The passengers — 33-year-old Geoffrey Dean of Castor, Alta., and Stewart and Jean Edelman of Saskatoon, who were both 72 — died in August after the Cessna 206 crashed during an attempted landing on Little Doctor Lake.
The pilot and a surviving female passenger were able to get out through the window of the front left-side door.
The TSB said the pilot dove back into the water to help the remaining passengers, but was unable to open the doors from outside the aircraft because they were locked from the inside.
The TSB said the passengers were found with their seatbelts undone, although it couldn’t say what efforts they had made to get out of the sinking aircraft.
The report said the impact forces were “well within the range of human survivability” and none of the occupants received immobilizing injuries.
The pilot and woman were rescued by a nearby boater within 15 to 20 minutes.
Little Doctor Lake, about 100 kilometres west of Fort Simpson, is accessible only by float plane in the summer and draws visitors from around the world looking for a wilderness experience.
TSB director of air investigations Natacha Van Themsche sent a letter to Transport Canada, saying she wanted to bring attention to a “significant safety issue” involving Cessna 206 series aircraft that are fitted with double cargo doors.
The TSB found one of the plane’s two adjoining exit doors was blocked by the aircraft’s extended wing flaps, which made opening the rear door more complicated.
The board says regulators in both Canada and the U.S. have known about the problem with the Cessna 206 since 1998, but no solution has been found and the plane continues to be licensed for up to five passengers.
The TSB says eight people have died since 1989 in accidents in which the wing flaps blocked the door.
“As shown in this occurrence, without functional exits, the time required to exit the aircraft may increase, which in turn increases the risk of death in time-critical situations, such as when the aircraft is submerged or there is a post-impact fire,” the TSB letter to Transport Canada said.
“The risks resulting from delayed egress from the aircraft remain high, and more defences are needed to mitigate this hazard.”
As of mid-February 2019, the TSB said there are currently 190 Cessna U206 aircraft, 50 Cessna TU206 aircraft, and 11 Cessna 206H aircraft being operated in Canada, in both private and commercial service.
The full report into the 2018 crash has yet to be released.
— With files from The Canadian Press