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.
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