The US Federal Aviation Administration (FAA) has concluded that "excessive" friction between the static and rotating portions of a certain seal inside the General Electric CF34 turbofan engine under certain high-power, high-altitude conditions is unsafe and must be corrected.

A final rule to be published tomorrow calls on US operators of some 2,450 CF34 engines to replace the original 4-step air balance piston seals with 8-step seals at the next life-limited parts replacement interval, a relatively inexpensive modification expected to take five hours per engine.

The issue gained prominence in the investigation of the Pinnacle Airlines Bombardier CRJ200 crash in 2004. Both pilots were killed during the repositioning flight after purposefully stalling the aircraft at 41,000ft and experiencing a dual flameout that resulted in core lock of both engines. The crew initially misreported the situation to air traffic controllers, and ultimately failed to restart the engines. The aircraft crash-landed short of the airport in a Missouri neighbourhood.

According the FAA, GE had found during the post-crash investigation "that under certain high-power, high-altitude engine shutdown events, interference between the rotating and stationary portions of the CF34's 4-step air balance piston seal can develop".

After the airworthiness directive was first proposed in July 2008, the engine manufacturer requested that the FAA remove references to "excessive friction" in the seal. Instead GE said it would be "more accurate" to say that by reducing the friction, the modification would enhance the ability to restart an engine after flameout. GE also asked that the FAA change the AD's incident description stating "both engines experienced high-altitude flameouts" to reflect the fact that the engines flamed out after the pilots performed a high-altitude stall and upset.

The FAA rebuffed GE on both requests however, saying that GE's proposed wording regarding "reduced friction" would suggest the seal change out "may not be adequate to address the inability to restart (the engines) due to the friction".

Commenting on the AD GE says it respectfully disagrees that there is an unsafe condition in the engine, noting the rulemaking is a response to "an engine condition that is extremely rare".

Source: Air Transport Intelligence news