Accident Sequence
On July 20, 2019, about 1620 Pacific daylight time, a Beechcraft F33A, N873BC, was substantially damaged when it overran the runway at Parkside Airpark (WA87) in Battleground, Washington. The pilot and a pilot-rated passenger were not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight.
According to the pilot, they were returning to their home airport after lunch at another airport. The pilot entered a descent from 5,500 ft mean sea level (msl) approximately 16 nautical miles from the destination. After leveling off at 1,000 ft, the pilot observed a windsock indicating a light easterly wind and entered the right downwind leg for runway 07. Touching down near the threshold at 70 knots with throttle near idle, the pilot noticed that airspeed did not decrease as expected during the landing roll, and engine rpm remained higher than normal.
The pilot attempted to reduce power by twisting the throttle knob counterclockwise but saw no decrease. As the airplane neared the runway end, the pilot used the mixture control to shut down the engine. However, the airplane overran the runway, crossed an adjacent road, and the left wing struck a metal pole before coming to rest, causing substantial damage to the left wing.
Throttle Control System
The airplane was equipped with a throttle control assembly that allowed fine adjustments via knob rotation (clockwise to increase, counterclockwise to decrease) and large adjustments via a pushbutton mechanism. The pilot typically used the pushbutton for takeoff and significant power reductions, and the twist function for minor adjustments.
Post-Accident Examination
An inspection and engine run were performed by the National Transportation Safety Board investigator-in-charge, a Federal Aviation Administration representative, and the engine manufacturer. Throttle and mixture continuity from cockpit to controls was confirmed. During an initial engine run, the engine operated smoothly without power interruptions.
A second run examined the throttle control mechanism. When the throttle was reduced from 1,800 rpm to 800 rpm using the pushbutton, the button became stuck in a halfway depressed position. With the button stuck, rotating the knob counterclockwise (normally reducing power) resulted in a minor increase in power. This test was repeated with the same result.
Disassembly of the throttle mechanism revealed that the spring and washer were present and correctly installed, with no debris or anomalous wear. The throttle cable was secured with a nut and washer, but the nut was loose and could be easily manipulated by hand. According to the manufacturer, a loose nut prevents the knob from being held against the throttle shaft, allowing the knob to spin freely when rotated, thus preventing throttle cable movement. If completely loose, the knob could separate from the shaft, but a loose nut would generally not affect button functionality if the knob was not near separation.
Airport and Performance
WA87 is a private airport with a single runway (east/west orientation) measuring 2,100 ft long and 25 ft wide. Performance computations using a temperature of 29°C, total weight of 3,100 lbs, and 10-knot headwind indicated a landing ground roll of about 710 ft.
Maintenance Records
A review of aircraft logbooks from May 5, 1998, to present showed multiple entries indicating lubrication of the "entire aircraft." Some entries noted that controls were "checked," but none indicated disassembly and inspection of the throttle control, which was a sealed assembly and not required to be disassembled.