NEW YORK (Bloomberg) - An Air Canada plane that nearly landed on a taxiway in San Francisco last month passed only 59 feet (18m) above the ground before aborting its approach and averting a crash, US investigators said on Wednesday (Aug 2).
The Canadian Airbus SE A320 may have missed one or more of the planes in its path on the ground by just a few feet in the July 7 incident.
The Airbus was preparing to land not on the intended runway but on an adjacent taxiway, where other aircraft had lined up as they waited for takeoff.
The pilots told National Transportation Safety Board (NTSB) investigators they thought they were about to land on one of two parallel runways at San Francisco International airport.
"They did not recall seeing aircraft on taxiway C" but "something did not look right to them", the NTSB said in an update of its probe.
The NTSB also released plots showing where the planes were at various times and images from an airport surveillance camera that showed the Air Canada plane passing close to the jets on the ground.
Although it was minutes before midnight, at least three of the planes on the ground were clearly visible and were well lit in the photo.
While the NTSB has not yet calculated how close the planes came from a catastrophic collision, the Boeing 787 that was first in line on the taxiway has a tail that is almost 56 feet (17m) high, according to the company's website.
That United Airlines plane had turned to enter the runway and the Air Canada plane appears to have passed just behind it, according to a radar plot released by NTSB.
The risk of collisions on the ground have been raised as a safety issue by the NTSB in the past.
The highest death toll ever in an airline accident occurred on the ground when two Boeing 747s collided on a runway in 1977 in Tenerife, Canary Islands, killing 574 people.
In the San Francisco incident, the pilot of the United plane had attempted to raise concerns about the Air Canada plane's flight path in radio transmissions.
"Where's this guy going?" the pilot said. "He's on the taxiway," he added a few seconds later as the Air Canada plane passed just behind him.
The Philippine Airlines Airbus A340 that was second in line on the taxiway also has a tail that is about 56 feet high, according to the manufacturer's website.
There were four planes on the taxiway, three of them wide-bodied aircraft that can carry more than 200 passengers.
The Air Canada pilots decided to climb away from the ground and abort the landing on their own, according to the NTSB.
An air-traffic controller in the tower, who was the only controller on duty at the time, ordered the plane to climb a few seconds after the pilots had already begun to do so.
At the time of the incident, Runway 28 Left was closed for construction.
The Air Canada plane was supposed to land on Runway 28 Right, which is parallel to the closed landing strip.
Both pilots said in interviews with investigators that they thought the runway lights they saw to their left were those of the closed runway, but the lights on the closed runway had been shut off, according to NTSB.
Runways mostly use white lights, while taxiways use green and blue.
From at least three miles (4.8km) away, the plane's flight path was pointed towards the taxiway, which is about 500 feet (152m) to the right of the lone operating runway, according to NTSB.
At about 0.7 miles (1.1km) from the airport, an Air Canada pilot radioed the tower and said there were lights on the runway, asking if they were still cleared to land.
The controller replied that the runway was clear.
The NTSB was not able to obtain the Air Canada plane's cockpit recorder.
The device was overwritten during subsequent flights, the NTSB said.
For at least part of its approach to the airport, the Air Canada plane was not visible to a radar system, designed to prevent collisions on the ground, NTSB said.
It is not clear whether that system was designed to warn against a plane landing on a taxiway.