Genx Icing Problem
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.
The problem is that flying through these ice crystals does not always seem to have an effect.
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.
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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
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
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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
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
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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
"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."
GE-powered 787s Getting Relief Icing Limitations | Ice Flight Envelope | Commercial Aviation content from Aviation Week
"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."
Thread Starter
from post #24 above
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.
From Aviation Week. It really is worth the subscription price. I have quoted some interesting reader comments after the article.
I'll wait and see where any further discussion goes.
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.
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.
Early Ice Crystal Problems?
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?
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?
Thread Starter
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?
Many similar events have been caused by inlet ice sheds on such type engines.
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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
"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."
Putting The Chill On Engine Core Icing Flight Tests | Technology content from Aviation Week
"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."
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Aviation Week Takes A NASA Core Icing Research Flight | Technology content from Aviation Week
"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."
"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."
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Third article only partial copy of the interesting part....
Following Clues To Solve The Icing Puzzle | Technology content from Aviation Week
"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."
Following Clues To Solve The Icing Puzzle | Technology content from Aviation Week
"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."
