An investigation into a crash that killed an Auckland flying ace has found his plane "departed from controlled flight" seconds before smashing into the ground, but has failed to determine why the pilot lost control.

Record-breaking aviator Wayne Matthews, of Greenhithe, was killed when his custom-built Thorp S-18 aircraft crashed at Whenuapai Air Base on the afternoon of April 26, 2008.

His passenger, 46-year-old Brent Baldwin of West Harbour, also died.

Mr Matthews, 62, had just completed a "touch and go" landing at the airport, when witnesses heard the aircraft "going quiet" while climbing steeply from the landing strip, the Civil Aviation Authority report says.

The plane was seen to fall into a steep, right-hand descent, before disappearing behind trees. Onlookers heard the aircraft hit the ground.

A former Air Force navigator, Mr Matthews held all the appropriate licences and endorsements, and had 745 flying hours, 501 earned in the crashed aircraft, known as Rosie 3.

Though the CAA report "could not establish with any certainty why the aircraft diverted from controlled flight", it did suggest two possible causes: a mishandled manoeuvre, or "a distraction sufficiently significant to cause a departure".

An autopsy found Mr Matthews suffered "significant" heart disease, "to the extent that an incapacitating event or distraction could not be ruled out".

"I cannot exclude the possibility of an abnormal heart rhythm which may have led to an episode of pilot incapacitation or distraction," report author Alan Moselen said. However, autopsy results alone were not enough to make a definitive ruling.

The investigation found no mechanical or structural faults and also ruled out bird-strikes or weather playing a role in the crash.

CAA spokesman Bill Sommer yesterday said the report had been passed to the coroner, and neither party had made further recommendations. The true cause of the crash would likely never be known.

On February 20, 2008, flying his Thorp, Mr Matthews set a record for circumnavigating NZ. He flew it non-stop solo on the 2097-nautical-mile flight in 15 hours and 53 minutes.

REPORT FINDINGS

* Wayne Matthews held appropriate flying licences and ratings.
* The aircraft was airworthy.
* No mechanical or structural defects found.
* Investigation could not establish the cause of the crash.
* The crash was not survivable.
* No safety recommendations proposed as a result of the accident.

Source: Civil Aviation Authority