A top Irish Air Corps flight instructor crashed into a mountainside with his trainee cadet after suffering an illusion, an investigation has found.
Captain Derek Furniss, 32, and Cadet David Jevens, 22, died when their two-seater light aircraft went down in Connemara's remote Crumlin Valley in October 2009.
Air accident investigators believe Capt Furniss - one of the military's most experienced teachers - succumbed to an airborne phenomenon known as somatogravic illusion.
The condition meant he believed their small Pilatus PC-9M plane was climbing steeply away from a mountain that unexpectedly appeared ahead when the plane was actually starting to nose-dive into it. His last words, captured on the black box flight recorder 26 seconds before impact, were: "Bad decision now."
Overwhelming sensations caused by disorientation and confusion as bad weather closed in on them probably caused the pilot to disbelieve his instruments and steer the aircraft into a peak known as Maum Dearg, near Cornamona village, it was found.
Capt Furniss, from Rathfarnham, Dublin, and Cadet Jevens, from Glynn, Co Wexford, were on a cross-country low-flying exercise with two other planes from Casement Aerodrome, near Dublin, to Galway and back.
A final report into the crash by the Air Accident Investigation Unit (AAIU) found that continuing the route in deteriorating weather, without reducing speed and in reduced visibility, contributed to the crash. The flight also broke minimum height restrictions in the moments leading up to the impact.
The AAIU made a number of recommendations, including a review on training for disorientation, qualifications of the officer commanding and regulations allowing flight instructors to authorise themselves.
The Defence Forces said it fully accepted the report's findings and would implement all the recommendations.
Defence Forces Chief of Staff Lieutenant General Sean McCann said: "I would like to extend my deepest sympathy to the families of Capt Derek Furniss and Cadet David Jevens on the tragic loss of their loved ones. I fully appreciate that the passage of time will not have dulled their absolute sense of loss."