2Degrees boss Eric Hertz wouldn't have been allowed to fly had he declared his mental illness before the plane crash that killed him and his wife - but it wasn't a cause of the accident, investigators says.
In report released by the Civil Aviation Authority today has been unable to pinpoint the cause of the fatal crash in March 2013, but says a number of factors played a part.
The plane fell into the sea off Raglan at Easter 2013, as the pair flew from Auckland to Timaru to visit their daughter.
The wreckage was recovered almost intact from the seafloor, 59 metres below the surface and the investigation has taken more than 2000 hours by authority staff as well as experts from the engine and plane manufacturers, Continental and Beechcraft.
The CAA has now concluded it is likely the plane lost power in the left engine for an unknown reason, going into a spin and never recovering.
Safety flags have since been raised about certain parts of the plane and how they're tested.
But investigators also said US-certified Mr Hertz had knowingly not told authorities in the US or New Zealand that he had been diagnosed with Generalised Anxiety Disorder or Major Depressive Disorder.
"Had the pilot declared the diagnosis of GAD or MDD, or the medication that he was taking for this condition, neither the FAA nor the CAA would likely have issued him with a medical certificate, meaning he would not be allowed to fly in either New Zealand or America," it said in a statement.
But it said it was not suggested this was a caused of the accident.
The CAA also said it had found there was limited oversight of permanently based, privately operated, foreign-registered aircraft in New Zealand, such as Mr Hertz's Beechcraft.
Mr Hertz had radioed air traffic controllers to say he had an emergency and was experiencing problems with both engines.
Radar evidence also showed that 30 minutes into the flight, after reaching its cruise altitude of 18,000 feet, the aircraft's ground speed decreased sharply before it hit the water about 90 seconds later.