A handling error and "casual attitude to CAA rules'' were factors leading to a gyrocopter crash which killed two men in Taranaki, a coroner has found.

Stephen John Chubb, 51, and Neville Ronald Adlam, 70, died instantly from multiple injuries when the small aircraft crashed at Hawera Aerodrome in November 2009.

A possible medical event may have also been a factor which led to the fatal handling error, Coroner Carla na Nagara found.

On the morning of the crash, the two men had completed several take-offs and landings from the Aerodrome in the UFO HeliThurster gyrocopter ZK-RAZ, belonging to Mr Adlam.

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They initially flew with Mr Chubb in the pilot's seat and Mr Adlam observing, then swapped seats and Mr Adlam completed several circuits as pilot.

The aircraft was then observed by a nearby farmer, Tanant Lester, dropping low and tilting to the left. He said the craft was making a ``chud chud'' noise, and kept veering left until it went nose-first into the ground.

Both men were killed on impact from multiple injuries sustained. Neither were under the influence of drugs or alcohol, but Mr Adlam had an underlying heart condition.

An investigation by the Civil Aviation Authority (CAA) found there was no mechanical problem with the gyrocopter, and the crash was caused by a ``bunt over'' or ``power push over''.

``This occurs when the gyrocopter effectively stops flying due to airflow going over the top of the rotor blades, rather than up through them.''

A gyrocopter is powered by air moving through the rotors rather than by an engine.

CAA investigator Alan Moselen said the bunt over could occur if the pilot pushed the control stick forward too rapidly.

Coroner na Nagara said the reasons for the handling error which led to the bunt over could not be established, but given Mr Adlam's significant medical history, a medical event could not be discounted.

It was also noted that Mr Chubb, although very experienced, was not a trained and qualified instructor and should not have been instructing Mr Adlam.

He was also not a qualified test pilot, and the gyroscope had not yet undergone its required 10 hours of test pilot flying.

Coroner na Nagara said Mr Chubb more than likely lacked the knowledge and experience needed to intervene to regain control of the aircraft.

"This case reflects a very casual attitude to CAA rules, to the pilots' peril. While I have no doubt both men were experienced pilots, had they coupled their experience with respect for rules governing flight - and pilot instruction - they may not have died as they did that day.''

After the crash, the CAA started a project to review gyrocopter operations and rules.