Accident Briefs: September 2019

The owner of the airplane experienced a partial loss of engine power during takeoff following a touchand-go landing, but was able to complete a 180° turn and land safely back on the runway. During a post-landing engine run-up, the owner was unable to duplicate the problem, so he taxied to his hangar and reported the problem to his mechanic. The mechanic, a commercial pilot, subsequently boarded the airplane, performed an engine run-up, which seemed normal, and elected to fly the airplane around the airport traffic pattern. A witness, who heard and saw the airplane on final approach, stated that the engine was making "small explosions" or "backfire"-like sounds. The airplane subsequently collided with a tree before it impacted the ground and a fence short of the 4000-foot-long runway and was consumed by a postcrash fire. 

Examination of the airplane revealed that the No. 4 cylinder exhaust valve was stuck in the "open" position due to excessive deposits from the combustion process. It is likely that the stuck exhaust valve resulted in the partial loss of engine power. Maintenance records revealed that the engine had not been inspected in accordance with a manufacturer service bulletin regarding stuck valves. Had the service bulletin been complied with, it is possible that the accident may have been prevented. Despite the partial loss of engine power that occurred during the previous flight, the pilot flew a traffic pattern that resulted in the airplane descending into trees about 1/4 mile before the runway threshold after the airplane experienced a partial loss of engine power during the accident flight. 

The pilot was diabetic, and although his blood glucose was likely not very elevated at the time he died, it was somewhat elevated on average over the preceding few weeks. Elevated blood glucose can cause blurred vision and subjective sensation of fatigue, as well as increased thirst and urination. Unless life-threatening, it does not directly impair decision-making or judgment; thus it is unlikely that the pilot's diabetes contributed to the circumstances of this accident.

Probable cause(s): A partial loss of engine power due to a stuck exhaust valve. Contributing to the accident was the pilot's decision to operate an airplane with a known mechanical deficiency and his failure to fly an appropriate traffic pattern that would have allowed the airplane to reach the runway.

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Skyforest, California/Injuries: 1 Serious, 1 Minor, 2 None 

The private pilot reported that, a few minutes after departure for a cross-country flight and as the airplane neared the top of a ridgeline, it encountered a downdraft, aerodynamically stalled, and then impacted terrain. One of the passengers reported hearing an aural tone, which was consistent with the stall warning horn, for several seconds before impact. 

Weight and balance calculations determined that the airplane was loaded near its maximum gross weight and had exceeded the forward center of gravity limit, which would have increased the airplane's stall speed during the accident flight. The calculated density altitude was about 6,550 ft, which likely reduced the available power and affected the climb rate. 

A video of the final moments of the accident flight showed the airplane about 25 ft above ground level when the airplane entered a high pitch attitude, followed immediately by a rapid descent, consistent with an aerodynamic stall. According to the pilot, the airplane was about 1,000 ft above terrain when it stalled; however, given the location of the camera and the pitch attitude that was observed, it is likely that the airplane was within about 50 ft of terrain as the airplane crested the ridgeline. The pilot should not have attempted to cross the ridgeline at such a low altitude; a higher altitude would have provided a clearance zone to avoid turbulence and downdrafts. The pilot's failure to cross at a higher altitude resulted in the airplane's encounter with a downdraft with insufficient altitude to recover from the stall. 

The pilot reported that there was no evidence of any preaccident mechanical malfunctions or failures with the airplane that would have precluded normal operation.

Probable cause(s): The pilot's failure to establish the proper airspeed after departure and to maintain adequate clearance from a ridgeline in high-density and downdraft conditions and his subsequent exceedance of the airplane's critical angle of attack, which resulted in an aerodynamic stall with insufficient altitude to recover. Contributing to the accident was the pilot's inadequate preflight weight and balance calculations, which failed to take into account the gross weight, high-altitude conditions, and center of gravity limit. 

NOTE: The reports republished here are from the NTSB and are printed verbatim and in their complete form.