Andrew Doyle/ZURICH
Early operators of Rolls-Royce Trent 700 engines powering the Airbus Industrie A330 twin are working with the manufacturer to eliminate surge, vibration and turbine disk corrosion problems. These have led to a higher than expected number of engine removals recently.
R-R has developed solutions for the surge and vibration issues. The modification work is causing logistical problems for the airlines concerned. Dragonair general manager engineering, David Lui, says: "We've been running very lean on spare engines." Coping with the extra shop work "had an impact on our operations".
There have been instances where the heads of bolts used to mount the high-pressure turbine to the compressor have sheared off due to differential thermal expansion. The failures are not considered a safety issue because the broken parts remain inside the turbine drum. This causes excessive vibration, however, and the engine has to be taken off the wing for repairs.
"Out of 32 bolts, one or two coming loose is not really a major [safety] issue, but it results in a very expensive piece of turbomachinery having to be scrapped," says Lui. "The fix is to have the bolts made of the same material as the compressor and turbine disks." The move involves switching to the same type of bolts as those used on the bigger Trent 800.
The surge problem has been caused by the separation of the lining on the second stage high pressure compressor, reducing compressor blade clearances. The remedy involves spraying on a new lining material during overhaul. R-R says only three surges are known to have occurred in unmodified engines.
The changes are already being included in new-build engines.
Lui says the high-pressure turbine corrosion issue has been a "long developing problem" stemming from moisture getting into the air cooling system for the turbine blades. The result is a "drastic reduction" in the fatigue life of the disks, to around 5,000 cycles from the expected 15,000, he says. "R-R has to do more rig tests before they find out the effect of the corrosion on the disks," he adds.
Source: Flight International
