Battleground, WA, USA
N873BC
Beech 33
The pilot selected the most favorable runway and flew his approach to land at his home airport. The airplane touched down normally near the runway threshold at the pilot's normal landing speed. Based on landing distance calculations, he should have been able to stop the airplane before the end of the runway. However, it was not until the airplane was about midfield that the pilot realized the airplane was not decelerating. At this point, the pilot attempted to reduce power further by twisting the knob of the throttle control counterclockwise, but the engine power did not decrease. As the airplane rapidly approached the end of the runway, the pilot used the mixture control to shut down the engine, but the airplane overran the runway end and the left wing impacted a pole, which resulted in substantial damage. Postaccident examination of the airplane revealed no anomalies with the engine and most of the engine controls except for the throttle control. During engine runs, the throttle knob push button became intermittently stuck in a depressed position. Normally, turning the throttle knob counterclockwise from this position would reduce power; however, during the postaccident engine runs, a corresponding minor increase in power was observed. Although disassembly of the throttle mechanism revealed a loose nut in the throttle control assembly, it likely did not contribute to the throttle failure, as the nut would not have affected the knob’s use to make power adjustments. Further, the throttle control could still be manipulated by rotating the throttle knob during the postaccident engine run. Based on the pilot's practices, it is likely that the pushbutton became partially depressed when the pilot reduced the engine power to descend while inbound to his home airport. When the pilot realized the airplane had not decelerated and attempted to make a minor power adjustment by rotating the throttle knob counterclockwise, the throttle control failed, and the pilot either experienced sustained or a subtle increase in engine power, which resulted in a runway overrun and impact with an obstacle. The reason for the failure of the throttle control assembly could not be determined based on available evidence.
On July 20, 2019, about 1620 Pacific daylight time, a Beechcraft F33A, N873BC, was substantially damaged when it was involved in an accident near Parkside Airpark (WA87), Battleground, Washington. The pilot and 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 his home airport after having lunch at another airport. The pilot entered a descent from a cruise altitude of 5,500 ft mean sea level (msl) approximately 16 nautical miles from his destination airport. He leveled off at 1,000 ft and observed the windsock as he overflew the airport, which indicated a light easterly wind. The pilot then entered the right downwind leg of the airport traffic pattern to land on runway 07. He touched down close to the runway threshold at 70 kts with the throttle near idle power. Approximately 700 ft into the landing roll, the pilot noticed that his airspeed had not decreased as it normally had during prior landings. Additionally, the engine rpm was higher than normal. The pilot attempted to reduce the engine power by twisting the knob of the throttle control counterclockwise, but he did not observe a decrease in engine power. As the airplane approached the end of the runway, the pilot used the mixture control to shut down the engine. However, the airplane overran the end of the runway, traversed an adjacent road, and the left wing impacted a metal pole before the airplane came to rest. During the accident sequence, the airplane's left wing was substantially damaged. The pilot stated that he normally uses the pushbutton mechanism on the throttle to reduce power after takeoff and for greater reductions in power during flight. He uses the twist function of the throttle knob to make minor adjustments in power. A review of the aircraft logbooks from May 5, 1998, to present showed multiple entries that indicated the "entire aircraft" had been "lubricated." Although some of the entries stated that the controls were "checked," none of them indicated that the throttle control had been disassembled and inspected; however, this was not required as the throttle control was a sealed assembly. The airplane was equipped with a throttle control assembly that allowed the pilot to make fine adjustments to the power setting. The throttle assembly was comprised of a compression spring, a knob, and the throttle cable, which was attached to a button at the end of the knob with an AN924-3D nut and an AN936-A616 lock tooth washer. To make a minor power adjustment, the pilot rotated the knob clockwise to increase power and counterclockwise to decrease power. For large power adjustments, the pilot depressed the button and either pushed the knob forward to advance the throttle or pulled the knob aft to retard the throttle. An inspection and engine run were performed at the owner's home by the National Transportation Safety Board investigator-in-charge, a representative of the Federal Aviation Administration, and the engine manufacturer. Throttle and mixture continuity were confirmed from the cockpit to the throttle and mixture controls. The engine was run for about 3 minutes at 800 rpm to warm up before an engine run-up was performed. The engine was then advanced to its maximum rated power momentarily before it was shut down. During these tests, the engine sounded smooth and continuous and did not display any interruptions in power. A second engine run was accomplished to examine the security of the throttle control mechanism. During this test, the throttle was reduced from 1,800 rpm to 800 rpm using the pushbutton mechanism, which became stuck in a halfway depressed position. With the button stuck in this position, an engine power reduction was attempted by rotating the knob counterclockwise. This movement would normally reduce engine power; however, only a corresponding minor increase in power was observed. This test was repeated multiple times with the same result. Figure 1: Throttle Control in Both Stuck and Normal Positions Disassembly of the throttle mechanism revealed that the spring and washer were present and had been installed correctly. The spring did not display any debris or anomalous wear that could have inhibited its movement. The throttle cable was secured with the nut and washer; however, the nut was loose and could be easily manipulated by hand. According to the manufacturer, a loose nut would prevent the knob from being held against the throttle shaft. This could allow the knob to spin freely when rotated by the pilot, thus preventing the throttle cable from moving in and out. If the nut was completely loose, the knob could separate from the throttle shaft. However, the loose nut would generally not impact the functionality of the button if the knob was not near the point of separation. WA87 was a private airport with one runway in an east/west configuration. The runway dimensions were 2,100 ft long and 25 ft wide. Performance computations of the airplane's landing distance were completed using a temperature of 29° C, a total weight of 3,100 lbs, and a headwind of 10 kts. These figures were plotted using the landing distance chart from the pilot's operating handbook, which showed the airplane's ground roll would have been about 710 ft.
A failure of the throttle control assembly for reasons that could not be determined based on available evidence, which resulted in a runway overrun and impact with an obstacle.
Source: NTSB Aviation Accident Database
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