, but it not impossible to find yourself possibly in a nasty situation. I have once experienced in a level D 738 sim following double engine flameout, high power nose high prior to failure, to a quick ensuing nose low dive. It shocked me the difficulty to raise the nose again and high loading on the manual trim handles. We recovered, but recall unloading the control column back pressure for a short time to help having read about such possible scenarios.




It is too early to say, but there could be lessons here not only just applicable to the Max but perhaps even the NG with regards to control loading whilst using manual trim. If we consider accidents including the 738 Fly Dubai nose dive, control loading implications to manual trim operation are important. After any stab trim runaway, it is essential that we have confidence in the effectiveness of the manual trim throughout the full range of movement and control loadings.




More attention is needed imo to why repeated AOA failures are happening on both 737 NG & Max variants.




For the crew operating these aircraft, there is little doubt that they were suddenly faced with multiple simultaneous failures and warnings. They had seconds to react to complex failures that could easily overload competent crews. If faced with Airspeed Unreliable, possibly altitude disagree, stick shaker, possible other master caution warnings, Stabiliser Runaway, increasing pitch down descent rate, followed by possible manual trim control being restricted by control loading (not proven yet) then they really did have a lot to deal with and the odds were stacked against them.




I think we should all be respectful to the families of the professionals that died trying in vain to regain control. It is too easy for armchair pilots to be judgmental without understanding the human factors and technical issues involved on the flight deck in these incidents. There is no evidence that crew training or that the airlines themselves had much if any involvement in outcome pf these accidents.
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Old 4th Apr 2019, 10:44
  #3075 (permalink)  
 
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So in sum, we have a non-DAL A system integrated with a secondary control system with no redundancy, monitoring, crew warnings or a dedicated means to disengage. Strap it to an aircraft where the trim system is more powerful than the primary controls and give it authority to move the stab at a far faster rate than normal trim and give it no limits of travel other than mechanical end-stops. If it goes wrong, force the crews to disable all powered stab control and equip them with manual trim wheels that are difficult and slow to operate at the best of times. Modify said wheels to make them slightly smaller and insert a more powerful damper to counteract the spring and backlash effect of the stab being moved at a faster rate than ever before - requiring an undocumented increase in physical effort. Finally inform the crews of this system's existence and get the company test pilots to retest all of this after a quick fatal crash, but do so only at a relatively slow speed. Second accident crew become the unexpected test pilots collecting a datapoint that suggests the trim wheel is impossible to move when at a higher airspeed. Cumulogranite awaits.

Apart from the flight envelop defining the configuration, CofG, AuW and airspeed/mach beyond which the manual wheels cannot be moved at an effective rate, what are we missing?
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Old 4th Apr 2019, 10:45
  #3076 (permalink)  
 
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[would he] not want to do the best possible work with the electric trim before cutout?
Yes... but as we know now, the MCAS system will keep cutting in... So the pilot reaches (4) in the check list. That step is clear... operate the the CUTOUT... If already trimmed down and speed a little high, the pilot is now in a very dangerous position. All following the FAA AD and Boeing check list.
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Old 4th Apr 2019, 10:46
  #3077 (permalink)  
 
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Originally Posted by SLF3 View Post
Anyone got a link to the report (or a scanned copy of it)?

All I can find are press reports as to what it says.....
...and many of those newspaper articles are referring to ET's press release as being the preliminary report...
https://www.documentcloud.org/docume...on-ET-302.html
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Old 4th Apr 2019, 10:50
  #3078 (permalink)  
 
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A few minutes ago the Ethiopian CAA website was still silent on this. There now only is minute information on the fact that there is an investigation and a ban.

In the case of recent Asian crashes the official websites were also hardly used or completely bypassed for either facebook and twitter reporting.

Lots of rules already, but another thing ICAO might take a look at. What is the minimum standard on reporting.

In many cases in any news situation the press gets an early view of things with an embargo till a certain time on publication. Might be the case here.

Another thing is the exceptional role the Ethiopian Airlines CEO has carved out for himself. He was one of the first on scene, shown handling evidence, reporting on the investigation, and continuing to do that. With all due respect, that is amazing, but in this case the only information available,...
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Old 4th Apr 2019, 10:51
  #3079 (permalink)  
 
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how would the ET302 pilots know
They might notice that the stick shaker was operating on a single side only. Reality is though the point of these devices is to encourage immediate action by the pilot (to avoid an impending stall), not to enter into some slow careful fault diagnosis.
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Old 4th Apr 2019, 11:09
  #3080 (permalink)  
 
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Originally Posted by quentinc View Post
They might notice that the stick shaker was operating on a single side only. Reality is though the point of these devices is to encourage immediate action by the pilot (to avoid an impending stall), not to enter into some slow careful fault diagnosis.
There were actually two failures on the MAX: The faulty AOA sensor data which triggered a whole range of spurious warnings, put the pilots in a high workload situation, which on its own was hazardous. Then MCAS comes along, and administers the coup-de-grace while the pilots are busy trying to make sense of the aircraft and their checklists.
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