The Transportation Safety Board's final report into the crash of the twin-engine charter flight is expected to be released today.
CBC News has learned investigators determined that the pilot was operating the aircraft under reduced engine power after noticing an oil leak, which caused a drag on the left side.
The imbalance in engine power contributed to what investigators called a mishandling of the aircraft.
The Northern Thunderbird Beechcraft King Air crash-landed on a six-lane Richmond street during rush hour shortly after taking off on Oct. 27, 2011.
Five men and two women passengers escaped after bystanders entered the burning plane, but the two pilots were trapped inside the cockpit. Both were pulled from the plane alive, but later died in hospital.
After the crash, some of the passengers told the media they noticed oil under the left wing of the aircraft when they boarded.
Shortly after takeoff, the captain said they would be returning to Vancouver airport because of an "oil issue." One of the passengers managed to snap a picture of the leaking oil.
As the plane returned to the airport, the pilot was asked by the control tower if he needed emergency equipment on the runway. He declined because an oil leak is not considered an emergency unless there is a loss of oil pressure, which there wasn’t according to the TSB report.
In the final stages of the approach, when the plane was at about 100 metres in altitude and just short of the runway, the pilot lost control of the aircraft, the report says.
As the aircraft rolled left and pitched down, it collided with a lamp post, clipped a vehicle and crashed on the busy roadway and burst into flames.
People from offices in the area and passing cars rushed to the scene and pulled the passengers out of the burning aircraft.
Pilots Luc Fortin and Matt Robic were trapped in the cockpit until fire crews arrived.
All of the passengers were hurt, some with serious spinal injuries and paralysis. Six of them eventually launched a civil suit against the airline, Northern Thunderbird Air.
Oil cap problem flagged in service bulletins
The TSB report says the crash was not ultimately caused by mechanical failures even though two mechanical issues contributed to the incident.
The crash was caused by a chain of events that momentarily led the pilot to lose control of the aircraft at a critical moment, the report concluded.
It also found the leaking oil was caused by an unsecure oil cap on the left engine. That did not cause the crash, but resulted in a "series of events" that led to the catastrophic event.
Two non-mandatory service bulletins were issued in 1995 and 2010 about that particular model of Pratt and Whitney engine and the oil cap, stating that service personnel were not latching the cap properly, which was causing mid-flight oil leaks.
The bulletins suggested modifications be done on the engine to fix the cap and restrict oil loss, but the report found the modifications were not done on this aircraft and the cap was often closed, but not latched by service personnel.
The report also noted there was no kill switch on the engine, so sparks from arching wires accelerated the fire that engulfed the aircraft.
Also on HuffPost