What happened
On March 19, 2011, at approximately 11:15 Pacific daylight time, a Cessna TU206G, registration N6169Z, was performing a local instructional flight at the Pierce County Airport - Thun Field (PLU) in Puyallup, Washington. The flight was being conducted under 14 CFR Part 91 in visual meteorological conditions with 10 statute mile visibility.
While the aircraft was in the airport traffic pattern, the crew performed pre-landing checklists, which included switching the fuel selector to the right tank and activating the auxiliary fuel boost pump while on the left midfield downwind leg for runway 34. As the aircraft transitioned from the base leg to the final approach at 900 feet mean sea level, the airplane began to sink. Despite the application of full throttle, the engine failed to increase power. The aircraft subsequently struck up-sloping terrain approximately 400 feet short of the runway approach end. The impact caused the nose landing gear to collapse and the aircraft to nose over, resulting in substantial damage to the forward fuselage and empennage. The two occupants, a certified flight instructor and a commercial pilot, sustained 2 minor injuries.
The investigation
Post-accident examination of the engine, a Continental TSIO-520-CCM, and the airframe revealed no mechanical malfunctions or failures that would have prevented normal engine operation. An investigation into the fuel system noted that the aircraft was equipped with a 2-stage electric auxiliary fuel boost pump. According to a representative from the engine manufacturer, operating this auxiliary pump at low power settings can flood the engine, leading to a loss of power. The manufacturer's approved checklist for this aircraft specifies that the auxiliary fuel pump switch should be in the OFF position during both descent and the before-landing phase.