Drunk first officer Katsutoshi Jitsukawa, 42, was arrested at the airport after failing a breath test just 50 minutes before his Japan Airlines flight to Tokyo was due to take off with him in the cockpit.
The driver of a crew bus alerted police after smelling alcohol on the pilot, according to reports.
The flight was operated by a Boeing 777 aircraft which holds up to 244 passengers.
A Metropolitan Police spokesman said Japanese Jitsukawa pleaded guilty to exceeding the alcohol limit at Uxbridge Magistrates' Court on Thursday.
Tests revealed he had 189mg of alcohol per 100ml of blood in his system, almost 10 times the 20mg limit for a pilot. The drink-drive limit in England, Wales and Northern Ireland is 80mg.
He was remanded in custody and will be sentenced at Isleworth Crown Court on November 29.
Japan Airlines issued an apology and pledged to "implement immediate actions to prevent any future occurrence", adding that "safety remains our utmost priority".