Pratt & Whitney is inspecting two PW4000 engines that suffered surges in at least two separate, but near-simultaneous incidents, despite recently being fitted with re-designed high-pressure compressor (HPC) stator vanes which were specifically designed to combat the problem.
"I have got two engines that I don't understand," says P&W executive vice president and chief operating officer, Robert Leduc, adding that diagnostic tests to find the cause will take "three or four" weeks. The problems cropped up in a PW4060 powering a Sobelair Boeing 767-300 and a PW4056 on a Singapore Airlines 747-400.
Both engines had recently completed 500 cycles after being fitted with a newly certificated "cut-back stator" kit which had shown a 6% surge margin improvement in rigorous tests, says P&W. The modification involves cutting back the trailing edge of HPC vanes on all 2.4m fan diameter engines.
Leduc, who stresses the events are not a "safety issue", says the engines recovered in both cases. P&W has duplicated the surge in the 767 engine which is now undergoing tests at the company's Wilgoos, Connecticut site to see if the changeover to the revised HPC introduced an unforeseen gas path change. The basic kit is intended to increase surge margin by improving streamwise flow at the base of the vane and redistributing airflow toward the outer diameter gas path wall at the blade tip area (Flight International, 16-22 May).
Retrofits, which have been cleared for 747s, 767s and MD-11s, have been stopped immediately, says Leduc. Around 120 are believed to have been accomplished, with the first only entering service in August. The retrofit is expected to affect 2,500 engines.
P&W is acting quickly to find the problem before it adversely affects the in-flight shutdown rate of the series which stands at 0.009, or roughly one per 100,000 hours. Of this, P&W says surges account for one per 450,000 hours.
The US Federal Aviation Administration is expected to issue an airworthiness directive by the end of the month, backing up operator bulletins that have already been issued.
These put cycle limits on the cutback engines, at which point P&W recommends replacement with new cutback kits or complete replacement with engines that have not been modified. Operators have to rotate modified engines to ensure no more than one is on any single aircraft.
Source: Flight International
