US aircraft safety investigators have called for grounding certain Boeing 787s and 747-8s powered by General Electric engines until they are inspected for cracks and also revealed that a cracked fan midshaft was discovered on a third engine last month.

The recommendations issued by the National Transportation Safety Board (NTSB) offer the clearest explanation yet for the rash of zero- or low-time GEnx engine failures since 28 July.

The NTSB letter sent to the Federal Aviation Administration also reveals that an analysis of the fan midshaft fractures do not point to metal fatigue as a likely cause. The fan midshaft connects the low pressure turbine to the fan and booster stages at the forward end of the engine.

Instead, the cracks in the critical engine component are "typical of environmentally assisted cracking of certain high strength alloys such as that used on the GEnx [fan midshaft]", the NTSB letter says. The NTSB is continuing to investigate what is triggering the environmentally assisted cracking. According to GE, such metals crack as a result of galvanic corrosion caused by a moist environment with the presence of hydrogen.

A potential trigger of the galvanic corrision could have been revealed earlier this week. In a statement issued by GE on 11 September, the company said it has changed the coating process for the fan midshaft on the production line as a result of the engine failures. GE says today that the new coating process changes the dry film applied to the midshaft, and replaces the lubricant used when a retaining nut is clamped to the midshaft.

The NTSB's letter to the FAA indicates all three engine failures discovered to date could be linked to the same cause.

The latest engine failure to be revealed actually was discovered during an ultrasound inspection on 13 August, but it was not disclosed to the public until now. The ultrasound check on the engine, which was installed on a 787 that had not been flown, revealed an indication of a "similar" crack on the fan midshaft.

The first such incident occurred on 28 July. An engine on an Air India Boeing 787 in Charleston, South Carolina, failed in low-speed taxi test. The crew was accelerating the aircraft through 40kt when the low-pressure speed rolled back on the No. 2 engine. The crew aborted the test and a visual inspection revealed the first stage of the low pressure turbine had shifted backward, colliding into trailing stages.

Six weeks later, a Boeing 747-8F operated by Air Bridge Cargo with about 1,200 flight hours and 240 cycles experienced a similar problem. As the aircraft accelerated through 50kt, the low-pressure turbine speed of the No. 1 engine dropped. The crew rejected the take-off and an inspection revealed "extensive damage" to the low pressure turbine, the NTSB letter says.

The NTSB highlighted the potential safety danger of repeated failures on low-time engines. The board's letter highlighted the "possibility that multiple engines on the same airplane could experience a [fan midshaft] failure".

In the event of an extended twin-engine flight over water, for example, the aircraft would have to operate on only a single engine for up to 5.5h, the NTSB letter says.

In response, GE says in a statement that it has already recommended repetitive inspections to the FAA and is close to completing ultrasonic inspections of the fan midshaft for all GEnx engines in service.

Boeing adds that is working with GE to inspect nine 747-8Fs within a "few days" to complete ultrasound inspections on the operational GEnx fleet.

[Updated at 21:45 in inlcude Boeing statement.

Source: Air Transport Intelligence news