In an Aviation Safety Advisory issued last week by the Transportation Safety Board, the agency sought to bring attention to Transport Canada a significant safety issue affecting Cessna 206 airplanes on floats. With flaps down, exit through the rear passenger doors is impeded.

The TSB cited a fatal accident that occurred on August 16 of last year, when a Simpson Air Cessna U206G (C-FNEQ) crashed into Little Doctor Lake in the Northwest Territories. The aircraft had departed from Fort Simpson airport (CYFS) on a sightseeing flight to Virginia Falls in the territories’ Nahanni National Park Reserve. On board were the pilot and four passengers.

The pilot was attempting to land on the lake when he lost control and the aircraft’s right wing contacted the surface, causing the plane to nose over and rest partially submerged in the water. The pilot and one of his passengers were able to escape and subsequently climb onto the floats, from where they were rescued by boaters 15 minutes later. The three remaining passengers were unable to exit the aircraft and drowned.

The resulting investigation discovered the rear-seat passengers had their seatbelts undone, and the rear double doors closed and locked. The aircraft’s flaps were in the 20° down position, preventing the forward portion of the rear door from opening beyond a few centimetres.

The investigation determined that the impact forces were well within the survivability range and none of the trapped passengers were injured in a way that prevented them from taking evasive action. It was reported that the passengers were wearing their seat belts at the time of impact. The pilot attempted to open the rear doors to rescue the passengers by diving below the surface but was unable as the doors were locked from the inside. The TSB cites the complex procedure for opening the double doors in Cessna 206s, made more complicated when the flaps are in a down position.

Since 1989 there have been five accidents in Canada and the U.S. in which the rear double doors were blocked by flaps in the extended position, resulting in eight fatalities. Currently there are 190 Cessna U206s, 50 TU206s and 11 206Hs being operated in Canada, counting both commercially and privately-registered aircraft.

“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 letter to TC states. “The risks resulting from delayed egress from the aircraft remain high, and more defences are needed to mitigate this hazard. TC may wish to reassess the suitability of the rear double cargo door as an emergency exit, as described in certification requirements for all Cessna 206 series aircraft.”

The letter finishes with a request for TC to inform the TSB of any action taken to address the concerns cited in the letter.

The  TSB’s investigation of this accident is still underway. The full text of the safety advisory letter to TC can be found here.