A network of pre-existing cracks caused a fatal aircraft crash near Whangarei in 2005, the Transport Accident Investigation Commission says.
Cracks up to 55mm long were found on the aircraft's vertical fin, the commission said in a report released today.
Top dressing pilot Peter Beatty, 49 and loader driver Greg Nash, 56, were killed when their Fletcher FU24-950 agricultural plane crashed at high speed into the Pukenui Forest, 5km west of Whangarei, on November 22, 2005.
The pair had been flying to Whangarei after abandoning plans to spread fertiliser on a farm property due to strong winds.
Their disappearance was not reported to police until 12 hours after they last made contact with anybody, and their aircraft was not spotted by searchers for another 12 hours.
The commission said the damaged area of the fin had been covered with a black protective rubber strip and would not have been noticed during a visual inspection.
"From an evaluation of all the evidence, the hidden damage to the leading edge of the vertical fin put the fin in an unserviceable and structurally weakened state.
"During the flight, normal aerodynamic loads on the weakened fin, probably from a combination of airspeed, pilot inputs, sideslip and wind gusts, caused the fin to fail and bring about the accident." The damage made the aircraft uncontrollable and there was no evidence any other failure, including a broken airspeed indicator, contributed to the accident, the report said.
The commission made several safety recommendations to the Civil Aviation Authority, including a "high priority" fin redesign and rewrite of the agricultural aircraft safety review.
Other recommendations included a review of the continued airworthiness programme for the Walter Fletcher aircraft and a clarification over division of responsibility for modified FU24 aircraft.
