A pilot thought his co-pilot was playing a joke on him by pretending to take a nap while the aircraft flew around an airport in northern England.
But in reality, his colleague, 57, had suffered a cardiac arrest, and died beside him.
The pilot only realised what had happened after landing the plane, the BBC reported on Wednesday.
A safety report by the Air Accidents Investigation Branch in Britain found that the co-pilot had passed a medical examination four months earlier.
He had been asked to join the short journey aboard a light aircraft around Blackpool Airport for safety reasons due to windy weather conditions.
The surviving pilot recalled how the pair were talking normally while the aircraft was gearing up for flight.
The co-pilot, who was also a flight instructor, had remarked: “Looks good, there is nothing behind you.”
Shortly after take-off, however, the pilot said his colleague’s head rolled back and he appeared have fallen asleep.
“The pilot knew the instructor well and thought he was just pretending to take a nap while the pilot flew the circuit, so he did not think anything was wrong at this stage,” the report stated.
When the plane turned, the co-pilot’s head slumped onto the pilot’s shoulder but “he still thought the instructor was joking with him” and continued to stay on the flight path.
It was not until he landed the craft that the pilot grew concerned, as the co-pilot was still unresponsive.
He alerted the emergency crew for help, but the man could not be revived.
An autopsy revealed that the co-pilot, who had a history of high blood pressure and had been taking medication for his condition, had suffered a heart attack.
The report stated that the man was “his normal cheerful self” that morning and that three people who had flown with him before the incident said he seemed well.
Britain’s Civil Aviation Authority said it will continually review health guidelines, adding that the rarity of accidents caused by cardiac events in flight suggests “the balance is currently about right”.