Faulty Rolls-Royce engine blades are deteriorating faster than expected, prompting additional groundings of Boeing's 787 jetliners for emergency repairs.

The discovery affects about 120 Trent 1000 turbines and has frustrated efforts to reduce the number of idled planes, lifting the number grounded for immediate attention to just under 40, a person familiar with the situation said.

Rolls-Royce discovered the issue with the Dreamliner's intermediate pressure turbine following incidents at Air New Zealand in December. Air-safety regulators will publish a formal requirement for repairs in coming weeks, said the person, who asked not to be named as no directive has been released.

"We continue to proactively manage a number of known durability issues within our Trent 1000 fleet and have made good progress in redesigning and replacing affected parts," London, England-based Rolls-Royce said by email. The issue concerns a minority of 787 engines that haven't already had the relevant blades replaced and may cause "additional short-term disruption," it said.

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The European Aviation Safety Agency didn't return calls for comment, while the U.S. Federal Aviation Administration said it's working on the issue.

Rolls-Royce shares closed 2.5 per cent lower at 966 pence in the U.K. capital, paring gains this year to 15 per cent. Boeing was priced 0.5 per cent higher as of 11:53 a.m. in New York.

The U.K. manufacturer has been battling a series of design faults affecting various parts of the Trent 1000 turbine for two years, with carriers including British Airways, Virgin Atlantic Airways Ltd. and Norwegian Air Shuttle ASA forced to lease in jets this summer as 787 engines came off-wing for repairs.

Dreamliners affected by the latest findings already faced shop visits for remedial work, so that Rolls's anticipated charge of 1.3 billion pounds ($1.5 billion) for emergency maintenance likely won't be affected. At the same time it could face a fresh bill for disruption to customer flights.

- Bloomberg