Air New Zealand flight 99 was climbing out of Auckland to Tokyo when passengers heard clunking and felt vibrations from the right-hand engine.

The plane was about 200km into the flight last December 5 when a part of the engine broke, spewing debris down the side of the plane. A report out today has revealed it caused damage to the underside of the wing, fuselage and horizontal stabiliser in the tail wing section of the plane.

Exhaust gas temperatures rose - a sign an engine is in bad shape - and pilots immediately shut it down, told passengers there had been an ''engine issue'' and, just 20 minutes into the flight, it would be turning back.

One of the 268 passengers on board (there were 14 crew) said there was no panic but when it landed in Auckland an airport fire tender hosed the engine down and ambulances were on standby.

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The preliminary investigation out today into the incident has details of the incident which led to the grounding of that and another aircraft which suffered a similar problem the next day on its way to Buenos Aires.

On the flight to Tokyo, small pieces of the turbine and stator blades which failed at the rear of the engine were blown out of the exhaust nozzle. The Transport Accident Investigation Commission report says besides damage to the titanium leading edge of the horizontal stabiliser, one of the main carbon-fibre wings was chipped and so was the fuselage of the then three-and-a-half-year-old plane.

There was no ''significant'' damage although it is understood the leading edge needed to be replaced.

The commission has found a known Rolls-Royce Trent 1000 engine problem — corrosion within the intermediate pressure turbine — had caused a turbine blade to fracture and fly from a disc.

There was serious damage to that part of the engine which was sent to Rolls-Royce's repair base in Singapore. The Argentina-bound flight incident the next day was not as serious but again there was significant damage within the engine.

Since the problem was first identified in Japanese airline ANA in 2014, vulnerable blades were being replaced but the Air New Zealand problems occurred earlier than Rolls-Royce modelling had predicted, said the commission's manager of air investigations, Peter Williams.

''Inquiries into these incidents suggest that the failures were consistent with a known problem with unmodified Trent 1000 engines. Rolls-Royce has been replacing engine components with a new design, and managing the safe operating life of unmodified engines using a risk-analysis model,'' said Williams.

After the December incidents Rolls-Royce reduced the number of flights Air New Zealand could make under its risk-analysis model.

The airline also voluntarily reduced the maximum time to a diversion airport for which it would fly its aircraft with unmodified engines.

Read the incident report:

Including the Air New Zealand incidents, there have been cases where eight intermediate pressure turbine blades have come adrift in Rolls-Royce engines, all caused by corrosion.
Rolls-Royce said it is likely that a combination of environmental and operational factors are involved and that these ''may be operator specific',' the commission report says.

The problem is different to another one identified in about 380 Trent ''package C" engines around the world. Problems with compressor blades cracking mean airlines including Air New Zealand are required to do more frequent checks, if necessary carry out repairs and have had range from diversion airports cut significantly.

As in the aftermath of the December incidents, the compressor problem has caused flight cancellations and a schedule shake up and forced the airline to draft in Portuguese charter operator Hi Fly.

Engine trouble

What happened?

Engines on two Air NZ Dreamliners suffered damage from turbine blades fracturing soon after take-off on successive days last December.

How dangerous was this?

They are designed to fly on one engine but there are new concerns about higher thrust needed causing the remaining one failing.

What will happen now?

These blades are being replaced in Rolls-Royce engines around the world. The Transport Accident Investigation Commission hasn't issued any new safety recommendations.

Is this the cause of current disruption?

No. The Trent engines have a problem in another part of them requiring more checks and range restrictions.