The deadly collision at Haneda Airport in Tokyo may have been caused by confusion over the air-traffic controller’s phrase “No. 1” during an abnormal period, aviation experts said.

Several former pilots and air-traffic controllers have focused on an exchange between an air-traffic controller and the captain of a Japan Coast Guard plane at 5:45 p.m. on Jan. 2.

That was two minutes before a Japan Airlines passenger jet landing at the airport hit the Coast Guard plane on a runway.

According to communication records released by the transport ministry on Jan. 3, the air-traffic controller instructed the Coast Guard aircraft: “Good evening. No. 1. Taxi to holding point C5.”

The Coast Guard plane responded: “Taxi to holding point C5. No. 1. Thank you.”

The C5 holding point is on the taxiway in front of the runway.

But the problem seems to stem from what “No. 1” meant, the experts said.

Hajime Naito, 71, a former captain at All Nippon Airways, said the Coast Guard pilot may not have been aware that the JAL plane had been given permission to land.

“If the Coast Guard plane misunderstood something, it may have been the term ‘No. 1,’” Naito said. “This term may have caused the pilot of the Coast Guard plane to rush to enter the runway.”

Yoichiro Hatta, 75, a former JAL pilot, offered a similar view.

“The pilot of the Coast Guard plane may have heard the term ‘No. 1’ and overinterpreted it to mean that he had received permission from air-traffic control to take off,” Hatta said.

According to Hatta, pilots must follow the instructions of air-traffic controllers.

In normal circumstances, if an air-traffic controller tells a departing aircraft that it is “No. 1,” it means it is next in line to take off.

The communication records for Jan. 2 show the air-traffic controller only instructed the Coast Guard aircraft to taxi to the runway stop position.

Despite this, the Coast Guard aircraft still entered the runway.

“It appears there was misrecognition by the pilot of the Coast Guard aircraft, who did not act as instructed,” Hatta said.

Hidetaka Tanaka, 41, who has 17 years of experience as an air-traffic controller and served as chief air-traffic controller at Chubu Centrair International Airport in Aichi Prefecture, said there is a different possibility.

At the time of the accident, Japan was scrambling to help Ishikawa Prefecture recover from a series of powerful earthquakes and a tsunami.

The Coast Guard aircraft was about to head for Niigata Air Base to provide relief for quake victims on the Noto Peninsula.

Before the collision, another civilian aircraft, which had communicated with the air-traffic controller, was waiting on a taxiway that was different from the one that the Coast Guard plane was on.

The air-traffic controller’s instruction of “No. 1” may have been a directive to give the Coast Guard plane priority for takeoff over the civilian aircraft, Tanaka said.

In emergency situations, such as disaster relief, air-traffic controllers may change the order of takeoffs, Tanaka said.

“It is possible the air-traffic controller gave priority to the Coast Guard aircraft heading for earthquake relief this time as well, designating it as ‘No. 1,’” Tanaka said.

He added that there was nothing odd about the communication records.

“An air-traffic controller could have noticed the Coast Guard plane’s approach onto the runway through ground radar screens or other means,” Tanaka said.

“Errors can happen,” he continued. “But usually, one of the parties involved notices it instantly and prevents an accident.”

According to investigative sources, the Coast Guard pilot was unaware that the JAL jet was landing.

JAL said at a news conference on Jan. 3 that the captain of the passenger jet "did not see" the Coast Guard aircraft.

“In this case, (an error) was missed by all eyes—of the air-traffic controllers and the captains and co-pilots of the Coast Guard and JAL planes--and the accident occurred,” Tanaka said.

Tomoki Kuwano, 83, is a former JAL pilot who heads the Japan Institute of Human Factors and has investigated the causes of numerous aircraft accidents.

“In this case, human error in communicating with air-traffic controllers was the likely cause of the accident,” Kuwano said. “But even if there is a single error, I want to see an examination of why a system was not built to prevent such an accident.”

The Coast Guard on Jan. 3 said the captain reported immediately after the accident that he “had received permission to enter the runway and had done so.”

But the transport ministry stated, “As far as we can see from the records, no permission was given to the Coast Guard aircraft to enter the runway.”

Five people aboard the Coast Guard plane were killed in the collision. The pilot was injured with burns.

All passengers and crew escaped the JAL aircraft before it burned to the ground.

Runway C, where the collision occurred, has been closed, and there are no prospects of it reopening soon.

The accident has continued to affect airline schedules during the busy New Year’s holidays. As of 4 p.m. on Jan. 4, 212 domestic flights to and from Haneda Airport had been canceled, according to the transport ministry.

(This article was written by Kazuyuki Ito, Yuki Nikaido and Eishi Kado.)