New Zealand's Civil Aviation Authority has grounded 21 helicopters because unapproved work done on their engines means they're not fit to be in the air.
Inspections at a company which does repair work on the Rolls Royce/Allison 250 series turbine engines - reported to be Oceania Aviation - found drilling and grinding work had not been okayed by Rolls Royce.
The engines are mostly fitted to Bell Jet Rangers and Hughes 500 series helicopters, with transport and agricultural operations affected. No Police Eagle or rescue helicopters are thought to be affected by the move.
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"The visits revealed departures from Rolls Royce-approved engine maintenance instructions that adversely affect the airworthiness of the engines," CAA director of civil aviation Graeme Harris said.
"The primary issue relates to unauthorised drilling and grinding work done on safety-critical components within the engine compressor section," Harris said.
Newshub reported the company involved was Oceania Aviation, a major New Zealand aircraft maintenance provider. An email to customers reportedly informed them the company's turbine factory had been closed temporarily following an audit by the CAA, and would reopen once the issues were fixed.
Grounding the helicopters is the best way to ensure all engines are made safe while minimising effects on the aviation sector, the CAA says.
There are 41 engines that will need parts repaired or replaced - 25 in New Zealand, 13 in Australia, two in Indonesia and one in Papua New Guinea. Other affected engines are part way through being overhauled at the maintenance facility.
A new Airworthiness Directive requires 20 compressor sections to be repaired within 30 days, or 50 flight hours, and 21 compressor sections affected by grinding to be immediately withdrawn from service for repair - meaning 21 helicopters are grounded from today.
Affected components must be replaced or fixed before these engines return to service.
Harris said the CAA was also looking closely at all the organisation's engine maintenance work.
"Further safety action may be necessary in due course but that is unknown at present.
"I have also taken action today to the maintenance organisation from conducting any further RR 250 series engine maintenance works until a detailed safety review can be completed."
Eight aircraft will be subject to the 30-day / 50-flight-hour limit for replacement of the compressor section and a further 17 will be grounded immediately - after being allowed to fly to a repair facility.
The firm has fully co-operated with the CAA investigation.