An All Nippon Airways passenger plane in Japan was forced to return to its departure airport on Saturday after the cockpit window was found to be cracked, an airline spokesperson said.

The flight returned to the city of Sapporo in Hokkaido without issue after the crack was discovered on the outermost of the glass’ four layers. Cabin pressure within the Boeing 737-800 remained normal throughout the flight, the spokesperson said.

“The safety of our passengers and flight crew is our priority and we apologize for the inconvenience,” the airline, which is Japan’s largest, said in a statement shared with HuffPost. “None of the other aircraft in the fleet are affected.”

Aviation expert John Strickland told the BBC that window cracks like this one are “not unheard of,” and that it could have been caused by something striking the glass, like a bird or a piece of hail.

“You might occasionally get a stress fracture too, from wear and tear,” Strickland said, “but that’s very rare.”

The plane is a different model from the Boeing 737 MAX 9, which suffered a mid-air scare earlier this month.

Part of the fuselage on one of the Boeing planes blew out while the craft was flying over Oregon on Jan. 5. A subsequent inspection of the plane found loose bolts and other “installation issues” on some of the Boeing 737 MAX 9 jets.

The Federal Aviation Administration as a result extended its grounding of Boeing 737 MAX 9 planes indefinitely on Friday so further safety checks can be performed. It also said it will tighten its oversight of Boeing.

Boeing on Monday announced that it will incorporate additional inspections into the 737 planes’ build process to ensure the planes’ safety. These inspections will include independent reviews by outsiders.

The recent scare made it “clear that we are not where we need to be,” Boeing Commercial Airplanes CEO Stan Deal said in a message to employees.

“We are taking immediate actions to bolster quality assurance and controls across our factories,” he said.