Two flight attendants were also given prison sentences of 45 and 60 days respectively.
The co-pilot, 38, was found to have a blood alcohol level of almost seven times the legal limit.
He told the court that he and his colleagues had downed two bottles of whisky before he began drinking beer.
“We lost control,” the man told the court on Monday, explaining his blood alcohol reading of 1.35mg of ethanol per millilitre of blood, far over the 0.2mg limit.
Pilot facing trial
The crew were tested by Norwegian police after an anonymous tip-off and the four accused were prevented from boarding the plane.
They had begun drinking at 15:00 on 7 August, the co-pilot told the court, and had agreed to stop at 20:00 but lost control of the amount they had drunk because they were tired after three consecutive flights.
He was still drinking beer shortly before 02:00 the next day, around four hours before his flight was due to leave with some 100 passengers on board.
He was initially sentenced to eight months in jail but that was reduced to six because he admitted the charges. He is yet to decide whether to appeal.
The plane’s 50-year-old captain is also being held in custody and faces trial on 17 September. He denies some of the charges against him.
Reports suggest his blood alcohol level was more than double the limit, at 0.54mg.
After the four crew members were arrested, air Baltic said it was introducing additional safety measures, including pre-duty breathalyser tests for all “safety-critical personnel”.
The airline said all four had been suspended and it was seeking their dismissal.