All Canadian Sea King helicopters have been grounded pending an investigation into an incident late Monday night at a Canadian Forces Base near Halifax, the Department of National Defence has announced.

A Sea King helicopter struck the tarmac with its main rotors just before midnight Monday during a landing at CFB Halifax in Eastern Passage, N.S., after a training flight.

DND confirmed that the helicopter experienced a "flight safety occurrence."

Lt-Col. James Hawthorne said the aircraft had landed and was in the process of being refuelled when the accident occurred.

"The aircraft landed safely after a training flight and after which time the tail lifted off the ground and the main rotor struck the ground in front of the helicopter. Some damage was done to the helicopter and some damage was done to the infrastructure at 12 Wing Shearwater," he said.

Hawthorne said it's not clear why the tail lifted, forcing the nose of the aircraft down.

"The answer I got from the pilot is that he doesn't exactly know what happened and he said it's the fastest thing he'd ever seen occur in a helicopter. So it caught him by surprise just like it took everyone by surprise at 12 Wing Shearwater," he said.

The main rotor consists of five blades about nine metres long, all of which struck the ground, sending debris and sheared off parts flying. A building on the base suffered dents, Hawthorne said.

The four crew members aboard the aircraft were not injured. Hawthorne credited the ground crew with having the "foreknowledge and thought" to recognize what was happening and to get out of the way.

This year marks the 50th year Canada's Sea King fleet has been in service. Hawthorne said he doesn't believe the age of the aircraft are a factor in the incident.

"The Sea Kings have provided incredible service for us for 50 years, coming up at the end of this month, and will continue to provide incredible service in the future," he said.

Hawthorne said the director of flight safety will be investigating what happened.

"The damage to the aircraft is quite extensive and it will take some time for us to determine how we're going to proceed forward with this aircraft, whether we're going to continue flying it the future or if it's no longer flyable," he said.

Hawthorne could not confirm whether the damaged helicopter was one of the group of three Sea Kings that have been seen flying in formation over the Halifax region in recent days in the lead up to the aircraft's 50th anniversary.

50th anniversary celebrations of the Sea King fleet are still expected to go ahead as planned July 31 and Aug. 1, he said.

CFB Shearwater is located across Halifax Harbour from the city itself.