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Old 26th Aug 2013, 22:53   #1 (permalink)
 
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Genx Icing Problem

This very serious generic type incident got buried by PPRuNe in the freighter forum

below is an excellent summary and a hope that it really can be addressed before it gets compounded into an accident.

I really don't see any of the big engines being totally immune from this until a level playing field design and cert standard is available

Boeing, GE Test Upgrades To Counter Engine Icing

Quote:
The July 31 incident, which hit an AirBridge Cargo 747-8F enroute from Moscow to Hong Kong, is the latest encounter of a high flying aircraft with the poorly-understood phenomenon of core engine icing. In this situation engines can surge and suffer power ‘roll-backs’ strike with little or virtually no warning because ice crystal clouds do not show up on weather radar. The problem is unusual because it generally occurs at high altitudes where atmospheric moisture levels are normally very low, and because it impacts the high pressure core of turbofans which were previously thought to be virtually immune from significant icing.
The AirBridge Cargo 747-8F was in darkness at 41,000-ft over China, near Chengdu, when it deviated to avoid a thunderstorm. According to Russian federal air transport authority Rosaviatsia, the aircraft entered an unseen area of ice crystal cloud not shown on the weather radar. Air temperature rose by 20 deg C to minus 34 deg C for a period of 86 seconds, and the crew switched the engine ice protection system from automatic to manual for around 10 minutes.
Around 22 minutes after flying through the warmer sector the aircraft’s No.2 (inboard left) engine surged and automatically restarted. The No.1 engine then experienced a speed reduction of 70% of N1. After landing at Hong Kong inspections revealed damage to the high-pressure compressor blades of the No.1 and 2 engines as well as the No.4.
Boeing says the flight test effort is focused on “verifying operational elements” of a change to the engine control software. The testing included monitoring the development of ice crystals on the GEnx-2Bs powering RC021, one of the company’s test airframes that has recently been used to evaluate fuel system upgrades and other performance improvements. The fully-instrumented aircraft was originally designated for 747-8I launch customer Lufthansa, but was retained as a test asset after the German carrier opted not to take the modified airframe.
The software changes to the GEnx-2B full authority digital engine control unit are designed to help the engine itself detect the presence of ice crystals when the aircraft is flying through a convective weather system. If detected, the new algorithms will schedule variable bleed valves to open and eject ice crystals that may have built up in the area aft of the fan, or in the flowpath to the core. The modification to the GEnx control logic leverages similar changes made to improve the ability of the CF6 to operate in similar icing conditions.
The ABC event is the latest in a growing number of engine icing incidents which have triggered recent changes in international certification requirements. Unlike traditional engine icing, in which supercooled liquid droplets freeze on impact with exposed outer parts of the engine as the aircraft flies through clouds, engine core ice accretion involves a complex process in which ice particles stick to a warm metal surface. These act as a heat sink until the metal surface temperature drops below freezing, thereby forming a location for ice and water (mixed phase) accretion. The accumulated ice can either block flow into the core, or shed into the downstream compressor stages and combustor, causing a surge, roll-back or other malfunction.
Until relatively recently is has been assumed ice particles would bounce off structures and pass harmlessly through bypass ducts, or melt inside the engine. Now there is evidence that there is an environment where there is a combination of water, ice and airflow which is susceptible to accreting ice. Like many of the other known core icing events, the ABC 747-8 incident occurred near convective clouds.
When incidents were first reported, investigators initially assumed supercooled liquid water, hail or rain was responsible because it had been lifted to high altitudes by updrafts. Most events were recorded above 22,000-ft, which is considered the upper limit for clouds containing supercooled liquid water. However, pilots reported that even though they were in cloud at the time, there was no evidence of the usual indications of trouble, including significant icing on the airframe or any other remarkable aspect to the weather.
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Old 27th Aug 2013, 08:30   #2 (permalink)
 
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These conditions are apparently very rare and elusive. I wonder how long Boeings test airplane has to fly before it encounters the conditions of the ABC 747-8 incident, if it ever finds them.

I understand that changes to the certification requirements are in the proposal phase. When adopted, they will only apply to new certifications. How do manufacturers qualify their engines for conditions that are never there when you need them?

An interesting element is that the ABC airplane was deviating around a thunderstorm, as was AF447. The pilot of AF447 had the impression that they were just below the top of the ice particle cloud they were entering. Satellite and radiosonde data indicate that cloud tops in the area were higher than 50,000 ft. Perhaps they were entering a new cell that was just building up below them, and the particular ice particles are unique to that situation?

Interesting also that the TAT probes iced up, but no airspeed anomalies.

Last edited by HazelNuts39; 27th Aug 2013 at 09:51.
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Old 27th Aug 2013, 08:49   #3 (permalink)
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Quote:
Originally Posted by loma
This very serious generic type incident got buried by PPRuNe in the freighter forum
- yes, clever that, since it obviously only affects freighter engines.
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Old 27th Aug 2013, 08:53   #4 (permalink)
 
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Quote:
These conditions are apparently very rare and elusive
Perhaps not as uncommon as you think. I just did a sim today and one of the discussion items was on "Ice Crystal Icing" (Boeing QRH title), the check captain said that there had been one or two occasions of it happening a month. Yes I know that amount of events is minuscule compared to the amount of GE engined aircraft that fly every day, but I wouldn't want to be there when it was happening.
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Old 27th Aug 2013, 10:11   #5 (permalink)
 
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Quote:
the check captain said that there had been one or two occasions of it happening a month.
Approximately 1 every 4 months.

Ref. AIAA 2006-206-739 Mason, Strapp & Chow
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Old 27th Aug 2013, 10:27   #6 (permalink)
 
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poor WX radar technique?

does not look so much like a technical problem of any engine / airframe combination but rather like some CB avoidance techniques have been lost....

I mean if the SAT changes for 30° C , then you are in an updraft just way too close above a CB...right?

anything can happen there....engines & pitots may ice up....you may experience compressor stalls ( without any ice) and a gross jet upset...

my theory since quite a time on these "ice crystal" engine things is, that flight crews, for whatever reasons, do not seem as conservative in lateral and vertical CB avoidance anymore....

Last edited by falconer1; 27th Aug 2013 at 10:33.
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Old 27th Aug 2013, 10:50   #7 (permalink)
 
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Quote:
if the SAT changes for 30° C , then you are in an updraft
The SAT didn't increase 30° C. The change in indicated temperature was 'probably' due to the TAT probe(s) freezing up.

From the same paper:

-- Events occur with little or no reflectivity at flight level - ice particles are very small & poor reflectors of radar energy

-- While avoiding weather radar returns - not flying through heavy storm cores

-- low to moderate turbulence - low updraft velocities

-- No airframe ice - ice particles hitting airframe do not accrete

Last edited by HazelNuts39; 27th Aug 2013 at 13:47. Reason: 'probably' added
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Old 27th Aug 2013, 10:52   #8 (permalink)
 
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thank you

for correcting me on the SAT / TAT thing...

however still think that it is primarily faulty WX avoidance...

Last edited by falconer1; 27th Aug 2013 at 10:52.
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Old 27th Aug 2013, 12:46   #9 (permalink)
 
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@BOAC

This from the Lufthansa website about their 747-8i:

Quote:
The Boeing 747-8 Intercontinental features new, state-of-the-art wings with improved aerodynamics and raked wing tips; new fuel-efficient, U.S.-manufactured GEnx-2B engines
So, clearly not only affecting freighter engines.
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Old 27th Aug 2013, 12:54   #10 (permalink)
 
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Quote:
Air temperature rose by 20 deg C to minus 34 deg C for a period of 86 seconds, and the crew switched the engine ice protection system from automatic to manual for around 10 minutes.
Around 22 minutes after flying through the warmer sector the aircraft’s No.2 (inboard left) engine surged and automatically restarted
HazelNuts, while I partially agree with your hypothesis, the report clearly states an air temperature rise, not an indication anomaly.

Last edited by cockygashandlazy; 27th Aug 2013 at 12:55.
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Old 27th Aug 2013, 13:03   #11 (permalink)
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Quote:
Originally Posted by Steve the Pirate
So, clearly not only affecting freighter engines.
- my comment was an obviously failed attempt at humour............
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Old 27th Aug 2013, 13:45   #12 (permalink)
 
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cockygashandlazy,

It's a journo writing. IMHO a temperature rise of 20 degrees C to minus 34 degrees C at 41,000 ft can only be explained by assuming an anomaly of the temperature sensor, which is a common occurrence in these incidents, see the Mason paper.

EDIT:: A TAT sensor is enclosed in a heated duct, susceptible to accretion of ice particles by the same mechanism as engine internal ducts and vanes.

Last edited by HazelNuts39; 27th Aug 2013 at 13:55.
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Old 27th Aug 2013, 14:02   #13 (permalink)
 
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well.....

Quote:
IMHO a temperature rise of 20 degrees C to minus 34 degrees C at 41,000 ft can only be explained by assuming an anomaly of the temperature sensor.....
not so sure about that, in fact there are known cases, especially from aircraft crossing the ITZ, were exactly such a SAT temp "anomaly" happened, due to convective action BELOW the aircraft's flight level, which in fact had created havoc with the engines and the flight path...in one case a G-IV experiened unstable engine ops ( I think it was a compressor stall) and departed controlled flight in a gross upset...

Last edited by falconer1; 27th Aug 2013 at 14:02.
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Old 27th Aug 2013, 14:37   #14 (permalink)
 
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falconer1,

In the cases you mention, how was it established that the TAT sensors were not affected by icing? Also the compressor stall is usually what causes the engine rundown. According to Mason's paper it occurs when the ice accreted on warm internal engine surfaces is released into the gas path.
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Old 27th Aug 2013, 14:54   #15 (permalink)
 
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Rare encounters ? yes

But obviously the results can be so severe, the encounters must be either eliminated or mitigated by the engines, etc.

I may be mistaken but I got the impression that the crews initial actions were in response to a serious event already underway.

Is there something they should have done before hand ?

From the engine standpoint, with the latest knowledge, the problem is actually aggravated by the increase in temperature as the air/ice gets compressed in the engine. It turns the ice crystals into little stick pellets that stick and accumulate to surfaces long enough to build up layers, temporary in nature.

The problems comes in when these temporary sticky buildups dislodge and cause downstream damage to the high speed compressor section, or screw up the airflow enough to cause the engine to surge or spool down.

Engines at high altitudes are quite sensitive to changes their FADEC logic wasn't expecting.

Given a chance for being activated, engine bleeds are your friend.

The FADEC logic actually has more to work with than just what the pilot senses from aircraft sensors.

For instance, the RPM and temp matches between the engine spools might be sampled fast enough by the FADEC to activate bleeds before the engine stops working.

The theories are there to be used but the verification may not be easy nor quick.
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Old 27th Aug 2013, 16:10   #16 (permalink)
 
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@BOAC

Quote:
- my comment was an obviously failed attempt at humour............
Ah, sorry, I'm normally reasonably good at spotting that.....
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Old 27th Aug 2013, 17:21   #17 (permalink)
 
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falconer1

TAT corruption is a common indicator of ice crystal icing - it's actually listed as such in the Boeing AFM. The heated TAT probe has proved to be very susceptible to corruption by ice crystals - characteristic of ice crystal icing is that it happens at very low temperatures - much colder than traditional icing. I've looked at lots of event data from ice crystal icing and we nearly always see TAT corrupted well before we see any other effects. BTW, they did try to redesign the TAT probe for ice crystals several years ago - doesn't appear to have helped.

Traditional anti-ice systems have addressed super cooled liquid water that hits the cold surfaces of an aircraft and freeze. Ice crystals don't do that - they just bounce off cold surfaces. What they can do is freeze on surfaces that are normally considered too warm for icing.

Quote:
Boeing, GE Test Upgrades To Counter Engine Icing After 747-8 Incident
Aviation Week.com 08/23/2013
Author: Guy Norris



Boeing and General Electric have flight tested an engine control software upgrade in a 747-8 which is designed to prevent the same form of ice crystal build-up that damaged three GEnx-2B engines on a Russian-operated freighter last month.
The July 31 incident, which hit an AirBridge Cargo 747-8F enroute from Moscow to Hong Kong, is the latest encounter of a high flying aircraft with the poorly-understood phenomenon of core engine icing. In this situation engines can surge and suffer power ‘roll-backs’ strike with little or virtually no warning because ice crystal clouds do not show up on weather radar. The problem is unusual because it generally occurs at high altitudes where atmospheric moisture levels are normally very low, and because it impacts the high pressure core of turbofans which were previously thought to be virtually immune from significant icing.

The AirBridge Cargo 747-8F was in darkness at 41,000-ft over China, near Chengdu, when it deviated to avoid a thunderstorm. According to Russian federal air transport authority Rosaviatsia, the aircraft entered an unseen area of ice crystal cloud not shown on the weather radar. Air temperature rose by 20 deg C to minus 34 deg C for a period of 86 seconds, and the crew switched the engine ice protection system from automatic to manual for around 10 minutes.

Around 22 minutes after flying through the warmer sector the aircraft’s No.2 (inboard left) engine surged and automatically restarted. The No.1 engine then experienced a speed reduction of 70% of N1. After landing at Hong Kong inspections revealed damage to the high-pressure compressor blades of the No.1 and 2 engines as well as the No.4.

Boeing says the flight test effort is focused on “verifying operational elements” of a change to the engine control software. The testing included monitoring the development of ice crystals on the GEnx-2Bs powering RC021, one of the company’s test airframes that has recently been used to evaluate fuel system upgrades and other performance improvements. The fully-instrumented aircraft was originally designated for 747-8I launch customer Lufthansa, but was retained as a test asset after the German carrier opted not to take the modified airframe.

The software changes to the GEnx-2B full authority digital engine control unit are designed to help the engine itself detect the presence of ice crystals when the aircraft is flying through a convective weather system. If detected, the new algorithms will schedule variable bleed valves to open and eject ice crystals that may have built up in the area aft of the fan, or in the flowpath to the core. The modification to the GEnx control logic leverages similar changes made to improve the ability of the CF6 to operate in similar icing conditions.

The ABC event is the latest in a growing number of engine icing incidents which have triggered recent changes in international certification requirements. Unlike traditional engine icing, in which supercooled liquid droplets freeze on impact with exposed outer parts of the engine as the aircraft flies through clouds, engine core ice accretion involves a complex process in which ice particles stick to a warm metal surface. These act as a heat sink until the metal surface temperature drops below freezing, thereby forming a location for ice and water (mixed phase) accretion. The accumulated ice can either block flow into the core, or shed into the downstream compressor stages and combustor, causing a surge, roll-back or other malfunction.
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Old 27th Aug 2013, 17:58   #18 (permalink)
 
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WX avoidance?

tdracer

thank you for your info...

Heard and read about such incidents and events for years, bizjets also had been involved..

Boeing actually had published excellent material on these problems

AERO - Engine Power Loss in Ice Crystal Conditions

AERO - Boeing Assistance in Airplane Recovery

but for all I think that I know, the "underlying fundamental" is that in all cases known so far the birds have been in a problematic weather situation...without being there nothing would have happened....

so maybe the only "way out" is to find better CB and WX avoidance techniques and / or have old proven techniques have a renaissance...like knowing how to use a WX radar....and making sure that one is a safe distance away, both laterally and vertically from severe convective weather...

I do not see any possibility of making modern airplanes and engines CB penetration safe...
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Old 27th Aug 2013, 18:19   #19 (permalink)
 
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Quote:
These conditions are apparently very rare and elusive.
They are not as rare as originally thought - significant densities of ice crystals near (tropical) convective storms appear to be relatively common.
The problem is that flying through these ice crystals does not always seem to have an effect.

Quote:
Satellite and radiosonde data indicate that cloud tops in the area were higher than 50,000 ft. Perhaps they were entering a new cell that was just building up below them, and the particular ice particles are unique to that situation?
They flew through a large-scale system that was composed of 4 smaller cells. At the time they flew by it was beginning to diminish, not build.
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Old 27th Aug 2013, 18:54   #20 (permalink)
 
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Quote:
At the time they flew by it was beginning to diminish, not build.
Satellite infrared imagery returns the temperature of the cloud tops, indicating the height of these tops. Yes, that height was reducing but, IMHO, that does not exclude the possibility that a newer cell was developing underneath.

See also BEA's 2nd Interim Report, para. 1.7.3:
Quote:
Analysis of the observations by the TRMM TMI instrument, the only one
operating in the microwave area, indicates the presence of strong condensation
around 10,000 metres altitude, lower than the altitude of the cumulonimbus
tops. This strong condensation would correspond to convective towers active
at this altitude, which confirms the strong probability of notable turbulence
within the convective cluster that was crossed by planned flight path of
flight AF447.

Last edited by HazelNuts39; 28th Aug 2013 at 15:31.
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