A colleague struggled to make sense of what he was saying before dressing him and moving him into a free seat.
The captain was alerted and, after speaking with Pentecost, called for medical professionals on board.
He was found to have dilated pupils, was unable to answer what year it was or who the US president was and drank 15 to 20 bottles of water during the flight, the court was told.
His behaviour caused a “huge distraction” for the captain and crew, though no passengers were aware of his condition.
The flight landed at Heathrow where paramedics met the aircraft and took Pentecost to Hillingdon Hospital.
A blood sample later showed he had methamphetamine and amphetamine in his system, and he was later dismissed from his job.