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Boeing amend 737 de-icing procedures & propose aircraft mods after near-stall inciden

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Boeing amend 737 de-icing procedures & propose aircraft mods after near-stall inciden

Old 9th Dec 2013, 07:13
  #41 (permalink)  
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"And no, it's not that dry" - actually it is pretty 'dry' - the same reason you do not necessarily require engine anti-ice at 40 and below!
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Old 9th Dec 2013, 08:01
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Not correct. You get some of the worst icing conditions on ground at extremely low temps and foggy conditions.

EFRO 090850Z VRB03KT 040V120 5000 BR FEW001 M24/M26 Q1021=

Minus 24, and close to fog.

EFRO 090823Z 0909/1009 VRB03KT CAVOK TEMPO 0909/0912 1200 BCFG SCT001
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Old 9th Dec 2013, 09:16
  #43 (permalink)  
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As Capt PS says, -24 is not -37! I would guess there would be no de-icing at EFKT during the day once any overnight condensate had been cleared.
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Old 9th Dec 2013, 09:42
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If you say so, and that is why this is happening?

EFKT 091020Z 00000KT 9999 BKN003 M35/M39 Q1021 R16/72//75

EFKT 091000Z 0910/0917 VRB03KT CAVOK
TEMPO 0910/0912 4000 BR BKN002
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Old 9th Dec 2013, 11:25
  #45 (permalink)  
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Yes, you get clouds in the arctic.

Let's rewind to your post #39. I take it you are seriously expecting to need to de-ice your nun here regardless?

EFKT 082050Z AUTO 30001KT CAVOK M37/M41 Q1015
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Old 9th Dec 2013, 11:59
  #46 (permalink)  
 
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Mist and fog does not form in bone dry conditions.
All i said was: No, those are not very dry conditions, and the weatherman agrees with me.
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Old 9th Dec 2013, 16:41
  #47 (permalink)  
 
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Is it not the case that you De-ice to get rid of contamination and you anti-ice to prevent contamination. The get rid of scenario is decided by sight & touch. The prevent scenario will depend on prevailing conditions. At -37c and CAVOK there might not yet be contamination. Will there become so is a judgement that thankfully I've not had to make. -20c was my worst.
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Old 9th Dec 2013, 20:36
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back to the aircraft control issue....

“Under certain circumstances it is possible that the input arms [to the power control units] may be exposed to fluid which in turn freezes solid and blocks [them],” says SHT, adding that Boeing was “not aware” of this potential problem before the investigation."

Can someone please tell me if this is the same as saying the aircraft experienced a jammed elevator, ie jammed yoke?

Or does the input arms exposed to freezing fluid somehow instead cause the elevator to deflect in one direction and not return?
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Old 12th Dec 2013, 00:20
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Anyone with ideas?

No one around that can shed any light on how a power control unit being blocked with ice can cause an aircraft to pitch up so aggressively in this manner?

Tightcircuit?

Safetypee?
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Old 12th Dec 2013, 09:26
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Its all to do with the power/pitch couple Hawk. The elevator was jammed but the pitch trim system (which adjusts the whole horizontal stabiliser) was still working. The report says that the autopilot adjusted the trim system nose up. I am not sure of the reason for that at that stage of the approach but it was probably just a response to reducing speed. This would normally have been countered by automatic nose down elevator but that was not available. As a result the nose rose and speed reduced further causing the auto thrust to increase power. The pitch up moment due to thrust increase is very strong in the 73 and can in extreme circumstances even overpower the elevator completely. In this case they had no elevator anyway so the nose would have kept on going up. The automatics would have been completely confused at that point and may even have added more nose up trim but the report doesn't tell us that.

Boeing altered the drills for stall recovery and nose high upset recovery to include the use of nose down pitch trim several years ago after a similar near disaster. In that case the elevator was not jammed but nevertheless the application of full power at very low speed caused the nose to pitch up uncontrollably.

Last edited by tightcircuit; 12th Dec 2013 at 11:38.
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Old 12th Dec 2013, 10:06
  #51 (permalink)  
 
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Khazan!

the application of full power at very low speed cause the nose to pitch up uncontrollably
Reading this post, can't help to think about the Khazan accident!
Even more in a 23,5k engine in a 500.
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Old 12th Dec 2013, 12:48
  #52 (permalink)  
 
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A speculative view assuming that the elevator was blocked with the autopilot engaged.
As speed reduced prior to GS capture the autopilot was unable to move the elevator to provide the necessary nose-up change in attitude, this inability would probably be interpreted as insufficient elevator response or a residual out-of-trim force by the autopilot, thus a trim follow-up input was commanded. The trim input continued because the 'blocked' force was not reduced.

As an analogy, the situation is like a pilot overpowering the autopilot nose down (hold the stick fixed when decelerating) resulting in an autopilot trim command; trim has far greater effect on pitch control than the elevator, thus the flight path deviation (most autopilots sense stick overpower and should disengage).

This doesn’t exclude a rare technical failure causing the trim to run, however there are multiple safe-guards against this, whilst for de-icing fluid freezing there are few if any.

When disengaging the autopilot the crew would have experienced the out-of-trim force requiring forward stick, which they were applying, but against a powerful trim input.
The key aspect of recovery is in recognising that a trim input is required (after disengaging the autopilot), but this is not always self-evident as indicated by similar low speed incidents and accidents.
Does modern training impart an expectation that aircraft will always be in trim, or not to appreciate stick-force cues (excluding certain FBW types)? Do pilots expect that centring the stick – by feel, to achieve zero force, will enable a low speed recovery – little awareness of the trimmed condition; is trim taught as a control system?

It would be interesting to know what aircraft types the incident pilots flew previously. I recall a similar incident in a conventional aircraft (not 737) where a nose-high overpower condition was induced due to poor control-feel awareness (previous / extensive FBW experience), yet the recovery was aided by the military FBW experience including controlling surprise and of low speed nose-high wing-over manoeuvres.

A de-icing fluid incident is described here: http://www.aaib.gov.uk/cms_resources...pdf_022992.pdf but note that the BAe146 has a servo-tab control system and if this is blocked there is some ability to move the elevator directly – an advantage of ‘wire’ control
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Old 12th Dec 2013, 18:22
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All very muddy to me, I guess we would need a detailed report to try to understand. Stuck elevator, ok that makes sense though the report didn't specifically refer to it as stuck/jammed. The power pitch couple I understand. However if the autopilot is in some kind of vertical mode, and the aircraft started to deviate from that and pitch up, I would expect the autopilot to gradually provide a nose down force to the elevator to command nose down. If that force is over a certain threshhold (which it would soon be if the elevator was jammed), then surely the trim system would start to trim nose DOWN, not up.

Must be more to this picture of course

Isn't there a simulator session for jammed elevator procedure that pilots would occasionally see?

thanks for your replies Safetypee
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Old 12th Dec 2013, 23:10
  #54 (permalink)  
 
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The thing is Hawk that if the autopilot loses use of the elevator then who knows how it will react? I believe that the autopilot uses the elevator the way we were taught to, ie select an attitude and then trim. Autopilot trim is a bit coarse and at a slower rate than normal trim. I suspect it just couldn't keep up with a situation it had never been programmed to deal with. Add the actions of the auto thrust which knows nothing of pitch or trim and was only concentrating on speed, throw in a bit of turbulence and system lag perhaps. There you have it.

Last edited by tightcircuit; 14th Dec 2013 at 09:19.
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Old 16th Dec 2013, 17:40
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So if I deice my 737 tomorrow, my manuals call for horizon stab full nose down. Is there a Boeing doc saying it should be otherwise. That seems to be what the report says. Have any of you had bulletins or manual changes?
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Old 16th Dec 2013, 18:58
  #56 (permalink)  
 
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Boeing Bulletin

There is a Bulletin which came out back in October, I think. Your lot must be slow at publishing them to crews. Especially on such a critical subject...

Prior to de icing, set stab to calculated take off position.
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Old 16th Dec 2013, 19:42
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...or 5 units NU!
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Old 17th Dec 2013, 23:35
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Just searched and I can't find the bulletin.
Can somebody post a link ? Cheers
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Old 18th Dec 2013, 08:34
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I can't find the BAOM reference, but here is the text from it:


IssueDate: October 16, 2013
Subject: Supplementary Procedure, Adverse Weather, De-icing/Anti-icing
Reason: To inform 737 flight crews that the stabilizer trim must be set to the calculated takeoff setting prior to de-icing the airplane.

Information in this bulletin is recommended by The Boeing Company, but may not be FAA approved at the time of writing. In the event of conflict with the FAA approved Airplane Flight Manual (AFM), the AFM shall supersede. The Boeing Company regards the information or procedures described herein as having a direct or indirect bearing on the safe operation of this model airplane.

Background Information
In the winter of 2012, a 737 customer reported an un-commanded pitch up on final approach followed by reduced elevator response rates during the upset correction. Root cause analysis determined that the reduced elevator rates were caused by ice accumulation on multiple elevator Power Control Unit (PCU) input levers. De-icing prior to takeoff caused fluid to enter the aft area that accumulated and froze on the PCU input levers during flight, which is believed to be the cause of this incident.

The current de-icing Supplementary Procedure calls for the stabilizer to be trimmed to full APL NOSE DOWN (AND), which positions the stabilizer leading edge to the full leading edge up position. Analysis and testing discovered that this position maximizes the amount of liquids that enter the tailcone near the elevator PCUs due to the opening in the tailcone sides left uncovered by stabilizer’s extreme leading edge up position. If enough fluid accumulates and freezes around the PCU input levers their movement may be restricted or jammed.

Follow-on analysis and testing of various stabilizer positions during de-icing demonstrated that setting the stabilizer trim to the calculated takeoff setting (in the green band) prior to de-icing significantly diminishes the amount of fluids entering the tailcone, reducing the likelihood of a similar event caused by ice accumulation on the PCU input levers.

Operating Instructions
Prior to de-icing the airplane, set the stabilizer trim to the calculated takeoff setting, rather than Full APL NOSE DOWN.

Administrative Information
Insert this bulletin behind the Operations Manual Bulletin Record page in Volume 1 of your Operations Manual. Amend the Operations Manual Bulletin Record to show bulletin "In Effect" (IE).
This bulletin will be cancelled in a future revision of the FCOM.
Please send all correspondence regarding Flight Crew Operations Manual Bulletin status, to the 737 Manager, Flight Technical Data, through the Service Requests Application (SR App) on the MyBoeingFleet home page.
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Old 18th Dec 2013, 23:43
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Our procedure call for Take Off trim setting...typically around 5 units
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