Andrew Doyle/LONDON
CFM INTERNATIONAL has been forced into a redesign of a turbine rear-frame (TRF) destined for use on all CFM56-5A/B turbofans, after cracks were discovered in the double-annular combustor (DAC) variant, powering Swissair Airbus A320s and A321s.
The problem was uncovered, by Swissair engineers in January, leading to the temporary grounding, of five A320/321s (Flight International, 31 January-6 February). The -5B DAC is designed to reduce nitrogen-oxide emissions, and was launched with orders from the Swiss national carrier and Austrian Airlines.
The problem only affects the -5B DAC because it is the first to enter service with tangential struts in the TRF, instead of the radial struts used in other -5A/Bs. Tangential struts are intended to increase the flexibility of the casing to cope better with varying operating temperatures, and were to be fitted to all CFM56-5A/Bs from the end of this month.
Pierre Drevet, programme director for the CFM56-5A/B, says that it is not clear whether similar problems will be encountered in the conventional -5A/B single-annular combustor (SAC) version, but that, "...to limit the number of tangential TRFs in service, we are going to re-activate production of a limited number of radial TRFs". Production of radial TRFs was closed down over a year ago.
In the meantime, some SAC engines will have to be delivered with the tangential struts, and aviation authorities are drawing up increased inspection requirements.
The DAC variant employs an inner and outer ring of fuel nozzles in the combustion chamber, and only the outer nozzles, are used at power settings below 30%, while the aircraft is taxiing. This results in a "temperature gradient between the hub and the external joint", which Drevet believes is "definitely an aggravating factor" in the strut problems.
The redesigned TRF with tangential struts should become available during the first quarter of 1997, says Drevet, coinciding with the introduction of an improved "DAC II" engine to meet emissions guarantees given to Swissair.
Meanwhile, Swissair is having to change each engine on its Airbus narrow-body fleet after just 1,200 cycles, so that the TRF can be replaced. This is because the cracks begin to occur after 1,200 cycles, says Heinz Bart, Swissair Technical Services' division manager for power plant customer-support business. "After 2,000 cycles all of the struts are cracked," he adds. "It's an annoying problem, which is now under control."
The CFM56-5C, which powers the Airbus A340, already has a TRF with tangential struts, although the presence of an additional low-pressure-turbine stage helps to reduce stresses in the TRF.
Source: Flight International
