Air NZ 787 engines still under scrutiny
Air New Zealand has been faced with further operating restrictions as a result of safety edicts over the risks of damage to Rolls Royce engines on some of its Boeing 787 Dreamliner aircraft.
Both the European and United States aviation regulators imposed broader limits in April on airlines using the engines because of worries that if one suffered blade cracking in flight the pressure on the other remaining engine to get the aircraft to safe haven might also cause it to fail.
The regulators ruled aircraft had to be at all times within two hours and 20 minutes of an airport to divert to rather than the previous five and a half hours.
That ruled out Dreamliner flights to North and South America and meant some changes in their use to Asian destinations.
The directives also cut total weights on the affected Dreamliner flights in certain conditions to ease pressure on any remaining solo engine in making it to safety, although Air New Zealand says it has found a way to work within that without affecting its full complement of passengers and cargo.
The extra safety measures limited Air New Zealand's use of its 11 Dreamliners, many operating with the affected Rolls Royce Trent 1000 C Pack engines.
A briefing from our Civil Aviation Authority to the Minister of Transport Phil Twyford on April 18, released to Newsroom under the Official Information Act, said the authority needed to tell him "of a new and serious engine problem on the Boeing B787-9 fleet operated by Air New Zealand".
The director of civil aviation, Graeme Harris, said the problem was not with the Intermediate Pressure Turbine which had been the subject of investigations and publicity since Air New Zealand had two separate engine problems in December on flights to Buenos Aires and Tokyo.
This time the issue was with Intermediate Pressure Compressors in the engines.
He explained Rolls Royce had found the compressor's second stage blades could suffer "cumulative fatigue damage that can cause blade failure and consequent engine shutdown" as a result of airflow conditions when the engine was at high thrust under certain temperatures and altitudes.
"While fatigue cracking of the stage 2 blades is a safety concern, in isolation it is no more significant that the IPT blade cracking reported earlier," Harris wrote.
"What adds an order of magnitude of seriousness to the IPC blade problem is the susceptibility of these blades to a resonant frequency vibration at high power settings.
"In the event of a single engine in-flight shutdown during the cruise phase of flight, thrust on the remaining engine is normally increased to maximum continuous thrust (MCT) for some period during the diversion to an alternate airport."
The concern is that "the loss of one engine will result in the remaining engine being operated at a high power setting which, because of the resonant frequency vibration of the IPC stage 2 blades, would result in the failure of the second engine before the diversion is completed".
"The probability of this dual failure would be greater if the IPC blades in the second engine were already cracked."
The CAA said the European Aviation Safety Agency and the US Federal Aviation Administration had issued airworthiness directives limiting the flight time such aircraft could be away from an airport to divert to - and imposing limits on total weight of passengers and cargo to limit the load on a single engine if such a diversion was necessary.
The report says Air New Zealand is one of four airlines most affected - with All Nippon Airways, British Airways and Virgin Atlantic - because it flies over long water routes.
Most US and Australian airlines with Dreamliners were fitted with General Electric engines and not affected.
The CAA notes Air New Zealand has taken measures to 'mitigate' the risks - which have included leasing aircraft from other operators and some changes to routes - but says the 140 minute diversion time imposed would rule out flights from New Zealand to North and South America. Its Dreamliners would still be able to be used on flights to Asia "although some less efficient routes may need to be selected to meet the diversion time".
The report to the transport minister contains blacked out sections on what Air NZ has done to mitigate issues and how many of its aircraft had been found to have cracks in the affected engines before the April safety directives.
Air New Zealand told Newsroom yesterday it "continues to be fully compliant" with airworthiness directives.
"Along with conducting ongoing engine inspections, we are currently operating with the maximum diversion time allowed under the FAA Airworthiness Directive (140 minutes)."
It noted some of its engines qualified for greater diversion times - for example one having been fitted with Rolls Royce Trent 1000 TEN engines not being subject to the new limit and still operating at 330 minutes.
A spokeswoman said Air New Zealand accepted the CAA's adoption of the two foreign regulators' views. "Safety is paramount and non-negotiable at Air New Zealand."
Air New Zealand had been able to get exemption to the FAA directive restricting take-off weight by seeking permission from the CAA to use a "specialist forecasting tool" which could pinpoint exactly where icing is likely to occur and the airline could plan flights to avoid it.
"The restriction was greater when atmospheric icing was forecast along the route. By using this specialist tool we have not needed to make any en route fuel stops in recent months."
Air New Zealand has addressed the engine safety issues since December by retiming some international flights, swapping aircraft types on some routes and leasing long haul aircraft from Boeing and EVA Air to manage its way through.
In April, after the EASA directive on the compressor, Air NZ said: "As a result of the checks two Air New Zealand 787-9 Dreamliner aircraft will be temporarily removed from service while engines undergo maintenance work at a Rolls-Royce facility in Singapore.
"Around 340 engines globally are subject to the checks and this is placing very high demand on Rolls-Royce’s maintenance facility meaning it may take a number of months before Air New Zealand’s engine repair work can be completed."
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