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SAN FRANCISCO — The wayward Air Canada plane that nearly caused an aviation disaster at San Francisco International Airport last month dropped off radar displays for 12 seconds in the moments before it approached four fully loaded passenger jets on the taxiway, according to new information released Wednesday from federal aviation officials investigating the incident.

The National Transportation Safety Board also released stills of a harrowing airport video showing the Airbus 320 nearly landing on the four planes awaiting departure on Taxiway C.

The Airport Surface Surveillance Capability (ASSC) system monitors incoming aircraft to ensure they are safely landing at SFO and 34 other airports across the country. The system, which provides a computerized visual to air traffic controllers, is designed to sound a warning from a loudspeaker in the tower if an airplane on final approach is heading for an occupied runway. But it does not warn for planes that may be incorrectly aligned to land on a taxiway, as was the case for the Air Canada plane.

Since last year, the Federal Aviation Administration has worked to upgrade the system to also alert towers to planes lined up to taxiways, where planes awaiting takeoff queue up.

“The agency expects to begin testing some modified systems in a few months,” a FAA spokesman said.

However, on July 7, when Air Canada Flight 759 mistook a crowded taxiway for its approved runway, nearly triggering one of the worst aviation disasters ever, the radar system offered no help.

Shortly before midnight that night, “the airplane flew too far right off course to be observed by the local controller’s ASDE-X/ASSC and was not visible on the ASDE-X/ASSC display for about 12 seconds,” the NTSB reported Wednesday.

By the time it reappeared on the air traffic controller’s radar display system, “it passed over the first airplane positioned on taxiway C,” federal investigators found.

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Editorial: Answers needed as SFO near-miss closer than thought A source familiar with the investigation called it a “blind spot” that is a half-mile from the start of Runway 28-Right and Taxiway C. The system, among other functions, uses the airport’s radar to follow incoming airplanes and is designed to warn controllers early enough in an approach so they can alert a plane of a possible collision.

It is difficult for air traffic controllers to determine from the tower whether airplanes are properly lined up on approach; however, the computer system can aid them, a source said.

“If that blind spot had not occurred, they would have had equipment that alerted them that Air Canada was not lined up for a runway,” the source said.

Runways 28-Left and 28-Right and Taxiway C are parallel to each other. In post-incident interviews, the Air Canada flight crew told investigators that they believed the lighted runway they saw was 28-Left, and that Taxiway C was actually their cleared runway 28-Right.

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But runway 28-Left had been closed for maintenance and featured a giant lighted “X” to warn pilots, along with other advanced warnings sent to flight crews, according to SFO officials and the NTSB.

“(The flight crew) also stated that they did not recall seeing aircraft on taxiway C but that something did not look right to them,” according to the NTSB.

The captain has over 20,000 total flight hours; about 5,000 of those as a captain in Airbus 320-series planes. The first officer has about 10,000 flight hours, more than 2,300 of those in Airbus aircraft.

Federal officials were not able to gather information from the airplane’s cockpit voice recorder because it had been overwritten.

NTSB investigators have not determined a probable cause for the incident, and the information released Wednesday does not contain analysis of the information, so no conclusions have been reached, the agency said.

NTSB preliminary findings and independent analysis of flight data show the Airbus 320 shortly before midnight on July 7 continuing its landing descent over two airplanes on the taxiway before finally climbing above two more planes awaiting departure. At its closest moment, the Air Canada flight was an estimated 51 feet above a fully loaded Philippine Airlines plane.

Federal investigators found that Air Canada was lined up to the taxiway starting at three miles out from the airport. Around 0.7 miles out, the pilot warned of lights on the runway. Six seconds later, Flight 759 dropped from radar.

As the plane was about 0.3 miles from the edge of the taxiway, the air traffic controller replied to the pilot’s concern over lights on the runway and affirmed the plane was cleared to land on runway 28-Right, not realizing the pilot was aimed at the taxiway. A United Airlines pilot warned over the radio that Air Canada was lined up on the taxiway, and the Philippine Airlines crew, the second plane on the taxiway, turned on its landing lights to try to warn Air Canada about the errant landing, according to the report.

The Air Canada pilot powered up its thrust when it was about 85 feet above ground, but the plane had dropped as low as 59 feet before it began to climb. After the radar black-out, the tower picked up the Air Canada flight again on its displays when it was over the first airplane, according to the report.

The pilot had already begun to pull up when the air traffic controller ordered a “go-around.” Two air traffic controllers were on duty that night, but only one was working in the tower at the time of the incident, according to the NTSB.

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