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. Last Updated: 07/27/2016

It's Hit or- Miss With Helsinki Radar

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Finnish aviation authorities were aware of serious glitches in the Helsinki airport's new air-traffic control radar — over the past six months, at least 20 planes, most of them from Aeroflot's fleet, simply did not show up on the screen.

But it was the Dec. 15 incident that really got them thinking.

On that day, an Aeroflot Ilyushin-62 took off from Helsinki airport and was instructed by air traffic control to assume an altitude of 7,000 feet. But the Il-62 somehow slipped off of the radar — and Finnish authorities say it also assumed the wrong altitude, rising to 7,500 feet.

For the next five minutes, the Ilyushin-62 was invisible to air traffic controllers. And its path was converging with a Finnair McDonnell Douglas MD-83 airliner approaching to land.

Five seconds before the planes' paths were to cross, the Il-62 suddenly reappeared on the radar screen — at the wrong altitude. At the same time, the Finnair flight crew's collision avoidance system blared a warning. All of it came much too late to make meaningful flight corrections — or do much beyond hold one's breath.

The Ilyushin squeaked under the Finnair plane by just 150 meters, half of the industry's minimum safety distance of 300 meters.

Finland's Civil Aviation Authority, among others, was not amused. It has launched an investigation into the near-miss.

CAA vice president Matts-Anders Nyberg, in a telephone interview from Helsinki on Thursday, estimated the CAA investigation would last for a month or two. Nyberg added that the CAA was seeking cooperation with Russian authorities and Aeroflot on investigating the matter.

The Helsinki airport handles 500 flights a day, but previous incidents in which planes did not appear on the new radar system installed this summer had never led to any such near miss, the Finns say.

Most of the 20-odd recorded incidents of planes slipping into involuntary "stealth" mode over Helsinki involved Aeroflot planes, although the radar has also winked off on flights by Finnair, SAS Scandinavian Airlines, the Siberian airline Sibir, St. Petersburg's Pulkovo Airlines and some private small Cessna craft.

Aeroflot performs seven flights a week to Helsinki, usually on Boeing-737s and Tu-154s, though it sometimes uses Il-62s as a backup. But if the Helsinki radar often misses Aeroflot flights, it does not seem to miss one or another make or model of plane.

The radar problem was first recognized in the summer, when the new radar failed to pick up transponder signals transmitted by some incoming flights.

But an official from Russia's State Civil Aviation Service said all radars that are certified by the International Civil Aviation Organization should be able to recognize the transmissions of an Il-62.

The ICAO did not reply to a request Thursday for comment.

The problem was noticed when the system failed to pick up signals by incoming flights.

Aeroflot said Thursday it was baffled by accounts of the Helsinki radar gremlins. An Aeroflot spokesman said the airline has not encountered any such radar failures at any other airport in Europe, including some to which Ilyushin-62s fly seven days a week.

The spokesman also said Aeroflot was surprised when the Finns sent the company a Dec. 26 notification of an altitude violation by its plane on Dec. 15. Aeroflot said that notification, 11 days after the fact, was the first anyone had said about the matter, and they maintain the plane never rose above 7,000 feet.

Alexander Gritsun, who heads the flying squad that operates Aeroflot's fleet of 13 Ilyushin-62s, and who also happened to be aboard the Ilyushin-62 on Dec. 15, said no reprimands of any kind had ever been made to the crew before Dec. 26 — either during the Helsinki takeoff and flight, or after the plane landed in Moscow.

"The controller did not contact the crew on any mishaps, so the pilot assumed that everything was alright," Gritsun said. "If something is wrong the controller should contact the crew. This didn't happen."

Gritsun said the Ilyushin had been told to fly at 7,000 feet and had done so, according to the plane's "black box" data recordings. He also said the Ilyushin's transponder that day had been in order, and that a recording of communications between Helsinki control and the Il-62 cockpit reflected all of that.

Nyberg countered that Helsinki radar spotted the plane at 7,500 feet. He said his two questions would be: Was the transponder turned on in a timely fashion? And: Did the pilot switch the altimeter from Helsinki airport pressure to standard air pressure, a step taken after a craft reaches an altitude of 5,000 feet?

Not having adjusted instruments for the air pressure could alone have accounted for the 500-foot discrepancy in altitudes, Nyberg said.

Gritsun, in turn, countered that black box recordings show the transponder was tested before the flight and was operating when it was supposed to be; that the pilot did indeed adjust the altimeter for pressure after 5,000 feet; and that all black box data would be shortly forwarded to the Finnish aviation authorities.

Nyberg said he welcomed the news that Gritsun and his colleagues were reacting swiftly to his agency's queries. He also added that CAA requests were sent out to airports in other Nordic countries to see whether they have encountered similar problems.