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ANALYSIS: US FAA orders repetitive inspections on GEnx fan midshaft parts

The US Federal Aviation Administration will require Boeing 787 and 747-8 operators with General Electric engines to perform repetitive ultrasonic inspections to detect the fan mid-shaft cracks that have appeared on three different engines since late-July.

The airworthiness directive, which becomes effective 21 September, accepts recommendations issued last week by the National Transportation Safety Board (NTSB), including a requirement to conduct an initial ultrasonic inspection on each aircraft engine before the next flight.

Boeing and GE have confirmed that the initial inspections have already been completed on the entire fleet of 787 and 747-8s powered by GEnx-1B and GEnx-2B engines respectively.

The inspections were ordered soon after the first fan mid-shaft on a 787 fractured during a low-speed taxi test in Charleston, South Carolina on 28 July. The fracture caused the first stage of the low-pressure turbine to move backward and collide with trailing stages, leading to a contained engine failure.

A second cracked fan mid-shaft was discovered in mid-August by ultrasonic inspection on another GEnx-1B, which is one of two engine options for the 787. Finally, on 11 September, an AirBridgeCargo 747-8 Freighter experienced an engine failure in Shanghai, China, an inspection revealed cracks on the fan mid-shaft.

This series of fan mid-shaft failures so quickly after both aircraft entered service was unprecedented. Only a handful of fan midshaft failures have been recorded on GE turbofans over a period of decades.

Within weeks, GE believed it had found the source of the problem, and it was relatively benign.

The GEnx uses the same kind of super-alloy for the fan mid-shaft that was introduced 15 years ago on the GE90. But GE switched to a new, lead-free coating for corrosion protection to be more environmentally friendly on the GEnx engine. It was this coating that failed to prevent cracks in the extreme operating conditions of the rotating fan mid-shaft under certain conditions.

GE has now switched back to the leaded coating for the GEnx fan mid-shaft. The leaded coating has already been introduced on the fan mid-shaft production line of GE supplier IHI, and also at GE's final assembly centre.

Although GE believes the cracking problem is now resolved, the episode will likely raise some questions for GE's engineering department. It remains unclear how the coating problem went undiscovered during certification trials for both the GEnx-1B and GEnx-2B engines. Both engines cleared a gauntlet of certification tests without noticing a problem that arose relatively quickly in operational service.

The coatings also highlight an increasing challenge for ultra-fuel-efficient turbofan engines. Fuel efficiency is achieved partly by increasing the level of compression of the core airflow. Higher compression translates into rising temperatures.

Super-alloys, such as the GE1014 super-alloy selected for the fan mid-shaft, are relied upon to absorb temperatures that would quickly melt conventional metals. But such conditions demand little margin for error, and even seemingly innocuous changes, such as a switch to a more environmentally-friendly coating, can have unexpected consequences.

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