Boeing says an airworthiness directive (AD) expected to be issued soon by the US Federal Aviation Administration, ordering inspections of the 777's back-up electrical generator and its oil servicing system, poses no threat to the aircraft's extended range twin operations (ETOPS) clearance.

According to unconfirmed reports, however, the FAA is considering imposing clauses in the AD which could affect ETOPS clearances for some operators. All makes and models of the 777 are affected by the variable speed constant frequency (VSCF) generator issue, regardless of engine make, although shutdowns have only occurred on Rolls-Royce powered aircraft. The FAA only says "we have not made a decision yet on the AD, or what it would contain". Should it prove to contain ETOPS restrictions, the result could disrupt long-haul, transcontinental 777 operations worldwide.

The AD will address problems with the Hamilton Standard-made VSCF generator. The unit suffered several failures, causing damage to engine-mounted gearboxes and prompting a major redesign (Flight International,18-24 August).

Boeing says a redesigned drive shaft, which will shear at lower stress levels to avoid causing secondary damage, is ready for installation on the production line. It is to begin appearing on new 777s this month. The manufacturer also plans to begin a retrofit of the 245 aircraft delivered to date.

Boeing insists that the VSCF issues, which are believed to have caused two in-flight shutdowns, are not ETOPS-related because it is only one of four independent electrical generation systems on the aircraft. "You can dispatch the aircraft for up to 10 days with the VSCF inoperative, even on ETOPS flights" says the company which expects the AD to "mandate what we put in all the original service bulletins". This essentially instructed operators to make more frequent inspections of the VSCF and its oil servicing system. The company adds that the VSCF is not on the 777 minimum equipment list - a crucial ETOPS parameter.

Industry observers believe that, despite Boeing's reassurances, the current focus on increased safety and ETOPS could be reflected in a restrictive AD. Coincidentally, the FAA is reviewing requests to extend the 777-200ER's ETOPS clearance to 207 minutes.

Source: Flight International