Civil Aviation Authority (CAA) director Graeme Harris said: "This action results from recent CAA inspection and monitoring visits to a maintenance organisation that conducts repair and overhaul of Rolls Royce/Allison 250 series turbine engines.
"The visits revealed departures from Rolls Royce approved engine maintenance instructions that adversely affect the airworthiness of the engines. The primary issue relates to unauthorised drilling and grinding work done on safety-critical components within the engine compressor section."
Rolls Royce 250 series engines were primarily fitted to Bell Jet Ranger and Hughes 500 series helicopters but did have some other applications, the CAA said.
Other affected engines were found in Australia, Indonesia and Papua New Guinea.
The CAA said it had been "working closely with the maintenance organisation and the engine manufacturer Rolls Royce to determine the safest means of ensuring all affected engines are returned to an airworthy state, while minimising the effects on the aviation sector".
The maintenance organisation had co-operated fully, CAA said.
Harris said: "The CAA is also currently conducting a more thorough review of all of the maintenance organisation's engine maintenance practices. This review commenced today. 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."
The impact of the safety action:
- 21 helicopters in the region will be grounded as a direct result of the Airworthiness Directive issued today
- For New Zealand-based aircraft, this Airworthiness Directive will primarily affect helicopter operators conducting air transport and/or agricultural operations
- 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 (albeit a non-revenue flight to a repair facility will be permitted)
