The Transportation Safety Board report released Tuesday states that moments before the crash, the pilot of one of the small planes lost sight of the lead aircraft in the four-plane flying formation.
After adopting a different flight path, the pilot tried to rejoin the formation but unintentionally placed his aircraft on a collision course with the lead aircraft, said the report.
The resulting crash sent both Cessnas spiralling towards the valley below.
"The flight profiles of both accident aircraft indicate that neither pilot saw the other aircraft in sufficient time to initiate effective and timely evasive action," concluded the report.
While the pilot of one of the planes, a Cessna 150L, was able to recover and land his plane in a farm field, the second pilot crashed into a shallow slough, killing himself and his passenger.
Killed were pilot Donn Hubble, 60, and Patrick Lobsinger, 70, the two occupants of the lead plane.
Just 15 minutes earlier, at about 4 p.m. on Feb. 9, 2011, the two Cessnas took off from Langley Regional Airport as part of a four-plane flying formation heading towards Chilliwack, B.C.
The report notes that three of the pilots in the group had previously flown together in formation but the pilot of the Cessna 150L was new to the group, although he had accompanied other pilots during formation flights.
Also part of the formation but not involved in the collision was a Cessna L19 and a Piper PA-28-180.
The report said that during a 15 degree right turn the pilot of the 150L lost sight of the lead aircraft, turned to the right and descended.
A few moments later, he turned left and climbed, searching for the leader and attempting to rejoin the formation and that's when the two Cessnas collided.
"The aircraft began to rotate and descend together, and fell out of control for several seconds," said the report.
The planes separated at about 120 metres above ground level, but the Cessna 150G broke up and fell into a slough.
Shortly after the accident, Harry Pride, an 85-year-old pilot, identified the deceased as members of the Boundary Bay Flying Club and a formation group called Delta Flight.
Pride called Hubble a "very serious, safety-minded person" and said Lobsinger was a teacher and a pilot.
"It's very demanding, you have to really be experienced and know what you're doing with an airplane," said Pride. "And it's like a team. As you go along, you feel really good if you do a good job and everything works out well."
According to the Transportation Safety Board report, formation flying in Cessnas is risky because the wings, which are located on the top of the plane, limit vision from the cockpit.
Formation flying is also challenging when planes with high wings, like the Cessna, and low wings, like the Piper, are flying together.
"Formation flying demands higher levels of skill, discipline and training than conventional flying," it added. "Without appropriate formal training to achieve those increased levels, the risk of in-flight collision is elevated."
The report also states that the risk of "inappropriate pilot actions" increase during loss-of-sight events if a qualified observer is not on board.
After the crash, Transport Canada issued a safety bulletin, identifying the hazards of formation flying and the importance of pre-flight planning and flying skills.