The Transportation Safety Board of Canada (TSB) says the design of an aircraft which crash-landed on a Dehcho lake in August 2018, killing three people, may have made it harder for passengers to escape.
A Simpson Air Cessna 206 floatplane lost control on landing at Little Doctor Lake when its right wing made contact with the lake's surface. The plane came to rest upside-down and partially submerged.
The pilot and one passenger were able to escape through a front door, but three other passengers could not find their way out and drowned.
In a safety advisory published on Friday, the TSB said the position of the aircraft's flaps – devices mounted on the wings to increase lift during flight – obstructed the rear cargo doors and represented a "significant safety issue."
Flaps on an aircraft can be moved into different positions by pilots according to different stages of flight.
A photograph of the recovered plane shows the flaps protruding downward. The TSB says this meant the flaps "blocked the forward portion of the rear cargo door."
While instructions on how to perform an emergency exit were posted above the door, the TSB's advisory expressed serious concern regarding the design's suitability in emergencies.
"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," read the advisory – sent to Transport Canada for its consideration.
"The risks resulting from delayed egress from the aircraft remain high, and more defences are needed to mitigate this hazard," said the TSB.
Around 250 aircraft of a similar design and type are currently operational in Canada.
The broader TSB investigation into what happened at Little Doctor Lake is ongoing, and this advisory represents the raising of a specific concern rather than the publication of a conclusive report.
The advisory suggests the conflict between the flaps and the cargo door was not the only factor as the occupants attempted to escape the aircraft.
"The impact forces [of the accident] were well within the range of human survivability and none of the occupants received immobilizing injuries," the advisory stated in an introductory passage.
"The pilot and surviving passenger exited through the window of the front left-side door. The pilot subsequently dove to assist the remaining passengers, but was unable to open the doors from outside the aircraft because they were locked from the inside.
"The investigation was unable to determine what egress action, if any, was taken by the passengers who were unable to exit the aircraft."
The TSB also stated the doors and door frames "were not damaged or distorted during the accident sequence. All doors and latches functioned normally."
'No suitable changes'
According to the TSB, this is the sixth time in the past three decades – in the United States and Canada – that flaps on a Cessna 206 have blocked the rear double door. In total, those six occurrences resulted in 11 deaths.
The TSB said Cessna had taken some steps to address the issue in previous years, but modifications advised by Cessna were not mandatory for operators and nor did they fully solve the problem.
Transport Canada, when certifying a later version of the Cessna 206, decided the rear cargo doors could not be considered an emergency exit "because [the assembly] was not readily accessible and the opening was not simple and obvious," the TSB stated. That finding did not, however, apply to earlier versions of the aircraft.
A study on emergency exits from submerged floatplanes was commissioned in 2006, but it ultimately indicated "there were no suitable design changes that could feasibly be applied to the entire Cessna 206 fleet," the TSB said. After that, the advisory added, "the file was put on hold due to other priorities and the absence of a clear way forward.
"Transport Canada may wish to reassess the suitability of the rear double cargo door as an emergency exit," the advisory concluded.
Cessna no longer exists, other than as a brand of Textron Aviation, which purchased the corporation in 1995 and halted Cessna's operations as a subsidiary five years ago.
Those who passed away in the August crash were 33-year-old Geoffrey Dean of Castor, AB; Stewart Edelman, 72, of Saskatoon; and Jean Edelman, 72, also of Saskatoon.