The Transportation Safety Board has ruled that a 2012 crash that killed all four people on a small plane near Kitchener-Waterloo was caused by an "unauthorized" aircraft spin.

The board reported the findings Thursday after a probe into the August 2012 crash of the Cessna 172S in Moorefield, Ont., which is located just north of the Kitchener-Waterloo airport.

According to the report the plane, owned by the Waterloo-Wellington Flying Club, left the airport at 6:15 p.m. on Aug. 24, 2012, and flew to Niagara Falls, then Toronto, before heading to a practice area north of the Kitchener-Waterloo airport.

About two hours later, the plane crashed into a field, killing the pilot and three passengers.

The investigation determined the plane entered a spin "in a configuration for which spins were not authorized" and could not recover.

The report also found there was a breach in the aerodynamic stall warning horn, but said its effect on the horn's operation could not be determined.

The TSB says since the crash, the flying club has brought in changes to its program, such as making clear its policy that pilots can only perform spins and other such manoeuvres when an instructor is on board.