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Ethiopian airliner down in Africa

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Ethiopian airliner down in Africa

Old 4th Apr 2019, 19:08
  #3141 (permalink)  
 
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Originally Posted by EDLB View Post
One common pattern between the Lion air and Ethopian air crash is, that the later/last manual trim up commanded from the coloum switches are in both cases only short blips, followed by 5 second trim down from the MCAS system.

I find it hard to believe that both pilots clinging on the control coloums for their dare life to get nose up do not try harder/longer on the electric trim as long as engaged and not cut out.

I assume that there is more to the story.
That the trim wheels didn't move because electric trim is not powerful enough at such high speed?
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Old 4th Apr 2019, 19:10
  #3142 (permalink)  
 
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Originally Posted by Just This Once... View Post
5 seconds after T/O with the stick-shaker going and a brace of captions is not the time to pull the power back.
It is time to put the flaps up? Would, perhaps, Airspeed Unreliable apply?
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Old 4th Apr 2019, 19:11
  #3143 (permalink)  
 
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Originally Posted by neila83 View Post
How much would you have reduced power by, with unreliable airpeed, stick shaker going off, low altitude, while you've seemingly lost pitch control? Bearing in mind no reference value is provided in the handbook, and reducing power pushes the nose down?
Well, if I were the pilot, I would have set power to 75% N1. This is the reference value which is provided in the handbook, AKA the unreliable airspeed NNC, and it is a memory item that pilots are expected to execute without needing to reference the QRH. As a note, the QRH specifically states that stick shaker, overspeed warning, and airspeed low alerts may sound erroneously or simultaneously, and this situation is regularly practiced in the simulator.
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Old 4th Apr 2019, 19:13
  #3144 (permalink)  
 
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Originally Posted by ecto1 View Post
That the trim wheels didn't move because electric trim is not powerful enough at such high speed?
Except that trim did move slightly (as expected given short blips) and the FDR shows the switch activation directly, 2 short blips about 5s apart.
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Old 4th Apr 2019, 19:17
  #3145 (permalink)  
 
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Originally Posted by Organfreak View Post
I've read the entire report and still see no mention of a bird (or other foreign object) strike to the left AoA sensor. Where did that tantalizing hint come from??? CVR?
Also, the media coverage (and even posts here) keep harping on the idea that all Boeing recovery procedures were followed, and yet they turned the trim cutout switches back ON in their desperation, which was certainly NOT part of the prescribed procedures. Seems as if that is some serious "dumbing-down" of the info.
The cut-out switches are not on the FDR plot. Whether they turned them back ON is pure speculation.
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Old 4th Apr 2019, 19:19
  #3146 (permalink)  
 
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Originally Posted by MurphyWasRight View Post
Why would pilots in a severe AND trim crisis merely blip the switches?

Is there any possibility that the extreme column forces could have interfered with the switches, either mechanically (including wiring etc) or biomechanically.?

By biomechanically I mean could the hand position and forces make the pilot believe he was pressing the switches when in fact he was not?
The FDR shows a 'bunt' with significant g-force due to the final MCAS stabiliser nose down trim. Until that point the trim was stable nose low but unvarying, so they may not have seen the need for (or possibility of) nose up trim. They may have severely underestimated the force of the nose down trim at high airspeed, compared to the earlier MCAS activation. Within 10 seconds they were experiencing negative 2g and 40 degrees nose down pitch on the FDR, so moving arms may have been impossible. During that time the aft forces on the yoke reached extreme values, and it is probably impossible to pull and trim at the same time. Hope that makes sense.

Originally Posted by threemiles View Post
The cut-out switches are not on the FDR plot. Whether they turned them back ON is pure speculation.
Not true at all. There is a long period where the trim is stable, and the cutoff switches have clearly been disabled, inhibiting MCAS. This is shown on the FDR. Later there is a sudden uncommanded nose down trim, consistent with MCAS. The only logical way this could have happened is for the switches to have been been enabled. To suggest that this is pure speculation, flies in the facts shown on the FDR.

Last edited by GordonR_Cape; 4th Apr 2019 at 19:43.
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Old 4th Apr 2019, 19:21
  #3147 (permalink)  
 
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Originally Posted by threemiles View Post
The cut-out switches are not on the FDR plot. Whether they turned them back ON is pure speculation.
I would state that it is pure inference, rather than speculation.

At 05:43:11, about 32 seconds before the end of the recording, at approximately 13,4002 ft, two momentary manual electric trim inputs are recorded in the ANU direction. The stabilizer moved in the ANU direction from 2.1 units to 2.3 units.

That could not have happened if the cutout switches had not been turned back on.

Last edited by slacktide; 4th Apr 2019 at 19:22. Reason: spelling
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Old 4th Apr 2019, 19:22
  #3148 (permalink)  
 
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Originally Posted by threemiles View Post
The cut-out switches are not on the FDR plot. Whether they turned them back ON is pure speculation.
Oh! I do apologize if I have posted wrong information. I had thought it was established fact.
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Old 4th Apr 2019, 19:28
  #3149 (permalink)  
 
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Originally Posted by slacktide View Post
Not only is turning the stab trim cutout switches back on NOT a part of the documented and approved recovery procedure, the FCOM specifically cautions against turning electric stab trim back on after it has been selected off.

"Ensure that the STAB TRIM CUTOUT switches are set to CUTOUT and stay in the CUTOUT position for the remainder of the flight."

It's disingenuous to state that the recovery procedure was followed, when it was only halfway followed, and other basic elements of pilotage were neglected. If they had monitored and controlled the airspeed within limits, and followed the published recovery procedure, they would have been able to manually trim the airplane using the trim wheel.

Regarding my language "monitored and controlled their airspeed." I know someone will pipe up about stick shakers and clackers and cockpit confusion. Yes, the left side airspeed was unreliable, and the left side stick shaker was going off. But the right side airspeed was not unreliable, and the BU airspeed was not unreliable, and the right side stick shaker was not going off. Isn't unreliable airspeed a well documented scenario which is regularly practiced in the simulator, with simultaneous shaker and clacker? Isn't it the second item on the very first page of the QRH? Are the appropriate initial pitch and power settings to be used during unreliable airspeed not memory items?
Couldn't agree more.

Memory Items for Airspeed Unreliable?
Memory Items for Unscheduled Stab trim?

It appears the aircraft was never put back in trim after the initial MCAS input and although an attempt was made with the cutouts the aircraft was out of trim. Turned back on again contrary to advice (accept this is speculation at present) and the system was allowed to trim forward again.

Unfortunately these points will be lost to the majority on here who simply want to flame Boeing.
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Old 4th Apr 2019, 19:31
  #3150 (permalink)  
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Thoughts.

This aircraft had multiple pitot static malfunctions in the days prior to the accident as per the tech log quoted in the report. Altimeter and VSI erratic.

If they had had a bird strike on the vane, they would have noticed the thump and said something. I think we can rule a bird strike out. And frozen things. It was 17 degrees C on the ground.

Just after takeoff the crew got an almost immediate
Stick shaker
Airspeed disagree
Altitude disagree
F/D disagree
Which constitutes an unreliable airspeed non normal
(Together with an anti ice caution)
And did not apply the memory items for that , and seemed to be prepared to continue to FL320.
The trim problems started once the flaps were retracted , however the flaps seem to have been retracted despite the left stick shaker being continually active and the aircraft in an unreliable airpseed state which calls for a pitch and power setting with the flaps down (if I recall, 10deg NU, 85%N1) but the power wasn't reduced from 94% either.
The stabtrim cutout switches seem to have stopped the MCAS from moving the stab, but then the switches were re-engaged (2 ANU trims were recorded 2,5 min after the stab trim switches were set to cutout) and this then allowed the MCAS to pitch it down again.
As regards thrust and pitching moments I personally would prefer to take my chances with speedbrake, idle thrust and a bit of a dip than two engines at 94% when I'm pointing downhill.
And I don't think I'd be getting ATC involved until I had the beast back under some sort of control.
This seems to be more than an MCAS thing, it may be a pitotstatic adiru thing.
If one sensor gets extreme readings or if there's a sensor disagree, then no system should rely upon that sensor data.
Systems shouldn't rely on one sensor only.
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Old 4th Apr 2019, 19:31
  #3151 (permalink)  
 
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Originally Posted by MurphyWasRight
Why would pilots in a severe AND trim crisis merely blip the switches?

Is there any possibility that the extreme column forces could have interfered with the switches, either mechanically (including wiring etc) or biomechanically.?

By biomechanically I mean could the hand position and forces make the pilot believe he was pressing the switches when in fact he was not?
The FDR shows a 'bunt' with significant g-force due to the final MCAS stabiliser nose down trim. Until that point the trim was stable, so they may not have seen the need for (or possibility of) nose up trim. They may have severely underestimated the force of the nose down trim at high airspeed, compared to the earlier MCAS activation. Within 10 seconds they were experiencing negative 2g and 40 degrees nose down pitch on the FDR, so moving arms may have been impossible. During that time the aft forces on the yoke reached extreme values, and it is probably impossible to pull and trim at the same time. Hope that makes sense.
The 2 blips occurred before the final MCAS action, in fact likely triggered it so the extreme final G load is not a factor. They were flying but with (great?) difficulty before then.

Not sure 3 units AND (from trace) while 'stable' is not correct. From the report it appears they tried and failed to use manual trim wheel after cutout and then (last ditch option) re-enabled it to attempt electrical trim.

Gets back to the question of why 2 short blips instead of continuous until collum force approaches neutral?
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Old 4th Apr 2019, 19:32
  #3152 (permalink)  
 
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AoA back to normal

Any ideas on why the faulty left AoA goes to values even lower than the assumed correctly working right AoA at the same time the terminal descent starts? MCAS works for a while, left AOA goes down, stick shaker is happy, MCAS stops.
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Old 4th Apr 2019, 19:34
  #3153 (permalink)  
 
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Originally Posted by GarageYears View Post
Really?

Aside from why the AP was successfully engaged, the three trim ND FCC commands (and one NU for good measure), DURING autopilot engagement are brief and likely STS driven - not MCAS. That comes after the AP is disconnected.

The first MCAS ND command occurs on deselection of the AP, and runs for 10 seconds as the system is expected to do. You can clearly see that in the FDR traces.
I'm not seeing it that way. With the AP engaged the 3x AND auto trim events kill the pitch attitude - from 8 deg positive decreasing all the way to zero by the time AP tripped-out. At that stage with the AP engaged, FD commanding a climb and with throttles parked forward with excess speed in hand the aircraft should have been going up like the proverbial. Indeed, the pitch attitude only becomes positive again when PF takes control and applies a generous amount of aft stick and ANU blips. Even then he does not achieve anything like the 8 deg positive achieved earlier.

Clearly I know what the aircraft 'should' be doing, but I'm just not finding it in the data provided.

If you see it differently please shout out as even after years of flight testing this has left me scratching my head.
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Old 4th Apr 2019, 19:37
  #3154 (permalink)  
 
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Originally Posted by MurphyWasRight View Post
Except that trim did move slightly (as expected given short blips) and the FDR shows the switch activation directly, 2 short blips about 5s apart.
Yeah, from 2.1 to 2.3 units, I know.

It could very well be that the initial blip moved the trim slightly and then stopped, they released and try again but no avail. Each square in the FDR is 3 seconds, so the second "short" blip is 2s or so. Between the two, looks to me in the 3s area. (one full square) Let's say 2.5 to be safe.

The final AND input by MCAS was 5s and moved 1.3 units. The last pilot input ANU was 2.5s (combined) and moved 0.2 units. Something odd is there.

It is a theory, I could be mistaken. But it's plausible to my eyes.
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Old 4th Apr 2019, 19:38
  #3155 (permalink)  
 
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Originally Posted by GarageYears View Post
Later two very short pickle switch inputs NU were input indicating the crew re-engaged the electric trim, but they were very short and had a very small effect on trim.
I am struggling to see any effect, and I didn't see any other indication in the report that the crew re-engaged.

I suspect that cutout switch position is not directly recorded and has to be inferred, possibly this is where the rumours of disagreements and CVR reading problems originate from - maybe there are sounds or speech on the CVR that could indicate re-engagement but it isn't definite, or agreed. Any reference to the disputed sounds may then have been excised from the report.

5 seconds later MCAS, having been reset by the crew electric trim input, and now being active again, ran for less than 10 seconds and seems to have been interrupted by the Left AOA value changing.
This MCAS signal got through, which some will take as proving the switch position was changed, there is so much else going wrong not sure I would agree (that it is proven). Also, this still doesn't look right per the descriptions we've had:
- MCAS is supposed to have less authority at higher speed (maybe that's it, it is shorter input?)
- If AOA reduces then MCAS is apparently supposed to put the trim back up, if no pilot trim input - yet there isn't and it doesn't
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Old 4th Apr 2019, 19:39
  #3156 (permalink)  
 
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Re: The FDR plot

Note the call-out box which states "Automatic Trim Command With No Change In Pitch Trim". However, there is another downwards blip of automatic trim command shortly before the crash, which clearly causes more nose-down trim. This must be the result of the MCAS being re-enabled. For some reason, the CVR transcript does not say so, but the pilots must have re-enabled the Cutout Switch.
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Old 4th Apr 2019, 19:43
  #3157 (permalink)  
 
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Originally Posted by MurphyWasRight View Post
Gets back to the question of why 2 short blips instead of continuous until collum force approaches neutral?
Could be a DFDR artefact. On another aircraft type I am more familiar with a continuous blip when the trim motor does not run just poles the DFDR data flag on and off again. This single pole will appear as a spike when graphed.

In the accident graph the 2 final electric trim blips do not appear to move the pitch trim, either due to cutout or aerodynamics overcoming the drive.
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Old 4th Apr 2019, 19:50
  #3158 (permalink)  
 
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Originally Posted by ecto1 View Post
Yeah, from 2.1 to 2.3 units, I know.

It could very well be that the initial blip moved the trim slightly and then stopped, they released and try again but no avail. Each square in the FDR is 3 seconds, so the second "short" blip is 2s or so. Between the two, looks to me in the 3s area. (one full square) Let's say 2.5 to be safe.

The final AND input by MCAS was 5s and moved 1.3 units. The last pilot input ANU was 2.5s (combined) and moved 0.2 units. Something odd is there.

It is a theory, I could be mistaken. But it's plausible to my eyes.
Unfortunately the plot has artificial slopes on binary events so without the raw data is hard to tell the actual length of short inputs so the total 'blip' could be significantly less than 2.5s.

Interesting idea that when they tried the electrical trim the system stalled so they tried again, hard to tell without raw data and CVR though.
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Old 4th Apr 2019, 19:51
  #3159 (permalink)  
 
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Originally Posted by infrequentflyer789 View Post
This MCAS signal got through, which some will take as proving the switch position was changed, there is so much else going wrong not sure I would agree (that it is proven). Also, this still doesn't look right per the descriptions we've had:
- MCAS is supposed to have less authority at higher speed (maybe that's it, it is shorter input?)
- If AOA reduces then MCAS is apparently supposed to put the trim back up, if no pilot trim input - yet there isn't and it doesn't
AFAIK MCAS uses the same rate of nose down trim of 0.27 units per second, it is the duration that varies, with a maximum of 9 seconds.
MCAS will 'unwind' the trim only when the AOA reduces below the 10 degree threshold, not when AOA reduces relative to its previous value.
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Old 4th Apr 2019, 19:52
  #3160 (permalink)  
 
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Originally Posted by Albino View Post
[ [Stab trim cutout switches.] Turned back on again contrary to advice (accept this is speculation at present) and the system was allowed to trim forward again.
Unless I misread the preliminary report, it appears that the "advice" that was followed was that of the FO, who advised that manual operation of the trim wheels was not possible. It's fairly easy to understand why the crew decided to flip those switches back on (if they did).
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