Accident Overview
On June 10, 2003, at 1245 central daylight time, an amphibious Cessna TU206F aircraft, registration N753, serial number U20603401, departed from Runway 20 at Swan River, Manitoba, en route to Dauphin. The aircraft was operated by the United States Department of the Interior and was being used for waterfowl data collection in cooperation with the Canadian Wildlife Service. On board were a pilot and a crew member, both employees of the Department of the Interior.
Sequence of Events
Immediately after take-off, the aircraft began to vibrate and engine power decreased. At that time, the aircraft was at about 75 feet above ground with approximately 1100 feet of runway remaining. The runway length is 4130 feet, with several ditches and a road crossing the departure path. The pilot assessed that there was insufficient runway remaining to land and that the engine was producing sufficient power to maintain flight. He decided to fly a right-hand circuit away from parked aircraft and terminal area buildings and return to Runway 20. The aircraft was turned to the right and levelled off. Engine power continued to decrease, and the pilot decided to make a forced landing in a plowed field ¼ mile west of the runway. On landing, the aircraft overturned and came to rest in an upright position. After the aircraft came to rest, the engine rpm suddenly increased; the pilot pulled the throttle back and the engine stopped. The pilot and crew member, wearing seatbelts and shoulder harnesses, exited the aircraft with minor injuries. The aircraft was destroyed by fire.
Pilot Information
The pilot held a valid commercial pilot licence and a second class medical certificate issued by the United States Federal Aviation Administration. He was instrument rated on single-engine land and sea airplanes. The pilot had held a commercial pilot licence since 1978, with a total of 9000 flight hours, including 1500 hours on the amphibious Cessna 206. In the previous 30 days, he had flown approximately 45 hours, 4 hours of which were on the accident day.
Weather and Airport Information
Weather at Dauphin, Manitoba, at the time of the accident: scattered clouds at 1600 feet and 3500 feet, visibility 9 statute miles, temperature 24°C, dewpoint 2°C, wind from 220° at 8 knots, altimeter setting 29.78 inches of mercury. Weather in Swan River was similar. The Swan River airport does not have dedicated emergency response services. Witnesses who observed the accident attempted to suppress the fire with extinguishers, but were unsuccessful. Several forest fire retardant spray aircraft waiting for the accident aircraft to take off attempted to extinguish the fire by discharging retardant, also unsuccessful. The fire was eventually extinguished by a fire truck spraying water from a nearby pond.
Flight Details
The pilot and crew member had been conducting low-level waterfowl surveys and were returning to Dauphin. En route, the pilot landed in Swan River and uplifted 120 litres of 100 LL fuel into the left fuel tank. Only the left fuel sump was checked before take-off; no contamination was found. The pilot did not perform a run-up before take-off, but no operating anomalies were noted. During the take-off roll, the engine achieved a maximum of 2700 rpm and a fuel flow of 23.5 gph. The Pilot's Operating Handbook Supplement for the Cessna 206 with a TCM 550 engine indicates maximum take-off rpm of 2700 and maximum fuel flow of 25.8 gph. The Cessna POH provides a landing distance over a 50-foot obstacle of about 1500 feet for a wheel-equipped aircraft.
Investigation Findings
Examination of the site indicated that the float tips had dug into the ground and bent up into the propeller arc. Damage to the top of the vertical fin and scratch marks on the upper wing surfaces indicated the aircraft had overturned on touchdown. The crew member reported seeing a steady stream of fuel from the left wing root after the aircraft came to rest. The aircraft was destroyed by a subsequent fire; therefore, fuel samples, fuel filter/sump condition, and instrumentation could not be obtained. The propeller departed the engine and was found 14 metres ahead of the wreckage, with approximately six centimetres of all three blade tips sheared off. Both nose-gear legs on the amphibious floats were broken off during impact, and all four landing-gear actuators were extended. The fuel selector valve was selected to the left fuel tank. A continuity check of the flight controls revealed no pre-impact anomalies. A cargo net was used to prevent movement of on-board materials.
A fuel sample from the local fuel station was analyzed and found to be 100 LL aviation gasoline with no contaminants. The fuelling station offered 100 LL and Jet B fuel from separate tanks with same-size nozzles. The fuel tank water separators/fuel filters had been replaced two days before the occurrence, and the operator reported no problems.
The engine and propeller were removed for further assessment. The engine was sent to Teledyne Continental Motors for teardown examination. The fuel manifold/distributor diaphragm steel shaft was extremely difficult to remove and coated with rust from contact with water. A rust-like stain was found on the manifold screen. The No. 1 and No. 2 fuel injectors were completely restricted. Initial inspection suggested partial restriction of No. 3 and No. 6 injectors, but subsequent inspection found them clear. Cylinders and pistons showed no indication of wrong fuel. No other pre-impact anomalies were found. The propeller and governor teardown revealed no pre-impact anomalies.
Analysis of the fuel manifold and injectors confirmed the steel shaft was coated with rust. The stain on the manifold screen comprised copper and zinc from corrosion. The No. 1 fuel injector orifice was restricted with copper and zinc. The No. 2 fuel injector restriction was a rubbery sealant, considered to have occurred during disassembly.
The investigation noted that landing straight ahead after power loss would likely have resulted in a runway overrun and probable collision with a road or ditch. The forced landing in the field provided a smoother surface with reduced impact forces. The crew's use of seatbelts, shoulder harnesses, and cargo net probably prevented more serious injuries.
The pilot elected not to retract the landing gear, increasing drag and likely contributing to the aircraft pitching forward on landing. The initial impact bent the right float into the propeller arc, damaging all three blade tips. Propeller separation allowed engine rpm to increase. Twisting and distortion of the airframe during the roll-over likely dislodged or cracked the fuel line at the left wing root, causing a fuel leak. The most likely scenario is that an electrical arc or heat source ignited the leaking fuel.
Water contamination in the fuel resulted in corrosion of the fuel manifold screen. Corrosion particles migrated to the No. 1 fuel injector, restricting fuel flow and causing the No. 1 cylinder to stop firing, contributing to engine power loss. Water in the fuel probably resulted in intermittent fuel delivery and partial fuel starvation. The water was likely in the fuel cells at take-off, but the source of contamination is unknown.