Graham Warwick/PARIS
GENERAL ELECTRIC Aircraft Engines has developed a solution to the fan-blade failure which has grounded GE90-powered Boeing 777 flight-test aircraft (Flight International, 14-20 June, P4). GE has until mid-July to restage the 3.6kg birdstrike test successfully, if Boeing is to deliver the first GE90-powered 777 to British Airways on schedule in September.
The loss of three composite wide-chord fan blades during the birdstrike test was initiated by the affected blades coming into contact with aluminium platforms, or flow-path spacers, located between the blades where they meet the hub.
The first blade to be struck apparently flexed sideways, causing the aluminium platform to cut through the composite fibres at its base, weakening the blade and causing it to break off when it rebounded. Two neighbouring blades were forced sideways and failed similarly, resulting in a fan imbalance exceeding design requirements. The fan is designed to withstand the loss of 2.3 blades.
The GE90 fan had already passed the same test at the rotational speed equating to the initial 377kN (84,700lb)-thrust rating and the test was being restaged at the higher fan speed required for the 409kN growth engine. In the first test, the bird was fired at a point halfway along the blade. At the US Federal Aviation Administration's request, the growth rig-test was staged with the bird aimed at the 75%-diameter mark.
The same failure mechanism is understood to have resulted in the loss of three blades in a GE90 blade-out test conducted at European Gas Turbines. This test was the first in which the composite blade was severed at its root, and not where it meets the hub, and was staged by GE to counter criticism of the way in which the blade-off test was originally conducted.
GE has designed a new platform with an aluminium centre-section, for airflow abrasion- resistance, and composite edges, which will deform when they come in contact with the flexing blade. Snecma's Villaroche fan rig is scheduled to be back in service on 12 July, leaving three days to conduct the birdstrike test.
The redesigned spacers would then be retrofitted to existing flight-test and production engines, which would allow certification flight-testing of the two GE90-powered 777s to resume soon after.
The blade failure is not related to a GE90 compressor-surge problem encountered in early May and fixed by modifying software controlling the variable-geometry vane schedule.
Source: Flight International
