Calgary's police force has two helicopters. (CBC)

Calgary police helicopters are reported to have flown too close to other aircraft at least 11 times in the last four years, a CBC News investigation has found.

Reports filed with Transport Canada dating back to 2006 say a police helicopter has flown closer than the mandated one nautical mile from other airplanes almost a dozen times.

In December 2009, a WestJet flight from Toronto was 400 metres above ground on its final descent into the Calgary International Airport when the cockpit's alarm that warns of a collision went off, ordering the plane to climb, according to one report to Transport Canada.

It appears the WestJet plane was forced to take corrective action because a Calgary police helicopter with its lights out and with "priority status" flew within 0.7 nautical miles of the Boeing 737. It seems the police helicopter diverted from its approved air traffic control route.

'We don't do anything without air traffic's knowledge and previous approval.'— Kevin Brookwell, Calgary police

Passengers would not have noticed anything out of the ordinary, said Richard Bartrem, a WestJet spokesman.

"It would have felt like any other approach, where we might have had to speed up a bit, depending on winds, or move slightly from left to right. But ultimately, it demonstrates to us that [the Traffic Alert and Collision Avoidance System] works and we were able to take that corrective action," he told CBC News.

Last month, one of the Calgary police's two helicopters came within 60 metres vertically and 1.8 kilometres laterally of a Beech King Air 200 owned by North Cariboo Air, sparking a probe by the Transportation Safety Board, the independent agency responsible for air safety in Canada.

"The risk would be, and the ultimate end would be, a collision between the helicopter and another aircraft and of course the ensuing disaster that would result from that," said Jon Lee, a manager with the board. "Secondary to that would be a near-hit or a near-collision, where an aircraft would have to take evasive manoeuvres and in doing so might injure the occupants of one of the aircraft

A WestJet plane on its final descent was forced to climb after a Calgary police helicopter was detected too close by. (Colin Hall/CBC)

Keith Johnson, safety program manager with the Airborne Law Enforcement Association, said a police chopper following the rules should never get that close to another aircraft.

"Sixty metres is pretty close," he said. "You would probably classify that as what we would call a near mid-air collision; in other words you don't have a collision but you certainly got real close and the risk factors were pretty high."

Calgary police stressed safety is the priority with its helicopters, and its pilots abide by all requests from air traffic control.

"The impression might be that we've got a bunch of rogue pilots that are flying around above the city and just kind of doing what they want because they are the police," said spokesman Kevin Brookwell. "But in actual fact, we are, like everyone else, heavily regulated.

"We don't do anything without air traffic's knowledge and previous approval."

However, in August 2008, the agency in charge of air traffic control reported that a police helicopter took off without clearance.

Air traffic control paramount

Air traffic control can choose to give a police aircraft priority and direct other aircraft to hold their approach or departure, a Transport Canada spokeswoman said in a written statement.

She emphasized that pilots must comply with all air traffic control instructions.

A special exemption allows RCMP pilots to fly without lights in circumstances such as surveillance or pursuit, she added.

The Transportation Safety Board wants the police, federal officials and the Calgary Airport Authority to get together soon to talk about making sure there are no more close calls.

Calgary police air program 14 years old

Calgary Ald. Diane Colley-Urquhart, who is also a member of the city's police commission, said Monday that the public should be informed when any incidents are investigated.

"If there are incidents of this nature, then I believe there should be full disclosure when they occur so that the citizens are aware, even if they're being fully investigated," she told CBC News.

Brookwell said the force will work on any recommendations that may come out of the Transportation Safety Board investigation.

The Calgary Police Service was the first municipal police agency in Canada to have a full-time patrol helicopter program in June 1995. It added a second helicopter in 2006.

The force's air services unit has four pilots, who can be civilians, and four full-time tactical flight officers, who are required to have at least three years of service with the Calgary Police Service.

With files from Brooks Decillia and Scott Dippel