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

Scary stuff


Norwegian near-stall to spur 737 modification

Scary indeed, how after almost 50 years of operating this type, we are inventing new deadly procedures.

In my opinion, horizontal stabilizer should be deiced manually, without fluid touching the most important flying surface. It is inverted airfoil anyway. Anyone deicing the bottom part?

near stall
 

"But it notes that, during the initial ascent, there was “no attempt” to disengage the autopilot, autothrust or manually adjust the stabiliser trim to nose-down."

UNBELIEVABLE !

I've been retired a long time but cannot imagine current crews are so "auto dependent"

This is not applicable to the problem that is being discussed. It might lead one to conclude that adding more heat would solve the problem. It's the residence time of the crystals against the probe. I liken it to light snow bouncing off a car windshield until you heat the windshield and a light layer of water-slush needs your wipers to become clear.

carlstraub

Have you flown the 737? Applying force to the control column (especially 920N) should disconnect the Autopilot.

Sounds to me like the crew averted an attempted Autopilot induced stall. I wouldn't call that "auto dependent"

No Safety recommendation yet from Boeing, so all pilots should apply normal published procedures until adviced.

SHT preliminary report here (.pdf):

http://www.aibn.no/Luftfart/Rapporte...-File&attach=1

to compare PT2 heating with is like to compare apples and oranges

very interesting incident and I'm glad to hear that a safe landing was accomplished.

set standard is right...enough force on controls should disengage the autopilot

often times in situations like this, the perfection of translation can come into question.

haven't we learned that in icing situations one should hand fly? of course that pre supposes that one knows how to hand fly on instruments well...

I always hated the deicing procedure for the tail of the 737...what a pain!

There appears to be similarities with control freezing or control difficulties due to de-icing fluids experienced several years ago.
The de-icing fluid manufactures have implemented changes to the fluid composition, but there is still some residual risk of ‘evaporative dry-out’ and ‘rehydration’ during flight, resulting freezing/blocked control systems. Many manufactures require inspection and/or wash-off action to remove accumulations of ‘gel’ from previous fluid application.

De Ice Fluid residue (Gel) in a control system bearing – not B737.

http://i41.tinypic.com/2zz8cqc.jpg

Indeed, I had colleagues back in 2001 that experienced something not dissimilar on a Fokker 100 (no unusual attitudes, but loss of longitudinal control )

wondering out loud if new 737 icing thing has any connection to russian 737 crash.

I really doubt it, but

It's seven strokes to one:

Fluid residue and rehydration can be a serious issue as highlighted by safetypee. A lot of work was done by the CAA 10 years ago following a number of elevator freezing incidents on the 146. It is worthwhile reviewing the CAA winter operations web site to refresh the memory on some of the hazards that are out there each winter. De-Icing and Anti-Icing Fluid Cautions | Air Operations | Operations and Safety

It's a pretty concerning incident. The sound of the trim wheel continuously running has been drummed into my head from Day 1 in the sim as a major cue that "something is not right"

That photo from safetypee does not fill my heart with warm feelings!

<<manually adjust the stabiliser trim to nose-down>> or would that be to nose-up if they had a jammed elevator?
Don't know how the B737 system works.

The aircraft subsequently landed safely after a 30min hold for the crew to conduct a systems check.

That is a very polite way of describing the practice of checking one's underpants for evidence of incontinence. Was this written by a British chap, perhaps? A model for us all.

I confess ignorance on such matters but that doesn't stop me from asking a stupid question.

If this is "de-ice" fluid, then it seems counter intuitive that it would freeze. What causes this fluid to freeze?

Basil,

The trim on the 73 (and I think all boeings) works by adjusting the angle of attack of the whole horizontal stabilizer. Trimming nose down with a stuck elevator will lower the nose. In fact in some circumstances, because of the large power-pitch couple the elevator is not powerful enough to lower the nose and as a result the stall recovery drill includes using nose down pitch trim if nose down elevator does not have the desired effect:

• Initiate the recovery:
• Hold the control column
firmly.
• Disconnect autopilot and
autothrottle.
• Smoothly apply nose down
elevator to reduce the angle of
attack until buffet or stick
shaker stops. Nose down
stabilizer trim may be
needed.

This is also the case for a nose high upset recovery:

• Disconnect autopilot and autothrottle
• Apply as much as full nose-down
elevator
• * Apply appropriate nose down
stabilizer trim
• Reduce thrust
• * Roll (adjust bank angle) to obtain a
nose down pitch rate
• Complete the recovery:
- When approaching the horizon, roll to
wings level
- Check airspeed and adjust thrust
- Establish pitch attitude

Exactly the same applies on the 757 and 767

In this incident the crew do not appear to have attempted either of these drills.

Baja, in very simple terms, de-icing fluid is a mix of glycol and thickening agent/dye. If this mix is retained in a crevice or bay, and/or not removed by the airflow during flight – parked overnight, then the glycol can evaporate. The residual powder may absorb water and expand by a factor ~x10, forming a ‘gel’, which can freeze.
Thus there can be a period without incident due to the absence of liquid water (sub zero / cold dry operations), but later exposure to wet conditions re-hydrates the ‘gel’ which may freeze during flight.