CFM International (CFMI) expects by July to begin delivering redesigned turbine rear-frames (TRFs) for CFM56-5A/B double annular combustor (DAC) engines on national carriers Swissair and Austrian Airlines Airbus A319s, A320s and A321s.
The revised TRFs should finally allow the two carriers to lift artificial flight-cycle limits on the engines which have been in place since early 1996 (Flight International, 13-19 March, 1996).
CFMI was forced into the redesign after cracks were discovered in the tangential struts of the TRF, caused by temperature gradients generated by the engine's DAC. As a result, the engines were initially removed after every 1,200 cycles for the TRF to be replaced, although this limit has now been raised to 2,400 cycles after modifications to control software helped to alleviate the problem.
Swissair is now performing an average of one engine change a week on the affected aircraft fleet, which consists of the carrier's 27 A319/A320/A321s, and the three A321s operated by its partner, Austrian Airlines.
"We have a limit of 2,400 cycles to inspect the TRF for cracks," says Armin Fuerst, chief of powerplant engineering at Swissair. "Depending on the length of the cracks, we remove the engine or continue to operate it. In general, we remove them [after 2,400 cycles]."
Fuerst adds that CFMI "-should be able to deliver the redesigned TRFs [from July] as we do the engine changes". CFMI has supplied three extra spare engines to the airlines to help them cope with the high level of changes.
TRFs with tangential struts were to have been used on all CFM56-5A/Bs from 1996, but following the problems in the DAC variant, limited production of the previous radial version was restarted. The tangential struts are designed to make the engine casing more flexible, to cope with varying operating temperatures.
Source: Flight International
