Crew members stopped two drunken Mexican pilots from leaving Costa Rica with an Aeromexico flight carrying 101 passengers to Mexico, a Costa Rican aviation official said on Monday.
"One of them, the co-pilot, tested positive for alcohol while the captain refused to do the test," Alvaro Vargas, deputy director of Costa Rican Civil Aviation, said.
The Boeing 737-700 had been due to leave Costa Rica for Mexico City on Saturday, but was delayed 24 hours to wait for replacement pilots, Vargas said, adding Costa Rican officials were investigating the incident.
"I don't think the passengers knew the real reason why the flight was suspended," he added.
Aeromexico apologised for the incident in a statement in Mexico City on Monday.
The pilots "were not in a fit condition" to fly and had been dismissed, at least until the end of the investigation, the statement said.
The passengers had been compensated for the inconvenience, it added.
According to Costa Rican newspaper Extra, the pilots had said they felt nauseous on arriving at the airport on Saturday, seven hours after attending a party in a San Jose hotel.
Aeromexico is part of the Skyteam alliance which includes Delta, Air France-KLM, Alitalia, Aeroflot, Korean Air and Vietnam Airlines.