Guy Norris/LOS ANGELES
CFM INTERNATIONAL (CFMI) plans to test a revised fan-blade retainer design on the CFM56-7B engine, following the failure of the original system during a crucial blade-off test which resulted in several blades detaching.
Despite the need for a new test, CFMI is confident that the engine will meet its scheduled certification target of 31 October. The engine is the first version of the CFM56 to have wide-chord fan blades and is destined for the new-generation Boeing 737 family.
The blade-off test was being re-run to confirm the integrity of a revised containment collar which was designed after blade debris had penetrated through the casing in an earlier test. CFMI says that the new containment system "worked well" in the blade-off test, but the fan damage occurred as the engine was wound down after the test.
Boeing, which has overall responsibility for the propulsion system and integration of the engine with the Rohr-built nacelle, says that it was "initially gloomy", over the results "-until we looked closely at the film and saw what had actually happened". Up to six blades are believed to have broken off instead of the one and a half or two which are normally taken out by such a drastic test. The solution involves strengthening the axial retainers on the dovetail assembly at the base of each root which prevent excessive forward and aft movement of the blade.
The strengthened "fingers" on the retainer device mesh with slots which are located between the dovetail assembly and the back of each blade. Snecma, which designed the retainer, is now performing static tests on the new design before the full whirl-test scheduled for 29 July. The revised retainer will have around 40% additional load capability.
The first member of the new generation Boeing 737 family to be flown with the engine, the -700, is scheduled to have its maiden flight in January 1997.
Source: Flight International
