A drunk pilot was found to be 10 times over the legal alcohol limit by police, despite passing the airline's breathalyser test.
The plane departed London for Tokyo's Haneda airport after a delay of one hour and 9 minutes, forcing the airline to operate the flight with two pilots rather than the customary three.
Katsutoshi Jitsukawa, 42, had nearly 10 times more than the legally permitted amount of alcohol in his bloodstream when he was arrested, CNN quoted the Metropolitan Police as saying on Thursday.
London's Metropolitan Police force say Katsutoshi Jitsukawa appeared at Uxbridge Magistrates Court in west London on Thursday and admitted to exceeding the alcohol limit.
Japan Airlines flight JL44 was due to take off just 50 minutes after tests showed that First Officer Jitsukawa had 189 mg of alcohol per 100 ml of blood in his body.
JAL Flight 44's departure from London at 7pm on Sunday local time was delayed by more than an hour. The limit for drivers in Britain is 80 milligrams.
The pilot later pleaded guilty before a court to being over the legal limit, and is expected to be sentenced on November 29. That is probably why he landed up at the Heathrow airport highly drunk and ready to board the flight.
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The incident came a day after another Japanese carrier apologised for multiple delays after a hungover pilot called in sick.
"We are certain (the in-house breath test) wasn't conducted properly", said Muneaki Kitahara, JAL's head of communications.
The British Airways pilot was jailed for eight months after reporting for duty while four times over the limit.
"The company sincerely apologizes to the passengers and to all affected by the employee's actions", JAL said in a press release.
The last-minute sickie forced the airline to delay five flights linking Okinawa island and smaller regional islands, affecting 619 passengers.
Under its internal rules, JAL limits two-pilot flights to routes of up to 12 hours.
The Transport Ministry has been requiring airlines to take measures to prevent alcohol-related incidents.