CEN12FA217 · Oliver Joseph ZODIAC CH 601XL

The pilot had previously flown the experimental, amateur-built airplane cross-country to a potential buyer; the sale was not completed and the airplane did not fly for several months until arrangements could be made to return the airplane. The pilot was ferrying the airplane on the return flight. A witness reported seeing the airplane in cruise flight when its engine “cut out” and its attitude dipped slightly then leveled; the airplane then nosed over and descended straight down, impacting the ground in a residential yard. Another witness reported that the engine “roared loudly” and sped up just before the airplane nosed over. A review of the airplane maintenance records revealed an annotation dated about 4 months before the accident indicating that the pilot had previously performed a precautionary landing due to a rough running engine. The reason for the rough running engine and any repairs made were not found in the airplane records.

A postcrash examination of the airplane’s engine showed heavy sooting and contaminants in the carburetor inlet, the inlet side of the carburetor slide, the outlet side of the butterfly valve, and the lower outlet portion of the venturi, which exhibited a distinct washing of the soot. The inlet side of the venturi had minor sooting. The outlet side of the slide was relatively free of sooting but exhibited a varnish or gum-type build-up. The slide was found in the closed position and did not move freely; however, it would move if a prying force was applied to the bottom of the slide. Sooting patterns on the outlet side of the engine components are usually indicative of a leaking intake valve, which could cause blowback of gases into the intake manifold and carburetor. If severe enough, this blowback could result in engine failure. In addition, the build-up observed on the carburetor slide was much more than normal and could have caused the slide to stick open during the accident flight. If the throttle were reduced while the slide was stuck, air flow would be reduced while fuel flow remained high, resulting in a rich mixture that could stop the engine. The engine would again run at full throttle but not at any reduced throttle setting. Although the carburetor slide was found in the closed position, a problem with engine operation would be expected with a slide that does not move freely. Based on the examination of the carburetor and other engine components, it is likely that the engine experienced a loss of power. The witness statements indicating the airplane’s sudden drop in altitude suggest that the pilot’s attention was focused on the engine problem rather than maintaining airplane control, resulting in the airplane entering a rapid descent and impacting terrain.

Verbatim NTSB analysis and probable cause from the NTSB dataset