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TSB credits warning system with preventing near-collision of executive jets


July 18, 2013   by Canadian Underwriter


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Canada’s Transportation Safety Board released Thursday the results of an investigation of an incident in which two executive jets nearly collided mid-air west of London, Ont. On March 8, 2012, American air traffic controllers had put them at the same altitude though they were travelling towards each other.

Canadian government says warning system helped pilots avoid crash

No collision occurred because the airplanes actually passed within a nautical mile laterally and 900 feet vertically from each other, which is far too close by aviation safety standards. Air navigation rules stipulate the airplanes are supposed to be separated by 2,000 feet vertically and at least five nautical miles laterally.

Less than a minute before they crossed paths, crews of both planes got alerts from their radar-based collision avoidance systems. Those systems use radar transponder signals and advise aircrew of “potentially conflicting aircraft that are equipped with secondary surveillance radar transponders. They provide alerts based on a “calculation of time remaining to the closest point of approach,” TSB noted in its report.

On the day of the incident, a Cessna Citation X, operated by XO Jet, was flying from San Jose, Calif. to Bedford, Mass. Part of its route took it through Canada, on a path from a navigation aid (known as the Peck very high frequency omnidirectional radio range, or VOR) north of Detroit to another VOR at London International Airport.

“At the same time, a Gulfstream V operated by Executive Jet Management was flying westbound from Windsor Locks, Connecticut, to San Francisco, California, on the same airway and flight level,” according to the TSB report. “The collision was averted after flight crews in both aircraft received warnings from their respective traffic alert and collision avoidance systems (TCAS).”

Both airplanes were flying at flight level 430, which is about 43,000 feet above sea level, depending on the actual air pressure. That cruising altitude is normally reserved for westbound traffic. So XO Jet’s Cessna Citation was put at an “inappropriate altitude for direction of flight,” or IAFDF, for “unknown reasons,” by the Oakland Air Route Traffic Control Center, according to TSB. The Oakland centre is a U.S. federal aviation administration facility based in Fremont, Calif.

“The United States air traffic controllers may assign an IAFDF when traffic prevents assignment of an appropriate altitude, as long as they have an agreement in place regarding such assignments with adjoining facilities,” TSB noted, adding that the FAA’s Cleveland Air Route Traffic Control Center has such an agreement with NAV Canada’s Toronto Area Control Centre.

According to the TSB investigation report, at 1:36 pm Eastern time, a controller from Cleveland contacted a controller from Toronto to hand off the eastbound Citation but did not mention it was at an inappropriate altitude. It was almost overhead the Peck VOR, not far from the Canadian border. Less than two minutes later, the controller in Toronto was relieved by another controller.

The “Oakville” sector of the Toronto ACC was staffed by a data controller, “focused mainly on coordination and data management,” and a radar controller, “focused primarily on communications and active control.”

“The sector was considered by the controllers as not very busy, and a handover briefing was not completed,” TSB stated of the data controllers. “The new controller began organizing the data board and formulating a mental traffic picture.”

Meanwhile, TSB stated, the westbound Gulfstream passed over the navigational aid near London airport and then, shortly before 1:41, the radar controller of the Toronto ACC Oakville sector was relieved. TSB stated a handover was done but no mention made that one aircraft was at an inappropriate flight level.

But within 40 seconds, the data controller told the radar controller of a “flight-progress strip for an inbound aircraft … at an IAFDF.”

By this time, TSB noted,  “the two aircraft were now 20 (nautical miles) apart and closing at a rate of 980 knots, or 16-1/3 nautical miles per minute,” according to the report.

Over the next 30 seconds, the Toronto-based controller told the eastbound Cessna Citation to descend 2,000 feet but did not get an acknowledgement. TSB records indicate the controller tried to get a hold of the Citation crew over the radio twice. By 1:42 p.m., the airplanes were less than 15 nautical miles apart and closing at nearly 1,000 knots.

At that time, the crew of both airplanes heard alarms from their collision avoidance systems, and the Toronto controller told the westbound Gulfstream to turn 25 degrees to the right. At the same time, the Gulfstream began to climb and the Citation began to descend as a result of the instructions the crew got from their collision avoidance alarms.

“The traffic alert and collision-avoidance system (TCAS) aboard both aircraft functioned as designed; it alerted the flight crews to the risk of collision, and provided them with resolution advice,” TSB noted. Less than a minute after the controller told the Gulfstream to turn right and the crew of both planes heard their collision avoidance systems, the planes crossed paths.

“The agreement between the west high specialty of the Toronto Area Control Centre and the Cleveland Air Route Traffic Control Centre, detailing approval requests for aircraft at an inappropriate altitude for direction of flight, was not followed,” TSB found. It also found that Canada’s “automated air traffic system situation display alert for an aircraft at an inappropriate altitude for direction of flight was disabled, and therefore did not alert the radar controller.”

Correction Notice:

An earlier version of this story incorrectly stated Flight Level 430 is 4,300 feet above sea level, not 43,000 feet ASL.