Air New Zealand expects almost all of its regional services to be operating on March 20 after the temporary grounding of its fleet of 11 ATR 72-500 turboprops caused major flight disruptions over the past few days.

The aircraft were grounded for unscheduled inspections after hairline cracks were found around the cockpit windows on one ATR 72-500 during routine maintenance. The crack was discovered in the frame 5 segment of the turboprop. Eight of 11 of the airline’s turboprops were found to suffer the cracking. Most of the inspections had been carried out by the evening of March 19.

The manufacturer notes that “these cracks can appear after a high number of cycles and can be detected during a standard maintenance check scheduled at 24,000 cycles (about 10 years of operation).” It adds that “the reason for the cracks is linked to stress concentration and fatigue propagation.”

ATR insists that “there is no safety issue with these cracks, as mechanical capability of cracked frame versus operational flight and ground loads was demonstrated.”

No special inspections are being called for. “Cracks at frame 5 reported location do not compromise the airworthiness of ATR airplanes,” the manufacturer tells operators in a safety message. The turboprop maker recommends operators “continue following the instructions of the ATR-approved maintenance program, which has proved to correctly address the issue.”

ATR also says it has modified the design to avoid the problem occuring. That fix has been implemented from aircraft MSN695 on for both ATR 42-500s and ATR 72-500s.

Air New Zealand’s ATRs, which have an average age of 10.9 years, have been operated by subsidiary Mount Cook Airlines since 1999.

Five of the ATRs will be operational by the end of March 20, and another two will return to service by March 22. Three ATRs have been “confirmed as requiring work” which is likely to be completed by March 25. The remaining aircraft is undergoing planned maintenance.

Mount Cook Airlines says it will be able to handle about 97% of the passengers booked on its flights on March 20, with the help of additional Air New Zealand mainline aircraft. Timing changes will be needed for some flights, however.

The carrier cancelled most of its ATR services – or about 60 one-way flights – on March 18 due to the inspections. About 3,000 customers were affected by the cancelations on that day. There were roughly 2,000 customers affected by cancelations on March 19.