CFM International hopes to receive approval next month for changes to the CFM56-7 engine that will provide a long term solution to problems experienced in the first full year of service on Boeing's Next Generation 737.

The issues were related to the engine's hydro-mechanical unit (HMU) which "-suffered some process problems and wear problems in various valves", says CFM56-7 programme manager, Wayne Adams.

The problems led, in two cases, to uncommanded acceleration to full power in flight, and to flame-outs on the ground.

"The HMU has been a problem, but during the last nine months we have been introducing manufacturing process improvements, and in the long term we have software and design fixes. Tests are on-going and we are looking at certifying the part for introduction into service by the end of March," says Adams.

One of the uncommanded accelerations was caused by contamination in the HMU. "We went back into the processes at AlliedSignal, [which makes the HMU], and solved it that way." The second incident was due to a problem with the fuel metering valve resolver. "We found a process issue which caused an intermittent failure, and we modified the software so that in the event of a failure, the dual track FADEC [full authority digital engine control] would track the good channel. We also improved the manufacturing process so that the little wire [which caused the problem] would not have the failure," says Adams.

There have been no uncommanded accelerations on the ground. The flame-outs were "-not related to the fuel meter, but were related to the HMU," says Bill Clapper, CFM executive vice-president.

"We have been pulling out all the stops. The changes already instituted have been very effective up to this point and reliability has improved significantly," says Clapper. At this stage, CFMI "-does not know" if the revised part of the HMU (P07) will be retrofitted throughout the fleet as the problems diminished with the process changes and revised FADEC software. The revised HMU, with the new part, will be flight tested by Boeing in a one-off acceptance test, following US Federal Aviation Administration certification.

By the end of January, about 350 CFM56-7s were in service on some 175 Next Generation 737s.

Source: Flight International