Boeing has advised GEnx engine operators that it is revising the 787’s and the 747-8’s flight manuals to prohibit flight within 50 nm of thunderstorms that may contain ice crystals. Following Boeing’s recommendation, Japan Airlines immediately announced it would switch aircraft on two routes. From April to November, GEnx-powered aircraft suffered six engine-icing events, according to a GE statement. All aircraft landed at their planned destinations, said the engine maker, and none of the incidents involved in-flight shutdowns–only temporary thrust losses.
Boeing and GE have begun working on a permanent solution under which Fadec software modifications for the GEnx-2B (on the 747-8) and GEnx-1B (on the 787) would detect the presence of ice crystals passing through the engine. The Fadec would then schedule the opening and closing of variable bleed valve doors behind the fan and booster, thereby ejecting any ice crystal accumulation before it enters the core.
The companies have conducted initial flight-testing with the new control software on a Boeing 747-8. The CF6 fleet had already “greatly minimized” the adverse effects of ice crystals through engine-control modifications similar to those under way on the GEnx engine, GE explained.
The two companies plan to certify the modifications on the control system early next year and make the software modifications available to operators in the first quarter. Plans call for the software updates to take about an hour to perform on wing.
GE asserts that the GEnx ice-crystal encounters occurred in conditions different from certification requirements. Strong convective weather can lift high concentrations of moisture to high altitudes, where it can freeze into ice crystals, Boeing explained. Such convective weather systems can cover a diameter of 350 nm.
Engineers now understand the principles of how ice crystals can create in-engine icing, sometimes inside the core. They bounce off freezing surfaces near the front of the engine and enter the core. Then, in the compressor, ice crystals melt on vane surfaces, creating a film of water. Particles keep impinging on the wet vane, cooling it to the point at which ice can form. Eventually, the ice accumulation breaks off and can cause various engine malfunctions, including flameout.