The young pilot of the micro-light aircraft in Fermoy watched in horror as his friend backed into the propellers of the aircraft.
17-year-old Gordon Kenny stumbled into the propellers during a routine test of the engine in a back yard and died instantly.
Neighbour Sean Geaney, in whose yard they were testing the aircraft, had bought the two-seater aircraft with his friend in Watergrasshill yesterday.
The yard at Macroney, Kilworth, has been cordoned off by gardai and a team of experts from the Air Accident Investigation Unit are examining the aircraft.