He described the terrain of the area where the plane went down as "mountainous, rolling hills".
"From what I understand, the plane had made contact with the (air traffic control), had asked a question, and when (the tower) answered, there was no response and they noticed it had dropped off the radar," trooper spokeswoman Megan Peters said.
Johnson said the plane was in the process of filing what's known as a "pop-up" Instrument Flight Rules Plan. That can mean the weather is worsening over the course of a flight and a pilot is switching from visual to instrumental navigation.
The aeroplane was under lease to the California-based West Valley Flying Club, according to the company's owner, Lee Price.
Johnson said the Knights, who were married, were travelling with at least one other aircraft.
- AAP