Boeing is working on an urgent redesign of the variable speed constant frequency (VSCF) generators on the 777 after a number of failures caused damage to engine mounted gearboxes.

The VSCF problem is receiving maximum priority as it directly affects extended range twin operations (ETOPS) just as the US Federal Aviation Administration is considering an extension of the twinjet's ETOPS clearance to 207min. The FAA is also preparing an airworthiness directive (AD) calling for mandatory inspections of the Hamilton Sundstrand-built VSCF and its oil servicing system.

The problem affects all models and engines. Boeing is conducting VSCF oil servicing flight test investigations on a Rolls-Royce Trent 800-powered 777 due for delivery to Singapore Airlines, and a Pratt & Whitney PW4084-powered aircraft due for handover to Japan Airlines. The company says: "We have found instances where the VSCF failed, resulting in damage to the gearbox. The drive shaft was designed with a shear section so if there is an internal mechanical failure to the VSCF, it will limit the damage. We are therefore redesigning the shaft to shear at a lower failure value." The problems cause secondary damage to the gearbox and other parts of the powerplant by introducing contaminants into the engine. Boeing says "around a dozen" VSCF failures have been reported.

There are two VSCF units mounted on each engine, though only one is in operation at any time. The VSCF acts as a back-up electrical generator, producing electrical power at variable frequencies and at variable voltages. The variable frequency power is converted into useable power for the rest of the electrical system.

Meanwhile, Boeing is investigating an unrelated sensor failure which caused the crew of a Continental Airlines 777-200 to perform a precautionary shut down of one of the aircraft's General Electric GE90 engines, believing that it had a low oil pressure fault. The incident occurred during the customer acceptance flight when the 777 was in Canadian airspace near Vancouver.

Source: Flight International