Operators of more than 560 Pratt & Whitney PW4000-powered Airbus A300/ A310s, Boeing 747s, 767s and MD-11s are starting inspections for potential surge problems.

The problems are restricted only to 2.37m (94in)-diameter fan versions of the PW4000. The move follows a suspected double surge event on a PW4056-powered Boeing 747-400, and a subsequent all-operators meeting for 747, 767 and MD-11 users held by Boeing on 3 March. The US Federal Aviation Administration, which also attended the meeting, is expected to issue an airworthiness directive instructing operators to begin inspections immediately.

Airbus is also understood to be taking similar action for operators of PW4152, 4156 and 4158-powered A310-300s, A300-600s and -600Rs. Although the engine has been prone to surge events since it was introduced, the double surge is believed to have led to calls for more urgent action from the FAA. Boeing confirms that a PW4460-powered China Eastern MD-11 suffered a double surge in July on a post-maintenance inspection flight, although no further details of the more recent suspected 747-400 event have been released. Boeing says: "We have been working with Pratt & Whitney on the PW4000 surge issue for some time. Measures to resolve those issues are currently being discussed with the operators."

P&W says the inspections will incorporate new "high fidelity techniques to identify any engine that has the potential to have compressor instability. We have an improved diagnostic system".

The new techniques will be used to map the surge margin, which appears to have deteriorated faster than expected, particularly on older engines. The stall margin is the separation between the stall line on a compressor stall-margin graph and the normal operating line.

P&W is working on a package to solve the problem. This includes redesigned, shorter chord stators in the high pressure compressor (HPC), three dimensional aerodynamic improvements to other sections of the HPC, and new software to reschedule the full authority digital engine control unit.

P&W says the design changes "are being verified" and are to be introduced in 2000. It declines to comment on whether the latest incident involved engines on test with the revised HPC stator design.

· According to the Airclaims CASE database, the inspection will affect up to 1,546 engines on around 565 aircraft. In all, P&W says the total fleet amounts to almost 2,000 engines, including spares, those in overhaul or on the assembly line. It believes that "only around 10%" are likely to be in the suspect area.

Source: Flight International