A Japanese airline says one of its jets nose-dived and rolled almost upside down earlier this month because the co-pilot hit the wrong controls while trying to open the cockpit door so the captain could return from a restroom break.

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An incident earlier this month in which an All Nippon Airways aircraft briefly flew virtually upside down after a copilot mistakenly operated a key steering mechanism has sent shock waves across the aviation industry.

“It’s usually impossible for a passenger airplane to fly in such a position,” one source familiar with the industry said. “It could have led to a serious accident.”

Japan’s Transport Safety Board said Wednesday that ANA Flight 140, operated by ANA’s group company Air Nippon, was on the verge of stalling after nosediving about 6,000 feet in 30 seconds on Sept. 6. The aircraft, carrying 117 passengers and crew members, was en route to Haneda Airport, which handles virtually all of Tokyo’s domestic flights, after departing from Naha.

According to the safety board, an analysis of the aircraft’s digital flight recorder indicated that the copilot, alone in the cockpit at the time as the captain used a restroom, mistakenly turned the rudder trim knob twice to the left for a total of 10 seconds.

The copilot apparently mistook the knob for the cockpit door lock switch as he tried to let the captain back in. The mistake is believed to have caused the airplane to tilt leftward and descend rapidly.

According to ANA, its aircraft usually tilt no more than 30 degrees when they roll, with the craft’s nose pointing up no more than 20 degrees and pointing down no more than 10 degrees. In the Sept. 6 incident, however, the aircraft rolled left and briefly reached a tilt of 131.7 degrees. Its nose pointed down 35 degrees at one point, the safety board also said.

“The figures are unbelievable, even in a case that requires such an urgent maneuver to avert a risk,” a source close to ANA said.

It was also revealed Wednesday that a stick shaker, a mechanical device to warn pilots of an aircraft’s imminent stall, was activated in the incident, indicating the ANA flight faced the risk of stalling.

On Sept. 7, the day following the incident, ANA discovered after analyzing flight data that the airplane had been at risk of stalling and had flown virtually upside down. However, the company failed to make it public.

ANA informed the media on the evening of Sept. 7 that it was still investigating the incident.

At a press conference Wednesday night after the safety board’s announcement, Shin Nagase, a senior executive vice president of ANA, said: “We had no intention to cover up (the incident). We couldn’t explain it, because it was being investigated by the transport safety board.”