A pilot continued flying after he thought his co-pilot was pretending to be asleep when he had actually died.
The pilot thought his colleague, a flying instructor who he knew well, was joking with him during the flight over Blackpool on 29 June last year, the Air Accidents Investigation Branch (AAIB) said.
Shortly after taking off, the 57-year-old instructor’s head rolled back as he suffered a sudden fatal heart attack, but the pilot continued to fly.
He only knew something was wrong after he landed the Piper PA-28-161 plane and the instructor was still resting on his shoulder and not responding, according to the report.
Before the short journey around Blackpool Airport, a second pilot had been requested for safety reasons due to windy weather, and the instructor agreed to join the flight.
While the plane was still on the ground, the instructor was talking normally with the pilot, the AAIB said.
The report added he had been his “normal cheerful self and there were no indications that he was feeling unwell”.
It said: “The pilot recalled that shortly after take-off from Runway 28 the instructor’s head rolled back.
“The pilot knew the instructor well and thought he was just pretending to take a nap whilst the pilot flew the circuit, so he did not think anything was wrong at this stage.
“He proceeded to fly the aircraft round the circuit. As he turned on to base leg the instructor slumped over with his head resting on the pilot’s shoulder.
“The pilot still thought the instructor was just joking with him and continued to fly the approach.”
The post-mortem examination concluded that the instructor, who had passed a medical four months earlier, died from acute cardiac failure.
The AAIB report said current medical assessments were acceptable but risks “can never be reduced to zero”.
It said it investigated the incident so lessons could be learned.
Its analysis added: “On this occasion he was flying with a qualified pilot who was able to land the aircraft safely.
“However, had this occurred on another flight the outcome could have been different.”