Genx Icing Problem

[lomapaseo]

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 (http://www.aviationweek.com/Article.aspx?id=/article-xml/awx_08_23_2013_p0-609559.xml)


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.

[HazelNuts39]

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.

[BOAC]

This very serious generic type incident got buried by PPRuNe in the freighter forum - yes, clever that, since it obviously only affects freighter engines.

[Offchocks]

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.

[HazelNuts39]

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

[falconer1]

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....

[HazelNuts39]

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

[falconer1]

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

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

[Steve the Pirate]

@BOAC

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

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.

[cockygashandlazy]

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.

[BOAC]

So, clearly not only affecting freighter engines. - my comment was an obviously failed attempt at humour............

[HazelNuts39]

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.

[falconer1]

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...

[HazelNuts39]

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.

[lomapaseo]

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.

[Steve the Pirate]

@BOAC

- my comment was an obviously failed attempt at humour............

Ah, sorry, I'm normally reasonably good at spotting that.....

[tdracer]

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.

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



http://www.pprune.org/graphics/factiva.gifBoeing 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.

[falconer1]

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 (http://www.boeing.com/commercial/aeromagazine/articles/qtr_4_07/article_03_1.html)

AERO - Boeing Assistance in Airplane Recovery (http://www.boeing.com/commercial/aeromagazine/articles/qtr_01_10/5/)

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...

[Nemrytter]

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.

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.

[HazelNuts39]

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:
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.

[tdracer]

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.Agreed that they are not as rare as once thought (and the frequency appears to be increasing - perhaps due to climate change/global warming). But given the thousands of flights that occur daily and the handful of actual events, it's still pretty rare.

There was discussion several years ago of using an engine company flying test bed, with a highly instrumented engine (including things like internal cameras to record ice formation) but it was ultimately rejected as too expensive and impractical due to the difficulty of finding the right conditions.

The prospect of doing fight tests in ice crystals using an FTB is being revisited in light of the recent GEnx events.

[Nemrytter]

Yes, I should have been more clear - I meant that the ice itself appears (relatively) common. The incidents it can cause are, thankfully, much less common.

As for instrumented engines - sounds like something Airbus should be examining for their 2014/15 campaigns in Darwin. I'm sure the A340 has plenty of engines to spare :)

[JammedStab]

Higher Altitudes Cleared For GE-Powered 787, 747-8 In Icing

Anti-core icing strategies emerge as FAA relaxes restrictions on GEnx-powered 747-8 and 787

General Electric is introducing a final series of software and hardware improvements to mitigate the threat of core icing on its GE90 and GEnx engines, and is using lessons learned from the modifications to ensure no ice-related surprises occur with the GE9X in development for Boeing’s 777X.

Solutions range from improved software for faster ice-crystal-icing (ICI) detection in the GEnx to hardware changes that reduce exposure to engine-icing issues in the GE90-94 by rehousing temperature sensors. The software change is particularly important for the GEnx-1B-powered 787 and GEnx-2B-powered 747-8 because it clears operators to resume flying at higher, more economical altitudes in areas of known icing.

Core icing remains a relatively little understood phenomenon that chiefly affects aircraft flying at high altitudes between 38,000-41,000 ft. in the vicinity of intense convective activity such as large thunderstorms or tropical storms. Several aircraft, particularly 747-8s have suffered uncommanded thrust loss, and even damage, after ice particles accreted to areas behind the fan before breaking off in slabs to be ingested by the compressor.

The ice particles, which are usually encountered at such altitudes only in the moisture-laden atmosphere of the tropics, measure around 40 microns in diameter and have a reflectivity of only 5% of a raindrop, making them hard to detect using standard weather radar. Following several incidents, the FAA issued an airworthiness directive to 747-8 and 787 operators in November 2013 prohibiting operation in moderate and severe icing. In the presence of known ICI conditions, crews were directed to detour 50 nm around convective cells or reduce altitude to a maximum of 30,000 ft.

That same month, Japan Airlines reacted by replacing 787s with 767s on routes from Tokyo to several destinations including New Delhi and Singapore, and suspended plans to use 787s between Tokyo and Sydney. The revised software, which Boeing flight tested earlier this year on a leased Ethiopian Airlines 787, will allow operators to increase operating altitudes in ICI conditions to 35,000 ft. for 747-8s and 37,500 ft. for 787s. GE is leading much of the industry research in this area and posits that the varying susceptibility between the engines is probably related to configuration differences. The GEnx-1B has one additional low-pressure compressor and turbine stage compared to the -2B. The 787 engine therefore operates at a slightly higher rpm and temperature than the 747 engine, making it harder for ice to form.

Boeing says: “These revisions allow our customers greater flexibility in fleet planning and flying more direct routes. We remain dedicated to working with GE and our customers to remove remaining flight restrictions. Only a small number of GEnx engines have experienced [ICI] inflight and none since 2013.”

GE certified and installed the initial software modification to the entire GEnx-1B and -2B fleets in mid-2014. “With the modification, there have been no further incidents. However, as we’ve continued to ground test for icing conditions both here and in Canada [at GE’s Winnipeg ice test facility], we refined the software logic for various operational nuances,” GE states.

The new software senses the presence of ice particles and automatically activates the inward-opening variable bypass valves (VBV) situated between the booster and high-pressure compressor, ejecting ice into the bypass duct. Although this is the same basic solution developed earlier by GE, the engine company says this latest load includes revisions to improve detection and VBV operation in various flight modes. Final operability and aircraft-level certification follow the completion of 787 flight tests; baseline software validation was conducted by GE on full GEnx engines—using a special device that produced ice crystals—during ground tests at Winnipeg. The company also conducted rig tests at the University of Dayton Research Institute in Ohio, and at NASA Glenn Research Center in Cleveland using a full GEnx booster.

The FAA has also certificated an anti-icing upgrade to the GE90-94, which involved the introduction of the first part for a GE large commercial engine to be created using additive manufacturing. The new part is the housing for the Goodrich-made PT25 temperature sensor located in the “gooseneck” between the fan and booster section and the inlet to the high-pressure compressor. The change is only being made to the GE90-94 because the larger GE90-115 does not appear to be vulnerable to the high-altitude icing threat.

The modification is the final step in a three-phase anti-core icing mitigation redesign for the engine, says Bill Millhaem, general manager of the GE90 and GE9X programs. “We previously released an improved stage 1 compressor blade, which was more robust, and we modified the variable geometry inlet guide vanes into the high-pressure compressor to reduce accretion onto those,” he says. “The third step is the PT25 sensor, which we have repositioned and given a new geometry to reduce accretion. The sensor used to be stuck in the flow path right in the gooseneck transition location of where ice builds up and then sheds into the compressor,” Millhaem adds.

GE chose to make the part using three-dimensional (3-D) printing or additive manufacturing techniques because “it happened to be the quickest way to put it out to the fleet. It also allowed us the flexibility to continue to iterate small design changes. That’s the great thing about 3-D printing. You can design, print it and run it in a couple of week. Traditionally it would take nine months to be able to do a simple design iteration, now it is two to three weeks, depending on the complexity, to do that,” says Millhaem.

Lessons learned from the CF6, GE90 and GEnx experiences are being fed into the design of the GE9X as GE continues anti-icing research with ongoing runs of the GEnx booster section at NASA Glenn. “With the -9X we are in the process of understanding the accretion process and tweaking the design to avoid parts [on which ice is prone to accumulate and then shed], as well as how we can use variable geometry to mitigate this,” adds Millhaem.

Higher Altitudes Cleared For GE-Powered 787, 747-8 In Icing | Technology content from Aviation Week (http://aviationweek.com/technology/higher-altitudes-cleared-ge-powered-787-747-8-icing)

[JammedStab]

From Aviation Week. It really is worth the subscription price. I have quoted some interesting reader comments after the article.

GE-powered 787s Getting Relief Icing Limitations | Ice Flight Envelope | Commercial Aviation content from Aviation Week (http://aviationweek.com/commercial-aviation/ge-powered-787s-getting-relief-icing-limitations)

"GE-powered 787s Getting Relief From Icing Limitations

Software update brings full icing flight envelope clearance into view for GE-powered 787s

Boeing and General Electric (GE) have begun delivery of modified engine control software that is expected to free the GEnx-powered Boeing 787 from all operating restrictions in high altitude ice crystal icing (ICI) conditions by year-end.

The move, if sanctioned by the FAA, will end limitations originally imposed on the GE-powered 787 fleet in 2013, when Airworthiness Directives issued by the agency prohibited operation in moderate and severe icing. A final load of new software is also being installed on GEnx-2B powered Boeing 747-8s, which have also been subjected to mandatory operational limits in ICI environments. The FAA took action after several incidents in which engines suffered unexpected thrust loss, and even damage in one case involving a GEnx-2B powered 747-8, while flying through or close to convective weather systems.

Under the original directive, in the presence of known ICI conditions crews were directed to detour 50 nm around convective cells or reduce altitude to a maximum of 30,000 ft. These limits were subsequently eased with the introduction of revised electronic engine control (EEC) software that allowed operators to increase operating altitudes in ICI conditions to 35,000 ft. for 747-8s and 37,500 ft. for 787s. GE and Boeing believe the latest software will enable all restrictions to be lifted.

The ICI phenomenon, sometimes referred to as core icing, chiefly affects engines powering aircraft at altitudes above 38,000 ft.—much higher than the conventionally understood form of airframe/engine icing, which affects aircraft flying through moisture-laden clouds at low and mid-altitudes. The ice particles, which form when convective weather lifts huge concentrations of moisture to high altitudes, measure only around 40μ in diameter and have a reflectivity of just 5% of that of a raindrop, making them hard to detect using standard weather radar. The particles enter the core, impinging on static surfaces such as vanes behind the core stage, and eventually form ice. The ice slabs then break off, entering the compressor and causing thrust loss.

To counter the problem new EEC software has been introduced to control an ice crystal anti-ice (ICA) system. This detects the presence of icing conditions and automatically cycles the variable bypass valves (VBV), situated in the GEnx engine between the booster and high pressure compressor, ejecting ice into the bypass duct. Although the fleet has not been affected by ICI events since initial versions of the software were introduced in 2014, GE has continued to develop refined software logic to cover for “operational nuances,” and pave the way for an anti-ice system that would ultimately allow the 747-8 and 787 to be cleared to fly throughout their entire flight envelopes in ICI conditions.

The system is designed so that each engine has independent, automatic anti-ice detection and reaction functions. The 787 ICA activates only above 30,000 ft. and in the latest version will activate for at least 30 min., more than double the time of the original software load. The system, which turns off when the aircraft descends below 28,500 ft., cycles the VBV every 35 sec. During each cycle the valve doors are open for 30 sec. and closed for 5 sec. Operator sources say that even when the ICA system is active the impact on fuel burn is insignificant.

Activation of the system, which is independent of the standard engine anti-ice system, is indicated on the 787 flight deck display by the symbol “ICA,” which appears above the N1 (fan speed) indicator of the relevant engine. Because the system logic for the activation of the bleed valves requires specific airflow and energy, the latest software also requires crews to maintain a higher Mach speed or thrust level when operating at lighter weights. For example, when approaching or flying through clouds or visible moisture at 37,500 ft., crews must maintain a minimum of Mach 0.87 if the aircraft’s gross weight is less than 320,000 lb. In contrast, maximum required speed for the same weight at 30,000 ft. is only Mach 0.79.

Boeing and GE expect that the latest version of the software for the GEnx-1B will be installed in all GEnx-powered 787s by the end of August. Boeing is meanwhile mapping out a follow-up program with the FAA to enable the lifting of all operational limits. The company adds that “extensive testing and analysis has been conducted and we’re working closely with GE, our customers and the FAA to permanently remove the remaining flight restrictions.” Operator sources indicate this is expected by year-end. However, the time line for the GEnx-2B powered 747-8 is likely to lag that of the 787; GE says the software was rolled out to the fleet more recently."

COMMENTS BELOW

"well, seems that in that case the GEnx engine seems to have a lower temp in the first part of the compressor…other engine models may have a higher temp in that front part of the core which may help that such conditions cannot develop that easily..

but in all fairness, ist seems to have happened in other engine / airframe combinations as well…maybe not that often..."

"the GEnx also uses a lean burn combustor (TAPS) which can be less stable under certain conditions and thus more prone to flameout."

"I think the 3 spool RR engines dont 'have the problem' due to their architecture. The compressor stages are shorter and run faster as the front fan is its own stage."

"Engine core ice accretion. The first noteworthy incidence, I know of, was a PWJT15D powered Beechjet that deadsticked a landing in Florida after a dual flameout. Personally, I am witness to numerous major shop level turbine engine events; it is not all that uncommon to see minor, unexplained, FOD indications in mid-stage compressor sections w/ no upstream indications. I believe core ice accretion common phenomenon across all turbine engine types/makes."

[lomapaseo]

from post #24 above


From Aviation Week. It really is worth the subscription price. I have quoted some interesting reader comments after the article.


The article is fine as is the direction of the corrective action. I'm not so sure of the single comment attached at the bottom of the article. It seems to infer just one persons understanding of a complex interaction and suspected differences with other engine models.

I'll wait and see where any further discussion goes.

[Derfred]

Oh... I saw the thread title and assumed it was about pilots in their 40's on drugs... :}

[safetypee]

Re# Tropical conditions: Don’t assume that ice crystals are restricted by geography; it’s more related to the airmass and conditions associated with very large storms.

Re instrumented engines: It is an expensive exercise as demonstrated by BAE/Honeywell late 1990s; BAe146 with instrumentation and camera in an unmodified engine alongside a modified engine to be exposed in the same conditions. This arrangement provided both a research base for the previously ‘unknown’ icing and a means of proving that the modifications worked given the difficulty in ensuring that the aircraft is actually in the ice crystal conditions relevant to the engine.

[tonytales]

I worked Aeronaves de Mexico's Britannia's at JFK way back when. They were Model 302 with only four tanks and so had to fly fairly high to make Mexico City - New York. They had continual problems over the Gulf of Mexico with "Bump Stalls" caused by ice crystal ingestion. I seem to remember they blamed alto-cirrus clouds as the source. An "intrascope" (early borescope) inspection was required and I saw more than a few twisted and bent compressor blades and on occasion what we called "corn-cobbing". I leave that to your imagination.

Realize the Proteus was a first generation engine with reverse flow internally. It only had glow-plugs instead of continuous ignition. Was this icing condition related to the present generation ice-crystal icing problems here?

[lomapaseo]

Realize the Proteus was a first generation engine with reverse flow internally. It only had glow-plugs instead of continuous ignition. Was this icing condition related to the present generation ice-crystal icing problems here?

No way of knowing at this point.

Many similar events have been caused by inlet ice sheds on such type engines.

[JammedStab]

Some interesting info on actual in flight tests. Their procedure for not having all engines affected similarly.....keep the power settings all slightly different. Actual experience of ICI included the smell of ozone which is listed in out QRH. Didn't mention the smell of sulphur which is also in our QRH. According to Boeing, their listed indications of ICI when it comes to smells is based on pilot interviews.

Putting The Chill On Engine Core Icing Flight Tests | Technology content from Aviation Week (http://aviationweek.com/technology/putting-chill-engine-core-icing-flight-tests?NL=AW-18&Issue=AW-18_20151023_AW-18_695&sfvc4enews=42&cl=article_4&utm_rid=CPEN1000000624045&utm_campaign=4119&utm_medium=email&elq2=2d9c5a90f67b488fa00bd77762d1b9d9)

"Putting The Chill On Engine Core Icing Flight Tests

NASA pilots develop test techniques for hazardous engine icing conditions

Flight testing for engine core icing by necessity involves deliberately flying a large research aircraft into atmospheric conditions that are known to cause turbofans to lose thrust, flame out and even sustain damage.

So how do you fly such a test program efficiently and gather useful data without endangering the aircraft, its engines and the crew? Although the CFM56-2 engines powering NASA’s DC-8 have no history of uncommanded power reductions, or “roll back,” test planners were aware that very few engines of any make have been purposely subjected to sustained and repeated exposure to high ice water content (HIWC) conditions.

“We need to make sure we don’t flame our engines out, so we worked with General Electric, Boeing icing experts, NASA Glenn and Langley,” says Wayne Ringelberg, lead HIWC project pilot for NASA. A former U.S. Air Force test pilot with heavy, multi-engine aircraft experience, Ringelberg worked with NASA Armstrong Flight Research Center chief pilot Nils Larson to develop an appropriate piloting approach.

“We ran a systems-safety working group just on the icing hazard mitigation to see what we thought the hazards are, and what we think we can do. It turns out a lot of it was ‘we don’t know for sure’ because the phenomenon is quite unknown,” says Ringelberg. “We had a sense we were probably not highly susceptible, but we just didn’t know,” he adds.

The agreed mitigation procedure involves staggering the four throttles as soon as the ice particle instruments indicate the aircraft has entered air with high ice water content above 0.5 grams/cu. meter. “Once we hit that level we are in it,” says Larson, who is also a former Air Force test pilot. “Every five minutes or so, you tweak them,” he adds. “We are flying on autopilot and the throttles are only staggered in the 2-5% range. We also only move one throttle at a time.” The slight variation in N1 (fan speed) is expected to be enough that if icing were to strike, only one engine at a time would be vulnerable. But there are no guarantees, says Larson. “Everyone’s guessing here, there are a lot of unknown unknowns.”

NASA flight engineer Tim Sandon continuously ran both anti-icing ignitor loops on each engine as a precaution. “We leave them on 20 minutes after exiting the ICI [ice crystal icing] area and leave them on prior to descent, as that’s a risk area,” says Ringelberg.

The campaign also is aimed at educating crews for “seat of the pants” HIWC warning signs. Even though nothing might be showing up on the weather radar, there are plenty of clues to be had, says Larson. “Other crews didn’t think they were in icing because sometimes on the windscreen it looks like water. That’s because the particles splatter when they hit the windscreen and melt. Sometimes there’s St Elmo’s fire and sometimes there’s a sound like a ‘whoosh’—similar to the sound of an emptying drain.” Descriptions of this rain-like effect baffled scientists for years, helping to compound the puzzle over HIWC. Pilots have also noted speckling on the windscreen, humidity changes, an ozone-like smell, crackling on the radios and a sound like rain on the cockpit roof."

[JammedStab]

Aviation Week Takes A NASA Core Icing Research Flight | Technology content from Aviation Week (http://aviationweek.com/technology/aviation-week-takes-nasa-core-icing-research-flight?NL=AW-18&Issue=AW-18_20151023_AW-18_695&sfvc4enews=42&cl=article_3&utm_rid=CPEN1000000624045&utm_campaign=4119&utm_medium=email&elq2=2d9c5a90f67b488fa00bd77762d1b9d9)

"Aviation Week Takes A NASA Core Icing Research Flight

Researchers fly into tropical storms to evaluate possible warning methods for core icing events

The warm subtropical waters of the Gulf of Mexico in late summer may seem an unusual place to hunt for a rare form of atmospheric icing, but this is prime research territory for NASA and the agency’s highly instrumented DC-8 aircraft.

Searching for super-cold conditions from the sweltering heat of a Florida airfield is as counterintuitive as the ice crystal icing (ICI) phenomenon NASA is trying to find, a condition in which ice particles accumulate inside the hot core of a jet engine. Also known as high ice water content (HIWC), the state occurs without warning, generally at high altitudes above normal icing levels, and can result in temporary power loss, surges, blade damage and, in some severe cases, engine shutdown.

Researchers believe that ice crystals start to melt and evaporate as they meet warm parts inside the engine, cooling core surfaces to temperatures below freezing. The cooling engine causes the melted ice crystal water to refreeze, and ice accumulates inside the engine core. At some point, slabs of ice come loose and are ingested, causing power loss or blade damage.

While the dangers of traditional icing at medium altitudes are well understood and easily countered, the high-altitude HIWC scenario continues to puzzle researchers. The number of core icing events appears to have mushroomed from virtually nothing to more than 150 known incidents over the past two decades, and the rate is increasing. The growing incidence of core icing has forced changes in aircraft operating procedures and prompted the creation of a new set of certification standards for engines and avionics.

Solving the HIWC mystery is important. Researchers theorize that ice core incidents are on the rise partly because more airliners are flying with greater frequency through mid-latitude and subtropical regions prone to intense convection. In addition ice crystals can affect aircraft data systems leading to errors in readings of temperatures, air speed and angle of attack. Icing is thought to have contributed to the loss of Air France 447, which crashed into the Atlantic after flying through storms in 2009.

Two international HIWC research groups are focused on tackling the icing problem. The North American HIWC study group involves NASA, the FAA, Transport Canada and Environment Canada, Airbus, Boeing and the Australia Bureau of Meteorology. The European high-altitude ice crystals (HAIC) consortium, which is coordinated by Airbus, brings together 34 industrial and research partners from 11 European countries and five from Australia, Canada and U.S. Both groups are coordinating research on three main objectives; understanding the physics of the HIWC process, developing new regulatory guidelines and developing HIWC detection methods.

Aviation Week was invited to join a NASA HIWC research flight from Fort Lauderdale, Florida. The test campaign forms part of what is “really a three-pronged approach to cracking this nut,” says Ron Colantonio, project manager for engine icing research at NASA Glenn Research Center, Ohio. “First we have to characterize the weather that’s causing the problem. Are the ice crystals small or big? How much is the water concentration? We don’t have a handle on things like that,” he says.

“Second, we want to know what’s happening in the engine. What are the physics? It’s not intuitive you can have icing inside an engine.” To reach these answers, and to develop a simulation tool for engine makers to use for testing designs against core icing, Glenn has modified its Propulsion Systems Laboratory (PSL) to replicate HIWC conditions. The facility has been tested with a fully instrumented Honeywell LF502, which was one of the earliest high-bypass turbofans to exhibit vulnerability to “roll back” caused by icing.

“We can ‘fly’ an engine from the ground up to 40,000 ft. The ice crystals are generated in front by spraying liquid water into a cold airstream. By the time it gets to the engine it is frozen,” says Colantonio. “It is the only capability in the world that can do this, and our hope is with this flight campaign we can see we are indeed replicating the right conditions in this facility. We think this is the new method of ground-based engine ice testing, and we are trying to develop new test methodologies and new ways to calibrate the facility. We need new diagnostics for ice crystals and water content. We are on the right path and we should have a solid capability in the next few years,” he adds.

With updated engine certification regulations on the way to cover core icing requirements, NASA is working with the FAA and other agencies to use the PSL as a means of compliance. “We have an icing research tunnel that manufacturers could come to from around the world,” says Colantonio.

The third target is to develop detection and warning methods, preferably by adapting existing onboard equipment. “The focus of these flights is to see if we can use remote-sensing capabilities, like the weather radar [to detect and avoid HIWC]. It is like the low-hanging fruit,” he adds. For the HIWC program the DC-8 was fitted with a Honeywell RDR-4000 X-band weather radar, which is designed to measure liquid water content and turbulence. “A lot of the signal is filtered and we are going into engine icing conditions and saving the raw radar data; we hope to see that we can detect ice crystals with the radar or at least infer if there are ice crystals there,” says Colantonio.

One puzzling question researchers hope to answer is why existing radars routinely fail to pick up ICI-like conditions at all, despite the very high water content. Air crews frequently report that weather radar at the time of engine thrust loss shows relatively benign green (low reflectivity) or even black, indicating no discernible threat. NASA Langley Research Center weather radar principal investigator Steven Harrah says: “The amount of moisture that’s deemed to be in these HIWC clouds is 3 grams/cu. meter or more, and if you converted that into precipitated rain, it would be 2 in. per hour, which is heavy rain. On a weather radar, that’s deep into the red, so we are scratching our heads and saying, ‘why aren’t we seeing this already?’ The systems work perfectly, so where’s the missing part?” The answer likely boils down to droplet size, says Harrah, but even the variations found within a typical convective cell or between different regions of the world cannot account for the discrepancy.

A potential answer is that the drop size distribution is skewed, and that although there is a lot of water held within the cloud, it is made up predominantly of particles too small to be picked up the radar. Weather radars operate on wavelengths optimized for around 1 mm, “which works very well, but for HIWC conditions that are maybe dominated by particles of 100 microns or smaller, that’s not so good,” he adds.

While dual-band radar could provide one answer, the preferred solution likely will be the less expensive route of adapting current radars with new signal-processing algorithms that will be able to “infer” the presence of HIWC. “We can measure convection and reflectivity in the atmosphere, where there is lot of moisture, so we can measure a high probability of HIWC at a certain point in space. That’s just a software change in the existing box,” says Harrah.

Specifically, researchers believe the location of the HIWC event can be inferred by bringing together evidence from the existing data or which can be gleaned from new modes. Convection can be mapped using a NASA-developed mode that measures vertical winds. Higher reflectivity at lower altitudes—indicating higher visible water amounts around the convective area—is another clue, while radial winds that advect moisture away from the convection “chimney” will also help locate the danger area. “Looking at those winds, we can infer where it should be and using outside air temperature and other parameters we can fine-tune it,” says Harrah. Preventing nuisance alerts will be key. “If pilots turn it off, that’s the worst possible result,” he adds.

For the test campaign, which included 10 flights over 20 days, the goal was to record both instrumented weather and standard radar data as the DC-8 flew in known HIWC conditions, and then see if by comparing the data a potential HIWC radar signature could be identified. The instrument suite included an isokinetic evaporator probe (IKP), a pitot-style forward-scattering device that measures ice water content, and three devices to gather data on particle shape and size. The trio was made up of a cloud droplet probe for particles 2-50 microns, a particle-imaging probe (PIP) for 100 microns to around 6.2 mm, and a 2DS stereoscopic probe for 10 microns to 1.2 mm.

“We’re trying to relate these ice water content-level measurements to what we what radar signatures are being recorded,” says Tom Ratvasky, in-situ probes co-principal investigator for NASA Glenn. “The data we are collecting and the technology we are working on for a more sensitive radar could be offered to assist the current fleet to avoid hazards. It will also help development of real-time nowcasting tools for detecting HIWC conditions, which the FAA is very interested in. The flights will also help with pilot education,” he notes.

For the HIWC campaign, the normally NASA Armstrong Flight Research Center-based DC-8 was temporarily housed on Florida’s Atlantic Coast. From here, researchers had the option of targeting convective weather over the Caribbean and western Atlantic or nearby Gulf of Mexico. On the day of Aviation Week’s 7-hr. flight, a line of thunderstorms was marching across the gulf and intensifying over Louisiana, providing promising conditions for engine icing.

Closing on the line of storms from the east, the aircraft was flown initially on the west side of the system’s stratiform clouds at 37,000 ft. Turning north, the DC-8 was flown through the edges of the storm, avoiding the areas of maximum turbulence red-painted on the radar just as any commercial airliner crew would do. Careful measurements of water content, droplet size, radar data, air temperature and other parameters continued throughout the flight, as the aircraft made several passes around a north-south oriented track through the storm. The aircraft then descended, and the pattern of observation was repeated at 34,000 ft. and 29,000 ft.

Good HIWC data was successfully gathered, and ice buildup on the air data and total air temperature probes around the cockpit was noted by the crew, even though the radar was indicating green or black. Several crews experiencing ice core events have also reported temperature anomalies, and for the first time on any research flight, the NASA campaign recorded total air temperature probe indication changes from well below freezing to freezing, and the temporary failure of the aircraft’s pitot tubes. Later in the campaign, the DC-8 also flew into Tropical Storms Danny and Erike, the first time that an aircraft equipped with both an ice water measurement suite and pilot weather radar was able to record conditions associated with such events."

[JammedStab]

Third article only partial copy of the interesting part....

Following Clues To Solve The Icing Puzzle | Technology content from Aviation Week (http://aviationweek.com/technology/following-clues-solve-icing-puzzle?NL=AW-18&Issue=AW-18_20151023_AW-18_695&sfvc4enews=42&cl=article_5&utm_rid=CPEN1000000624045&utm_campaign=4119&utm_medium=email&elq2=2d9c5a90f67b488fa00bd77762d1b9d9)

"The focus of many of the 1990s’ thrust-loss studies had been on the AlliedSignal (later Honeywell) LF502-powered Bae 146, including a 1992 incident in which an Ansett-operated aircraft had lost power on all four engines over Western Australia. However, it was a 2002 event in the U.S. involving a McDonnell Douglas MD-82 in which the aircraft descended to 17,000 ft. before being able to restart its engines that produced one of the biggest clues. Although not caused by ice building up in the core of the aircraft’s Pratt & Whitney JT8D-217 engines, the evidence showed ice particles had blocked the inlet of a pressure sensor which sent an erroneous message to the autothrottle. The MD-82 was also equipped with SLD ice detectors, but because these did not trigger, the event became a turning point in the understanding that engine failure was more likely linked to ice particles.

“It was a new discovery, but in fact it wasn’t quite new,” says Strapp. “They knew about it in the 1950s because they had a problem with the Bristol Britannia and flameouts in its Proteus engines.” The issue began in April 1956 when two of the four turboprops on a BOAC Britannia flamed out at 20,000 ft. over Africa on a route proving flight to Nairobi. Kenya. The event was a mystery as the engine had successfully passed through intense ice- certification tests in Ottowa and, just as in recent events, no airframe icing was present. The only clue to the presence of ice particles was a thin white “witness line” along the null point on the leading edges.

As a result, Bristol, Rolls-Royce and the certification authorities “did a lot of work back in the 1950s characterizing the atmosphere. It was work that, in essence, we repeated. But it was no longer traceable and we didn’t know how accurate it was,” says Strapp. “However, they knew a lot about ICI [ice crystal icing] and by time we got onto HIWC in 2004 this was not common knowledge. We didn’t think you could get icing from just dry ice crystals. You can get the same conditions at turboprop altitudes if you are flying in the tropics, and they were,” he adds. Part of the reason the lessons were forgotten was the unusual reverse-flow configuration of the Proteus and the fact that the more popular pitot-style engines that succeeded earlier generations were not susceptible to ICI. “It went off people’s minds,” says Strapp."