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Russian authorities are investigating an serious icing incident in cruise during which three engines on a Boeing 747-8 freighter sustained damage and two suffered a loss of thrust.
Federal air transport authority Rosaviatsia identifies the aircraft involved as an AirBridge Cargo 747-8F, registered VQ-BGZ. The 747-8F is powered by four General Electric GEnx-2B engines.
It had been operating between Moscow Sheremetyevo and Hong Kong on 31 July, and was cruising at 41,000ft at the time.
The incident occurred at night in the vicinity of Chengdu, about 1.5h before landing, as the aircraft deviated to the right of its intended flightpath in order to avoid a thunderstorm.
Cruising in darkness at 41,000 ft., on July 31 near Chengdu, China, the crew of an AirBridge Cargo Boeing 747-8F were beginning to prepare for the descent into Hong Kong when they deviated to avoid a thunderstorm clearly depicted on the weather radar.
Even if they had been able to visually check their surroundings, they would not have noticed anything unusual about the area they penetrated in the outflow region of the anvil cloud trailing the relatively distant storm. There was no sign of airframe icing, nor any echoes from the radar.
Yet the cloud was full of undetectable ice crystals that—within minutes of the encounter—caused significant damage to three of the aircraft's four engines, one of which lost thrust while another surged. The AirBridge Cargo (ABC) crew had unwittingly come face-to-face with core engine icing, a poorly understood phenomenon that has been striking a wide variety of aircraft and engines on a growing scale since the 1990s. As well as surges and mechanical problems, the previously unrecognized form of icing inside engines causes thrust loss, or power “roll-backs,” with virtually no warning.
According to Russian federal air transport authority Rosaviatsia, chief investigators of the 747-8F event, the crew saw at least one typical clue to the phenomenon. Entering the area of ice crystals, the total air temperature (TAT) rose by 20C to -34C for 86 sec. The crew reacted by switching the engine ice-protection system from automatic to manual for about 10 min. But approximately 22 min. 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 (low-pressure rotor speed). After landing at Hong Kong, inspections revealed damage to the high-pressure compressor blades of the Nos. 1 and 2 engines, as well as to the No. 4.
Within weeks of the latest event, Boeing and General Electric flight tested an engine software upgrade specifically designed to counter the ice-crystal buildup. GE says the software changes to the GEnx-2B full-authority digital engine-control unit will 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.