Canada's newest military helicopters have been grounded except for search and rescue work, after cracks were found on a tail rotor.

The 15 Cormorants fly out of bases in Ontario, British Columbia, Newfoundland and Nova Scotia. The first cracks were found in a craft in Newfoundland during a routine maintenance inspection. If the crack spreads, the blade could potentially come off.

Though the crack was found on only one chopper, the decision to ground the entire six-year-old fleet was made Saturday. The only exception will be essential search and rescue work.

"The cause of the cracks is under investigation and inspections of that area of the aircraft have been increased," the Air Force said in a statement on its website.

"The situation will be re-evaluated as more details from the investigation are made available."

This is the third time this year the choppers have been grounded, the most recent time because of cracks in their tail rotor hubs.

Earlier this month, it was revealed the Cormorants require much more maintenance than originally believed, costing the Air Force millions of dollars.

The fleet, ordered in 1998 for search-and-rescue work, were supposed to require about seven hours of maintenance for every hour of flight, according to their manufacturer, EH Industries.

But the machines actually take up to 22 hours in the shop for each hour in the air. EH Industries has since revised its maintenance estimates, saying the aircraft initially needs about 12 hours for each hour of flight.