Accident: 1½ mi. NE of Renton, WA. 04-Aug-2005. Whilst reportedly operated by Sound Flight N741DB collided with a Cessna 150, N66234, while preparing to enter the downwind leg of the airport traffic pattern for runway 33. The collision occurred about one and one-half miles northeast of the airport. Both aircraft were under the control of a developmental air traffic controller who was being supervised by a certified controller. The Cessna pilot contacted the tower controller and was instructed to enter a right downwind via the forty-five. The controller advised, "...traffic ahead and to your right, one thousand six hundred, turning southbound, type unknown." The pilot did not respond. About 15 seconds later the pilot of another DHC-2 Beaver, N715JR, contacted the controller and was advised that the Cessna was passing behind him, was no factor, and to enter right downwind for runway 33. The pilot acknowledged. Approximately 20 seconds later the pilot of the accident DHC-2 Beaver made his initial contact with the controller, who replied, "…ok, you have traffic twelve o'clock, two miles, a Cessna, one thousand three hundred southbound." The pilot advised he was looking for the traffic. About 17 seconds passed when the controller instructed the pilot of the accident DHC-2 Beaver, "…enter downwind via the 45, you'll be following that Cessna ahead and to your right." The pilot responded, "Ok, we'll be landing on the water," and the controller replied, "Roger that, he's landing on the hard surface, you'll be doing a low approach over the water, understand." About 40 seconds passed, when the controller asked the pilot of the accident DHC-2 Beaver if he had the Cessna in sight, to which the pilot replied, "…ah, we've got an aircraft on downwind ahead of us in sight," with the controller acknowledging, "roger, follow that aircraft." The pilot acknowledged, stating later in his statement, "The Beaver ahead of me was given clearance to proceed as requested. I informed the tower that I had the traffic ahead and on downwind in sight and was told to follow him." About five seconds later the pilot of the other DHC-2 Beaver reported entering downwind and would be landing on the water, and approximately 30 seconds later the controller instructed the Cessna to proceed direct to the downwind. Less than five seconds passed before the controller advised the Cessna pilot, "and you have traffic off your right wing, has you in sight, one thousand six hundred, floatplane." The pilot of the Cessna replied, "…traffic in sight." Approximately 15 seconds later the controller confirmed that the accident DHC-2 Beaver would be landing on the water, with the pilot replying, "…affirmative. We have the aircraft on base." About five seconds later the controller asked the pilot of the accident DHC-2 Beaver, "…do you have the traffic that's passing underneath you still?" The pilot responded, "ah yah, base to final landing on the water." The Cessna's right wing glanced off the Beaver's left front float bow bumper before impacting the forward inboard side of the right float as the Beaver's propeller penetrated the upper outboard section of the right wing. A subsequent impact with the Cessna's empennage rendered the airplane uncontrollable, resulting in a steep uncontrolled descent prior to impacting a vacant school building. The pilot of the Beaver transmitted the collision information and subsequently landed on grass alongside the Renton runway, with no injury to the occupants but with substantial damage to the airplane. The Cessna crashed and was destroyed with two fatalities.
The National Transportation Safety Board determined the probable cause(s) of this accident as follows: The failure of the Cessna pilot to maintain visual separation and the failure of the DHC-2 Beaver pilot to understand the air traffic advisory information provided, resulting in a midair collision. A factor contributing to the accident was the inadequate traffic advisory information provided by air traffic controllers. SEA05FA158A.