( CNN ) -- Dutch airline Transavia said it has launched an investigation after a Boeing 737 pilot was locked out of the cockpit and his first officer was later found asleep at the controls . The incident took place in September , when the airliner was en route from Greece to the Netherlands , a top Dutch safety investigation agency said . The 737 landed safely in Amsterdam as scheduled , the airline said Wednesday . According to a Dutch Safety Board report released Wednesday , the pilot stepped out of the cockpit to take a bathroom break about 2Â½ hours into the flight . When he returned a short time later , the pilot used an intercom to ask his first officer to open the door . There was no answer , the report said . Eventually , the pilot alerted the crew and was able to open the door himself . That 's when he found the first officer asleep , according to the report . `` It 's a serious incident , '' said Wim van der Weegen of the Dutch Safety Board , `` What makes it serious is the combination of the pilot being unable to access the cockpit and the first officer being asleep . `` By 'serious incident , ' I mean the flight was in danger , '' he said . The Dutch Safety Board will decide whether to open its own inquiry when the airline 's investigation is finished , van der Weegen said . Laws regarding pilot breaks during flights vary from country to country . For U.S. carriers , sleeping while at the controls is a violation of FAA regulations . Flights longer than eight hours require a relief pilot on board to take over when pilots sleep . U.S. airlines also require a flight attendant to be in the cockpit when the pilot or first officer take bathroom breaks , in case the person flying the aircraft becomes incapacitated . Alaska Airlines pilot loses consciousness midair .
first officer found sleeping at controls of a Transavia 737 , says Dutch airline . Returning from a bathroom break , the pilot could n't immediately enter the cockpit . He eventually gained access , and the plane landed safely in Greece . The incident took place in Wednesday en route from Amsterdam , Dutch officials say .