The single-engined SOCATA TB-10 Tobago aircraft came down in a paddock on Wednesday morning, about three kilometres south of the Mt Gambier airport.
The 78-year-old pilot from Mt Barker and both passengers, a 43-year-old woman and a 16-year-old girl from Mt Gambier, were killed on impact, with the front of the plane extensively damaged.
Reports said the plane was on an Angel Flight, which are used to help country people access specialist medical treatment.
Four Australian Transport Safety Bureau investigators have been sent to the scene and are expected to begin their inquiries on Thursday.
“While on site the team will be examining the site and wreckage, gathering any recorded data and interviewing any witnesses,” the ATSB said in a statement.
The crash occurred as fog blanketed the Mt Gambier area, with the plane’s flight schedule indicating it was travelling from the regional centre to Adelaide.
It’s the second serious plane crash in South Australia in the past two months with a Cessna Conquest operated by charter company Rossair crashing near Renmark in May.
Killed in that crash were the company’s chief pilot Martin Scott, 48, fellow pilot Paul Daw, 65 and Civil Aviation Safety Authority inspector Stephen Guerin, 56.
The ATSB said the flight lasted only 60 to 90 seconds with early indications suggesting the plane came down steeply.
An interim report on the cause of the Renmark crash is expected in the next few days.
A similar report on the Mt Gambier crash is due within 30 days.
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