For the second time in four months, HMCS Windsor is heading back to Halifax so crews can find out what went wrong with the sub.

The latest issue arose on Friday when the supercharger on one of its two generators broke — possibly delaying the submarine's participation in a NATO exercise in Norway this month.

Early Friday morning the sub picked up and dropped off personnel in St. John's, said Capt. Jamie Clarke, commander of Canada's submarine fleet.

The crew started the sub's diesel generators, but one of them shut off without warning. They tried again and the problem persisted.

'Don't really know how it happened'

"The supercharger on one of the generators broke," Clarke said in a phone interview from his office in Victoria, B.C.

He stressed that no one was hurt and no emergency procedures were initiated.

Clarke compared the submarine's supercharger to those found in some cars and trucks. The device increases the density of air — namely oxygen — inside a combustion engine, making it run more efficiently and generate more power.

"That blower assembly is what has broken," Clarke said. "At this point, we don't really know how it happened."

Redundant system

The diesel generators recharge huge batteries in the submarine's fore and aft. When the sub dives, it relies entirely on battery power.

The system is redundant, allowing the submarine to operate with only one generator, Clarke said. He did not have specific details on how the sub's performance has been affected. The submarine is returning to Halifax so crews can fully assess and fix the problem.

"When things break on our subs, we get it fixed as soon as possible and get them back out to sea," Clarke said.

HMCS Windsor was heading to Norway to participate in a navy training exercise with other NATO countries. The sub was scheduled to arrive there in two weeks. The total delay resulting in a trip back to Halifax for repairs is not yet known.

Clarke said the cause, cost and remedy for the problem won't be known until the the navy's maintenance personnel can do a full inspection when the sub arrives in port sometime Monday.

Journalists just got off the sub

The supercharger broke just two weeks after members of the media were on board the sub.

In late May, the navy invited journalists to join senior commanders on board HMCS Windsor for overnight trips. The idea was to showcase the capabilities of Canada's submarine fleet.

The military has recently been eager to highlight various programs and their value, with expenses under the microscope. A federal Defence Policy Review is underway to determine the Department of National Defence's priorities for the future.

Halifax-based HMCS Windsor is Canada's only fully-operational vessel in the four-sub fleet. On the west coast, HMCS Victoria is being used for training, HMCS Corner Brook will be in an extended maintenance period until 2018 and HMCS Chicoutimi may return to service next year, navy officials say.

Supercharger separate from battery issue

In February, a cell in one of HMCS Windsor's batteries overheated and began emitting smoke. The sub visited a port in Virginia, where Canadian navy officials disconnected the problematic cells from the power system and HMCS Windsor returned to Halifax.

The sub's batteries are scheduled to be replaced this year.

Even with the disconnected cells, HMCS Windsor passed the navy's performance test and was deemed ready to be sent back out to sea.

The supercharger failure and the overheated battery are separate issues that will both have to be addressed, said navy officials.

HMCS Windsor — along with all of Canada submarines — are "complex pieces of machinery," Clarke said.

"The key now is to come home and have it fixed," he said.