The Transportation Safety Board of Canada found that the two pilots who crashed above Dewdney in February, 2011 while practising formation flight in Cessna 150s did not discuss procedures in case they lost sight of each other.
The TSB also found that there are limited vision angles in the cockpits of high-wing planes such as the Cessna 150.
Pilot Donn Hubble, 60, of Langley and passenger Patrick Lobsinger, 70, of Surrey died after their blue Cessna collided with another plane midair, then crashed into the Nicomen Island Slough just off the Lougheed Highway near Dewdney.
The second damaged plane piloted by Paul Knapp, 57, of Pitt Meadows landed in a nearby field. Knapp was uninjured.
"Formation flying is a challenging activity requiring high levels of skill and discipline," said Bill Yearwood, the manager of Air Investigation Operations for the Pacific region.
"Without appropriate training and thorough preflight briefings, there is an increased risk of in-flight collisions, and these collisions often cause fatal accidents."
The two Cessna 150s were part of a four-aircraft formation flight.
One was the lead aircraft and the other was behind and to its right. During a right turn while in formation, the aircraft on the right lost sight of the leader. While trying to regain sight of the leader, the aircraft climbed, turned left, and struck the lead aircraft.
The two planes were briefly joined together, then descended out of control until about 400 feet above ground level where they separated.
The lead aircraft broke up and fell into the slough. Both pilot and passenger succumbed to their injuries.
The other pilot regained control of his plane and landed without injury in a nearby field, but without engine power.
While formation training is required for events such as air shows, the only regulatory requirement for casual formation flight outside of air shows is a pre-arrangement between pilots intending to fly in formation.
The TSB reports the group had a pre-flight briefing on join-up procedures after takeoff and breakout procedures.
However, the pilots' briefing didn't go over procedures in case of loss of sight of an aircraft or returning to formation.
The investigation also found that flying in formation in high-wing aircraft such as the Cessna 150 poses an elevated risk of loss of visual contact due to limited cockpit vision angles.
Since the accident, Transport Canada issued a bulletin warning of the risks of formation flying.
It highlights the importance of pre-flight planning and flying skills in reducing the associated risks.