ERA25LA012 · Piper PA28 160

During a familiarization and check-out flight, and after completing several maneuvers about 25 miles from the departure airport, the pilots elected to return to the departure airport. After turning on an approximate 8-mile final approach, the pilot had the engine set to a low power setting, the carburetor heat on, and switched the fuel tanks. After the airplane had descended to about 1,000 ft mean sea level (msl), the pilot reported that the engine’s throttle was unresponsive. The instructor took control of the airplane and attempted to perform a forced landing in a grassy area beside a road. During the approach for landing, the airplane impacted a light pole and an automobile. Postaccident examination of the wreckage revealed that the both wings, the fuselage, and empennage had sustained substantial damage as a result of the accident. An initial examination of the wreckage found that the fuel selector valve was in between the two “OFF” detents; however, the airplane had been recovered before the wreckage could be examined and it could not be determined whether the as-found position was indicative of its position at the time of the accident. Testing of the valve in this position revealed that flow from the valve was completely obstructed. Further testing revealed that the valve’s detents were positive, and that it functioned normally when the left and right fuel tanks were selected. Further examination of the airframe’s fuel system revealed that the main vent system for the left fuel tank was obstructed; however, the vented cap for the left fuel tank was unobstructed, which would have allowed proper venting of the left fuel system. The carburetor underwent additional testing on a production test bench and was disassembled and examined. The carburetor performed normally with a slightly leaner fuel flow when tested at two of the test points. Further examination revealed that the throttle valve shaft and bushings displayed wear, allowing side motion of the throttle shaft, which could have allowed a small amount of bypass air to enter the throttle body, and lean the fuel/air mixture. It is likely that the slightly lean postaccident tests were a result of the worn throttle valve shaft and bushing; however, this small amount of leaning would not have prevented normal operation of the engine. The weather conditions at the time of the accident were conducive to serious carburetor icing at low power settings. According to the pilot, the carburetor heat was used during the final approach. The position of the carburetor heat control and valve could not be confirmed during the postaccident examinations due to the impact damage to both the control and the carburetor airbox; therefore, the functionality of the carburetor heat could not be confirmed. Overall, the wreckage examination was unable to identify evidence of any preimpact mechanical malfunctions or failures of the airframe or engine would have precluded normal operation. The impact-related damage to the airplane and engine otherwise prevented a test run that could have checked the various systems as a whole.

Verbatim NTSB analysis and probable cause from the NTSB dataset