A plane belonging to Japan's domestic carrier All Nippon Airlines (ANA) returned to its departure airport after a crack was reported in the cockpit window during flight.
The crack appeared in the outer layer of the four layers of window surrounding the cockpit.
There were no injuries to anyone on board.
ANA flight NH1182 was flying from Sapporo in Hokkaido to Toyama on the main island of Honshu in Japan.
The plane, a Boeing 737, landed at New Chitose Airport in Sapporo at approximately 12:10 local time (3:10 GMT).
A spokesman for ANA, Japan's largest airline, said the crack was discovered as the plane passed over Hakodate.
“The crack was not something that affected flight control or pressure,” the spokesman said.
The plane returned to the airport where it made a safe landing.
Aviation expert John Strickland said the cause of the crack was still unknown.
“These things happen sometimes. Maybe something has hit the window, for example a bird, or a large hailstone, which has never been heard of before.”
“You can sometimes get a stress fracture as well, due to wear and tear, but that's very rare,” he added.
He said the airline will likely have to replace the entire window, not just the broken layer, to make sure the plane is completely safe.
“These things do happen, and it's impossible to say how frequently they happen.”
There were 59 passengers and six crew members on board. Alternative flights were arranged for passengers.
This is the second accident involving a Boeing 737 aircraft in as many weeks. The ANA flight was not a Boeing 737 MAX 9, but rather an earlier version that was “by no means old,” Strickland said.
All Boeing 737-9 aircraft have been grounded by the US aviation regulator (FAA) after an Alaska Airlines plane suffered an explosion that toppled a cabin panel from a new plane in midair, leaving a gaping hole in the side of the fuselage last week. .
That plane, which was carrying 177 passengers and crew, was forced to make an emergency landing in the US state of Oregon.
The Federal Aviation Administration on Friday extended the grounding of Boeing 737 Max 9 aircraft indefinitely to conduct more safety checks and said it would tighten oversight of Boeing itself.
The Federal Aviation Administration said that the 171 planes of the same design as the plane involved in the accident were forced to remain on the ground “for the safety of American travelers.”
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