The US Federal Aviation Administration has approved plans by General Electric to conduct the critical fan blade-off tests for the GEnx engine for the Boeing 787, which involves detaching the blade further along its length, rather than at the root as in normal blade-off tests.
The first GEnx engine to run, on 19 March last year, is now scheduled to be sacrificed in the destructive "blade-out" test in April. The test, one of the last for FAR Part 33 engine certification, proves the inherent design of the engine and its structural ability to cope with the catastrophic loss of a blade at full power.
By allowing the test to be conducted using a blade detached further away from the root - and therefore shorter - GE is able to maximise the weight advantages of using composite materials for both the blades and fan case.
The FAA approval is therefore a vital milestone in its entire GEnx development strategy, particularly as this involves extending the use of lightweight composites to include the fan case for the first time.
The FAA Special Conditions approval covers all currently planned 787 versions, and is based on the precedent established in 1995 when the FAA agreed on a similar set of rules for the GE90. Special conditions applied to the GEnx therefore include an engine fan blade containment test "with the fan blade failing at the inner annulus flow path line", says the FAA.
The agency also requires that GE "substantiate by test and analysis, or other methods acceptable to the administrator, that a minimum material properties fan disk and fan blade retention system can withstand, without failure, a centrifugal load equal to two times the maximum load that the retention system could experience within approved engine operating limitations".
GE is also required to "establish an operating limitation that specifies the maximum allowable number of start-stop stress cycles for the fan blade retention systems". This evaluation, says the FAA, "shall include the combined effects of high cycle and low cycle fatigue", and will eliminate the fan blade's limit life once it has successfully demonstrated 100,000 cycles. The blades must also undergo lightning tests.
Source: Flight International
