A flight instructor had a heart attack while flying over England last summer, and the pilot at the controls first assumed his coworker was playing a prank.

With the instructor's head drooping over his shoulder, the unwary pilot completed the flight and touched down at Blackpool Airport in Lancashire on June 29, according to the New York Post.

The UK Air Accident Investigation Branch published a report of the incident earlier this month, indicating that the pilot first thought the 57-year-old instructor was pretending to have fallen asleep.

The unidentified instructor, who had been medicated for hypertension for 20 years, died suddenly from abrupt heart failure four months after passing a routine medical, per The Straits Times.

Due to the high winds, he was requested to join the brief flight over Blackpool Airport in a small aircraft. The surviving pilot said they were engaged in typical conversation as they prepared to take off.

"Looks OK, there is nothing behind you," the co-pilot, who was also a flying instructor, noted.

However, the pilot reported that shortly after liftoff, his colleague looked to have fallen asleep with his head curled back.

Read Also: Israel Raids Palestine's West Bank

Pilot Realized It Was Not Joke But It Was Too Late

According to the UK investigation, the pilot "knew the instructor well" and felt he was only joking "while the pilot flew the circuit," so he didn't suspect anything was odd, per the Daily Mail.

The co-pilot's head fell into the pilot's shoulder as they banked, but he still thought the instructor was playing a prank, and they proceeded on their course.

After he landed on the plane, the pilot saw that his copilot was still unresponsive, and he began to worry.

He called for aid, but his colleague already died.

The UK Air Accidents Investigation Bureau said it constantly examined health recommendations and the 'rarity' of cardiac incidents in flight indicates that "the balance is currently about right'.

Related Article: Caught On Video: Joe Biden Stumbles