Italian investigators have disclosed that there was no engine de-pairing requirement in place for the Rolls-Royce Trent 1000s fitted to a Norwegian Boeing 787 before one of its powerplants failed after take-off from Rome.

The engines were subject to a modification – under a service bulletin designated 72-H818 – which introduced an intermediate-pressure turbine blade with a different parent material and coating, to protect against corrosion and fatigue due to sulphidation.

A separate service bulletin had set out a de-pairing life to protect against the risk of dual engine failure.

Italian investigation authority ANSV says Rolls-Royce used statistical models, based on blade sampling and fleet data analysis, to derive a blade life limit for different groups of serial numbers.

This enabled a further bulletin to set a blade hard life which was significantly lower than that at which certain in-service engines – those which had yet to receive the newly-developed blades – would have to be de-paired.

As a result, says ANSV, Rolls-Royce agreed with the European Union Aviation Safety Agency to remove the de-pairing requirement.

But the failure of the left-hand engine on the Norwegian 787-8, on 10 August, occurred 1,210 cycles after the installation of its intermediate-pressure turbine module – failing 200 cycles before its hard life limit of 1,410 cycles.

The aircraft’s other engine had performed more cycles – a total of 1,337 – and was even closer, just 103 cycles, to its own limit of 1,440.

ANSV says this showed the hard life limit was “not sufficient” to avoid a detrimental effect on safety, and left the aircraft vulnerable to dual engine failure, particularly given that a remaining operative engine “undergoes overall higher solicitations”.

It has recommended that EASA evaluate de-pairing criteria for engines yet to receive new blades, in order to avoid two such engines being installed on the same aircraft.

ANSV acknowledges that Rolls-Royce, on 19 September, revised the service bulletin for managing these engines, reducing the blade life limit for specific engine serial numbers. EASA endorsed this revision through an airworthiness directive on 18 October, which takes effect from 1 November.

Norwegian’s 787 sustained damage not only to the engine exhaust cone but also the left wing, flap fairing, horizontal stabiliser, fuselage and main landing-gear tyres. Debris shed from the powerplant damaged a total of 28 cars and three building awnings.

The inquiry into the Norwegian incident is continuing, says ANSV, with teardown of the engine and its intermediate-pressure turbine blades, a failure analysis of the blades and other engine parts, and examination of certification and airworthiness criteria.

But ANSV also points out that the new blades developed by Rolls-Royce have generated “satisfactory results” against premature failure during tests, although it cautions that testing is a “continuous process involving sampling from the in-service fleet”.