N8656K

On August 2, 2021, about 1458 Alaska daylight time, a Stinson 108-1 airplane, N8656K, was substantially damaged when it was involved in an accident near Sterling, Alaska. The private pilot and three passengers were uninjured. The flight was operated as a Title 14 Code of Federal Regulations Part 91 personal flight.

The pilot reported that he was taking some friends for a local flight. The useful load of the airplane was 766 lbs, and he estimated the takeoff weight was less than 690 lbs, which included the passengers and about 120 lbs of fuel. He stated the takeoff roll took longer than normal; however, the airplane seemed to climb normally with one notch of flaps until about 100 ft above ground level. The pilot said that at that point, the engine did not feel like it was producing full power; the airplane would no longer climb; and it began a descent toward trees located off the departure end of the runway. The airplane subsequently impacted the trees and came to rest at an estimated 45° nose-down angle. The airplane sustained substantial damage to both wings and the fuselage.

A video of the takeoff taken by witnesses near the runway showed the airplane from the start of the takeoff roll until just before impact. The main landing gear were airborne just before the tailwheel, and the airplane entered a nose-high attitude and ascended to about tree top level. About 6 seconds after takeoff, the airplane began to descend and continued to descend in a nose-high attitude until the recording stopped. The sound of the engine was consistent throughout the length of the video with no audible anomalies noted. Additionally, the windsock visible in the video indicated a slight right-quartering tailwind at the time of takeoff.

Figure 1. Airplane during takeoff

During a postaccident engine examination of the Franklin 150-B4 engine, all bottom spark plug ignition wire caps were found to be finger tight. The top spark plug from the No. 6 cylinder was finger tight and exhibited signatures consistent with exposure to high heat on the outside of the plug. The crankshaft was manually rotated, and the cylinders were checked for suction and compression. The Nos. 2 and 3 cylinders exhibited weak compression, and it was difficult to detect suction on these cylinders.

When the oil sump was removed, a small metal gear, a portion of a cotter pin and pieces of metallic debris were found in the sump. The camshaft displayed scoring consistent with foreign debris inside the engine. When the fuel strainer was removed, organic material/debris was located within the screen and in the bottom of the bowl.

The small metal gear was later determined to be from the original mechanical tachometer, and it fell into the oil sump when the original tachometer was replaced with a digital tachometer.