Go Back  PPRuNe Forums > Flight Deck Forums > Tech Log
Reload this Page >

ICE crystals

Tech Log The very best in practical technical discussion on the web

ICE crystals

Old 21st Jul 2009, 22:43
  #41 (permalink)  
Warning Toxic!
Disgusted of Tunbridge
 
Join Date: Jan 2005
Location: Hampshire, UK
Posts: 4,011
Likes: 0
Received 0 Likes on 0 Posts
It doesn't happen. It hasn't happened in nearly 40 years of 747 operations up and down African through the ITCZ, across the Indian Ocean through monsoons and hell, and through storms in Asia. BA has been operating up to 114 widebody aircraft on worldwide routes. I've done my share flying them. The rule is below -40, you can forget icing. I've not known of a single case to contradict it- had it happened, we would have changed our procedures. My procedures are as outlined, and as far as I know, BAs are as well.

The proof being thrown at me that it does happen seems to consist of airey-fairey theoretical papers and unsubstantiated accident theories, in particular AF447 which appears to have had substandard systems, but nobody really knows. When the advice and official procedures are changed, I will gladly go along with the new procedures. But until then, some of you may as well save flapping your lips together, because you are spouting unproven theories that I do not accept or believe. Is that clear enough for some of you? I tried to say it nicely, but that's how it comes!
Rainboe is offline  
Old 21st Jul 2009, 23:31
  #42 (permalink)  
 
Join Date: Jun 2009
Location: east of 10° west
Age: 62
Posts: 104
Likes: 0
Received 0 Likes on 0 Posts
OK, Rainboe

may I offer, without any hard feelings, some middle ground that we could maybe agree to..

I remember when I heard of the Beechjet events. My first thought at that time was, well, those poor folks obviously must have had some sort of fuel contamination, may be a water and / or strange water/PRIST issue, whatever..

The Beechjet events still do not seem as clear cut, as remember that a short time after that, PWC issued some Service Bulletins for those engines in Beechjets and C-560s, one is half ( single channel) FADEC with hydromechanical back-up and the other fully hydromechanical on the FCU side, and those SBs actually rerouted some hot bleed air to a sensitive part of the FCU that was presumed to maybe have become "frozen", albeit as PWC stated in a high humidity environment..

so the Beechjet events actually may not fall into the "iced compressor" category, but then who knows..

Now to the Boeing, NASA. FSF studies AND some revised AFM info on Airbuses and Boeing etc. regarding possible "Compressor Icing"..

Guess we can agree that the stuff in and close to highly convective weather at altitude is touchy..

could we agree now on the following: new engine technology, that employs highly tuned and efficient compressors, may in fact be more susceptible to airflow disruption by the phenomenon described in the Boeing AERO article than older tech engines....?!

air traffic has increased immensely over the last decades, the altitude bands in which we fly are a tad different from the ones 30 - 40 years ago, and last but not least ( my favorite theory, as you may have found from my other posts), could it be that more airplanes at altitude fly closer to convective weather than ever before ( maybe due to some false sense of safety by modern crews using modern weather avoidance equipment..)???

Now , could all these factors taken together mean, that we start to see stuff, that simply had not been encountered on a statistically significant basis 30 - 40 years ago???
falconer1 is offline  
Old 22nd Jul 2009, 01:43
  #43 (permalink)  
 
Join Date: Dec 2002
Location: UK
Posts: 2,451
Likes: 0
Received 9 Likes on 5 Posts
My involvement with ‘ice crystal icing’ started in the late 80’s with engine ‘rollback’ events involving the BAe146 aircraft; but only with the ALF 502 engine. At that time, the cause of the problem was not known.
As events accumulated, numerically and severity, investigations and tests associated the issue with ‘weak’ engine anti-icing, large Cbs, high altitude (>FL300), and large ISA+ deviations.

Ground tests in engine/icing tunnels together with modeling provided a consistent theory linking ice particles (seen in other work), from which modifications evolved.
Flight tests in European icing conditions repeated the normal icing certification parameters (Appx C) and checked that very severe SLD was not the cause.
Flight tests in / near large Cbs were conducted in Panama to measure atmospheric conditions and induce a rollback; no rollbacks occurred. Subsequent tests in the US in similar conditions did induce a rollback on an unmodified engine and proved the effectiveness of the modifications on another.
These tests are summarized in the ‘Strapp’ report (The Ice Particle Threat to Engines in Flight - http://www.pprune.org/tech-log/37634...ml#post5027449) under ‘Commuter Transport Aircraft’.

The essential elements of the theories were substantiated with the work by Strapp, and that of Stromberg (University of Manchester/UMIST), Maltby (BAe/now RR?), et al – Honeywell engines etc, using the BAe146 test aircraft. Fully instrumented engines including one with cameras inside the engine identified ice formation and severity over a range of ‘ice crystal’ conditions.
The relatively small 502 engine suffered ice blocking of the first stage compressor stators which reduced the airflow, rolled back the N2 and thence N1 with increasing EGT. The rollback conditions were only susceptible in a narrow ranger of atmospheric conditions, depended on the duration of the encounter, and engine air off-take – airframe anti-ice, bleed-band setting etc. The conditions were associated with TAT probe anomalies due to freezing, but no pitot problems were seen.

No airframe icing was encountered during these tests – little or no liquid water – classic icing theory. Some light icing was seen on the windshield and wipers where the crystals melted on the heated screen and refroze at the edges; the screen appeared wet / very wet like flying in heavy drizzle.
There were some extremely damaging hail encounters in the tops of the Cbs, but not much fine ice crystal content, this was more in the outflow and anvil regions. Hence aircraft avoiding, or ‘cutting the corner’ around Cbs were more likely to encounter the problem. Interim restrictions on the 146 required Cb avoidance by a wide margin (20 nm IIRC) and subsequently no flight above FL280 in ‘icing conditions’ (< -10C in visible moisture). Problems still occurred due to HF and ‘interpretive’ issues, misjudging the distance to the storm (core centre / edge of the ‘red’) – use of WR, and lack of an airworthiness mandate. Also, the conditions were very difficult to detect. The ‘ice’ cloud structure was very wispy, almost non existent, and gave little or no return on radar.

Following sever multiengine events, ADs were issued and a mandated modification program instigated; all engines were modified by improving the aerodynamic design of the first stage of the compressor and heat flow in the inlet guide vanes, amongst other minor improvements.
The very similar ALF 507 engine was not affected; it had two stages of at the front of the compressor vs one, and was further reinforced by the introduction of FADEC (bleed control).

BAe/Honeywell did liaise with other engine manufacturers who also reported high altitude ‘unexplained events’ – (manufacturers do talk to each other about safety matters). However the nature of the problems on the ‘big’ engines were not the same or severe (at that time) as the 502 engine; the differences are described in the report under Large Transport Aircraft.

Although the ‘threat’ does not appear to be universal to all engines or aircraft, the issues of severity and exposure to ice crystal conditions, the inconsistent nature of the atmosphere, and changes with the ‘high tech’ evolution of engines should not (IMHO) lead to any complacency; it may be that we have yet to see a large enough storm or for someone to misidentify the hazard applicable to thier aircraft.
safetypee is offline  
Old 22nd Jul 2009, 07:54
  #44 (permalink)  
Warning Toxic!
Disgusted of Tunbridge
 
Join Date: Jan 2005
Location: Hampshire, UK
Posts: 4,011
Likes: 0
Received 0 Likes on 0 Posts
Very good. Nothing more than 'theories' and 'theoretical papers', usually involving other types and engines I don't know! Meanwhile my manuals remain unchanged. I will continue to operate to the official advice in my manual. Can you tell me when it has let the system down? Not 'suspicions' please.

The OP asked a question. It was answered with the official line. Some of you then went off on one with unsubstantiated theoretical stuff usually involving other engines/types. He wanted a simple answer!
Rainboe is offline  
Old 22nd Jul 2009, 08:06
  #45 (permalink)  
Per Ardua ad Astraeus
 
Join Date: Mar 2000
Location: UK
Posts: 18,579
Likes: 0
Received 0 Likes on 0 Posts
The OP asked a question.
- yes inded he did - he asked about something that is not in the engine icing section of your manual ('lifted' verbatim - and unchanged - from the BA 737 Classic manual of the 1980's.)
theoretical stuff usually involving other engines/types.
- which 'other engines/types' are you on about? Did he specify? I have put my reading glasses on and looked and I see
But what about the hot sections of the engine?
What do you see?
BOAC is offline  
Old 22nd Jul 2009, 14:50
  #46 (permalink)  
 
Join Date: Mar 2002
Location: Florida
Posts: 4,569
Likes: 0
Received 1 Like on 1 Post
The OP asked a question. It was answered with the official line. Some of you then went off on one with unsubstantiated theoretical stuff usually involving other engines/types. He wanted a simple answer!
Ah, but there in lies the true power of PPRune Technical Forum. It is not just a PM type one on one but a true discussion forum including drifting "off Topic" now and then and providing useful information to many more than just a single poster. Tis a shame that the thread got personalized, but if one reads beyond the personal coments there is much information and truly balance opinions within.

Typically the opening question is often open ended and subject to interpretation by the readers and responders, hence the broad viewpoints.
lomapaseo is offline  
Old 22nd Jul 2009, 17:51
  #47 (permalink)  
 
Join Date: Feb 2005
Location: Correr es mi destino por no llevar papel
Posts: 1,420
Received 0 Likes on 0 Posts
Originally Posted by Rainboe
It doesn't happen.
It happens so seldom that the large majority of the pilots can justifiably say: "It never happened to me." Saying that it doesn't happen at all is somewhat untrue.

I will continue to operate to the official advice in my manual.
Very good, sir. It seems to me that current procedures (inflight restart, unreliable airspeed) are quite capable of coping with possible unfortunate effects of encountering nasty ice particles in large concentrations.

Incidentally, official lines may or may not bear resemblance to real world. Can you count changes of the manuals and procedures during your career, brought on by the demise of the unsuspecting?
Clandestino is offline  
Old 22nd Jul 2009, 17:59
  #48 (permalink)  
 
Join Date: Dec 2002
Location: UK
Posts: 2,451
Likes: 0
Received 9 Likes on 5 Posts
There appears to be some difficulty in recognizing what has previously been theory and is now accepted as repeatable evidence, or that the established facts might apply to range of aircraft and engine types as circumstances change.

An earlier ‘peer reviewed’ article at http://www.specinc.com/publications/...craft_1998.pdf provides an interesting ‘operational’ view in the summary; vis:-

Based on the available meteorological data from research aircraft, it appears that only the most vigorous thunderstorms. i.e. super cells and complexes of thunderstorms, are associated with anvils that contain IWC (Ice Water Content) > 1 gm m^3 in regions outside of the main core of the storm.
There is currently insufficient data to determine the threshold concentration of IWC that will induce rollback. Based on the fact that literally billions of hours have been logged by commercial turbofan engines in common cirrus and anvils, with IWC typically <-0.2 gm m^3 it would appear that these clouds do not contain sufficient IWC to cause rollback problems.
Also, it appears that rollback may be more likely to occur the longer the engines are exposed to relatively high mass concentrations of ice particles and snow at very cold temperatures.
If indeed the TAT temperature rise and associated power loss reported in 1974 was an incipient rollback, it should be noted that this is the only reported incident of this kind associated with a Boeing 747, and this could possibly be a result of the large extent of high concentrations of IWC in the anvil remnants of Tropical Storm Kerry.

While we are not in a position to make recommendations to pilots, we can concur that the standard practice (see FAA Circular 00-24B) of avoiding the regions of high radar reflectivity by at least 20 nm is advisable. In terms of rollback avoidance, the main reason to remain as far as possible from the region of high radar reflectivity is that measurements show that the IWC typically drops off sharply as a function of distance from the storm center. Also, any regions containing convection, i.e. rising cloud parcels or overshooting tops could contain super cooled liquid water, which may exacerbate the rollback problem.
Lastly, the probability of rollback incidents appears to increase with duration in areas of relatively high IWC, so that minimizing the time of encounter in these thunderstorm anvil regions may be advisable.


The more recent Boeing position, particularly slide 6, links the 100 (plus) Large Transport Engine events to ice crystals.
IMHO this presentation answers the originating questions.

Note that engine manufacturers do not sit back without action; - General Electric Company (GE) CF6- 80C2A5F Turbofan A300, but also 747/767 if applicable.
Whilst there may not be specific advice in airframe manuals because defensive measures are already in place, this should not be taken to mean that a problem cannot be encountered.
We all require continued professional enquiry into atmospheric threats, particularly where we might not know how close to the edge of a safe boundary we are currently operating, e.g. a small change in operating procedures for air bleed during the cruise – driven by economics, might cross an ‘icing’ boundary.

In many BAe146 rollback incidents, the FDR showed that crews were unaware of other incipient events; a single engine event was often a multiple event. So in the larger engines with more tolerance to such conditions crews might encounter ice crystals and accrete ice, but due to the different concentration or duration of encounter they do not suffer visible symptoms.

The latter might be theory – possibly just speculation, but it is something that I wish to bear in mind when flying in such conditions.
safetypee is offline  
Old 22nd Jul 2009, 22:24
  #49 (permalink)  
 
Join Date: Nov 2004
Location: Here, there, and everywhere
Posts: 1,117
Likes: 0
Received 12 Likes on 7 Posts
Originally Posted by Clandestino
It happens so seldom that the large majority of the pilots can justifiably say: "It never happened to me." Saying that it doesn't happen at all is somewhat untrue.
Similar to the refusal to believe by some on the proper way to operate modern de-ice boots.

Or just the belief that if it doesn't affect any of the types at my airline, it does not exist.

Well look at this....

Regulations.gov

"The FAA proposes to adopt a new airworthiness directive (AD)
for certain Boeing Model 747 airplanes and Model 767 airplanes. This
proposed AD would require revising the airplane flight manual to advise
the flightcrew to use certain procedures during descent in certain
icing conditions. This proposed AD results from reports of several in-
flight engine flameouts, including multiple dual engine flameout events
and one total power loss event, in ice-crystal icing conditions. We are
proposing this AD to ensure that the flightcrew has the proper
procedures to follow in certain icing conditions. These certain icing
conditions could cause a multiple engine flameout during flight without
the ability of the engines to be relit, and consequent forced landing
of the airplane."

"The requirement to activate the engine anti-ice prior to descent in
visible moisture with total air temperature less than 10 [deg]Celsius
(C) and greater than -40 [deg]C already exists for airplanes that are
not equipped with a primary in-flight ice detection system, which is
designed to automatically activate wing anti-ice and engine anti-ice
when the airplane is in icing conditions. However, the primary in-
flight ice detection system does not detect ice-crystal icing;
therefore, the engine anti-ice would not be activated during these
icing encounters. There is no requirement to activate engine anti-ice
at temperatures below -40 [deg]C, and this proposed AD would require
activation of engine anti-ice at temperatures below -40 [deg]C.
This condition, if not corrected, could result in a multiple engine
flameout during flight without the ability of the engines to be relit,
and consequent forced landing of the airplane."

So if by chance the 747's operated by Rainboe have CF6 engines, he will be doing what the revised manuals will tell him to do even if he still refuses to believe what many manufacturers, regulators and investigative agencies already know. I suppose if BA has different engines, then this problem is still just an unsubstantiated theory and does not exist.

Last edited by punkalouver; 23rd Jul 2009 at 01:40.
punkalouver is offline  
Old 22nd Jul 2009, 23:33
  #50 (permalink)  
 
Join Date: Dec 2004
Location: Between Vedex and Murag!
Posts: 291
Likes: 0
Received 0 Likes on 0 Posts
Well spotted
shortfuel is offline  
Old 23rd Jul 2009, 08:00
  #51 (permalink)  
 
Join Date: Jun 2001
Location: UK.
Posts: 4,390
Likes: 0
Received 1 Like on 1 Post
I'm out of the business now but little question for someone who understands core icing intimately - if such exists:
What effect does switching on engine anticing have on ice crystal icing?
Basil is offline  
Old 23rd Jul 2009, 08:27
  #52 (permalink)  
Moderator
 
Join Date: Feb 2000
Location: UK
Posts: 14,202
Received 46 Likes on 24 Posts
I don't think that engine anti-icing is likely to have much effect, because that is primarily targetted at ice formation on intakes, not solid ice particles going through the compressor.

Rainboe does make some valid points in that at the moment there is limited knowledge and that a lot of what's there is in research papers, not aircraft manuals - so at the moment sticking to published operating advice is definitely the safest thing to do. That said, those research papers have been prepared by some world-class researchers and subjected to peer review of other world-class researchers before being published, so they are a heck of a lot more than guesswork.

It's clear (for example from the work by Walter Strapp and others quoted by SafetyPee) that there's an issue - hence the FAA/NASA research action that's ongoing. The issue is that *some* rollbacks have happened - albeit 100 in 10+ years is such a rate that most pilots have never knowingly seen it. It's a much smaller issue than conventional icing, if it wasn't we'd all know much more about it. But, I for one am morally entirely behind the researchers trying to understand this better because 100+ rollbacks in a finite period is still a lot, and with enough more there's a risk of an aircraft being lost eventually. That justifies the effort of researching it, and I'd anticipate the subject eventually finding it's way into affected MoMs (starting presumably with those listed by Punka Louvre) and the ATPL syllabus - although I'd guess not for a few years yet whilst the icing boffins try to understand it a lot more (and no doubt publish a fair number more papers!)

G
Genghis the Engineer is offline  
Old 23rd Jul 2009, 12:56
  #53 (permalink)  
 
Join Date: Nov 2004
Location: Here, there, and everywhere
Posts: 1,117
Likes: 0
Received 12 Likes on 7 Posts
Originally Posted by Basil
What effect does switching on engine anticing have on ice crystal icing?
According to the proposed AD:

These
conditions can cause ice crystals to
accumulate in the core flow path of the
engine during low-power conditions,
such as idle or idle descent. The
accumulated ice sheds during throttle
increase and is ingested into the engine,
causing the combustor to flameout
resulting in an in-flight flameout and
potential damage to the high pressure
compressor due to ice impact. The GE
CF6–80C2 and CF6–80A series engines
models have similar compressor
designs.
Activating the engine anti-ice
increases the flameout margin and
reduces the potential for multiple
engine flameouts by increasing bleed
flow and idle speed. Engine anti-ice also
assists with relighting the engines by
turning on the igniters on airplanes that
are not equipped with autorelight.
However, in several of the subject
engine flameout events, the engine antiice
was already on when the engines
flamed out. In each flameout event, the
engines relit and continued to operate
normally for the remainder of the flight.
punkalouver is offline  
Old 23rd Jul 2009, 13:05
  #54 (permalink)  
 
Join Date: Dec 2002
Location: UK
Posts: 2,451
Likes: 0
Received 9 Likes on 5 Posts
Genghis, et al; some aircraft have a combined engine intake lip and first stage compressor anti icing system. The146 is an example; the weakness of the compressor element contributed to the ice crystal problem.
The anti ice OFF situation was considered. In ‘theory’, the ice would bounce off an unheated blade / stator and proceed to the ‘hotter’ parts of the engine without problem. However, in this, there are at least two considerations. First, that the air entering the engine and compressor is heated by ‘intake’ compression and thus the first stages could still accumulate ice – note that some engines suffer fan blade icing in similar conditions. Second that in other circumstances not far removed from ice crystal conditions, the IWC of the atmosphere became liquid (and/or super cooled) before entering the engine, again leading to the possibility of icing. The research suggested that an altitude change of 2000 ft below the ice crystal layer might be sufficient to cause this – a change from an unknown problem to a known problem, but each requiring different anti-ice system selections and the crew not having sufficient information to decide which.
Thus for the 146 there was a need to need to fix the root cause - modify the engines; then follow the manufacturers procedures – use anti icing.

The absence of specific procedures/advice in other aircraft (IIRC the 146 has none) may reflect that other manufacturers heeded the research information and modified their engines (search for ADs), particularly those with evolving designs. Manufacturers still rely on ‘airmanship’ for the avoidance of Cbs by a sufficient margin – but what is ‘sufficient’.

Note that the 100+ events appear to be on Boeing aircraft only; I don’t have figures for any others except that the 146 had 50+ and that engine manufacturer’s shared data of events was a very (disturbingly) large file. Obviously, this excluded non-reported or misidentified events, and any that were incipient, ‘close to the edge’ and not seen by the crew.

Perhaps of interest a ‘what if’ question was asked about the use of airframe anti-icing in ice crystal conditions. Did the ice melt, runback, and refreeze in a more hazardous position. At that time, there was insufficient evidence for any practical operational conclusion.
safetypee is offline  
Old 23rd Jul 2009, 14:47
  #55 (permalink)  
 
Join Date: Nov 2004
Location: Here, there, and everywhere
Posts: 1,117
Likes: 0
Received 12 Likes on 7 Posts
Actually Beechjet and Airbus incidents have been mentioned on this thread.

Hmmmm.....Rainboe mysteriously disappeared from this thread. What a surprise.

Last edited by punkalouver; 11th Aug 2009 at 02:48.
punkalouver is offline  
Old 23rd Jul 2009, 16:28
  #56 (permalink)  
 
Join Date: Dec 2002
Location: Where the Quaboag River flows, USA
Age: 71
Posts: 3,407
Received 3 Likes on 3 Posts
The RR BR710 engine on GLEX, G500/550 and B717 have had several ice crystal incidents. I cannot say for the B717, but some have occurred above FL410. The engine rolls back while under FADEC EPR control, presumably ice crystal icing on the EPR probe. The advised solution is to switch to N1 control.

When I was at Eastern we had a B727 at FL330 enter clouds when the TAT indicated no potential for icing and promptly all three rolled back to a near-idle condition. They managed to restart them, but the company advised crews operating in tropical conditions to not hesitate to turn on engine anti-icing even if not strictly called for.

While I understand Rainboe's point of view, we should be open to the fact that the atmosphere holds a lot of surprises that, perhaps, only the dead know for sure. Just because the information is in technical publications and hasn't made it into flight publications, isn't to dismiss it. OTOH, we shouldn't be making up procedures in the cockpit, but where is the harm in using engine anti-icing when under these conditions?

GF
galaxy flyer is offline  

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Thread Tools
Search this Thread

Contact Us - Archive - Advertising - Cookie Policy - Privacy Statement - Terms of Service

Copyright © 2024 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.