Air New Zealand has confirmed that three of its ATR 72-500 turboprops that have undergone inspections will "require additional work" and be put out of service for a week until 25 March.

"Hairline cracks were also found in the cockpit window area of these aircraft," a spokeswoman tells Flightglobal Pro.

Only two aircraft from the 11-strong ATR 72-500 fleet of regional subsidiary Mount Cook Airlines were in service on 19 March.

The operator expects three more aircraft to be put into service by 20 March, followed by an additional two by 22 March. One turboprop remains in the hangar for pre-planned maintenance.

The carrier grounded its entire fleet of of the type on 18 March after discovering hairline cracks around the cockpit windows of one aircraft during routine maintenance.

"A window had to be replaced as part of maintenance and when the panels were removed, that's when the cracks were discovered. The cracks are, however, not associated with the windows," says the spokeswoman.

An ATR spokesman says the carrier decided to ground and check the aircraft because they did not know the causes nor the consequences of the cracks, "but the problem does not affect safety of the aircraft".

He added that ATR is providing technical support to the carrier, but could not give more information on the cause of the cracks.

"Because this is not affecting safety, no specific checks related to this particular issue have been [required of] other operators," he says, adding that the European Aviation Safety Agency is aware of the situation.

More than 5,000 passengers have been affected as a result of Air New Zealand's grounding.

Mount Cook expects to accommodate 97% of its passengers on 20 March as more turboprops become operational after inspection, and with the help of Air New Zealand's aircraft operating some of its services.

Mount Cook's 68-seater ATR 72-500s are 10.9 years old on average and operate to 10 destinations around New Zealand.

Source: Air Transport Intelligence news