Engineering & Research 415-D N3940H accident near Beaver Falls, Pennsylvania

Probable cause

The owner/pilot’s failure to follow engine manufacturer recommendations for engine oil changes and spark plug cleaning and rotating when operating the airplane with 100 low lead fuel, which resulted in carbon buildup in the engine and the total loss of engine power due to the No. 3 cylinder exhaust valve being stuck open.

NTSB analysis

According to the pilot, he departed for a flight to his home base airport, and, while the airplane was climbing through 200 ft above ground level, the engine began to run roughly. While he was attempting to turn back toward the airport, the engine experienced a total loss of power, and he subsequently made an emergency landing in a nearby field. No anomalies were noted with the carburetor or the induction, ignition, and fuel systems; 100 low lead fuel was found in the header tank.

A postaccident examination of the engine revealed that the No. 3 cylinder exhaust valve was stuck in the "open" position and that the Nos. 2 and 4 cylinder spark plugs were carbon fouled. Although an engine manufacturer service bulletin (SB) stated that 100 low lead fuel was an acceptable alternative fuel, it also indicated that "the amount of tetraethyl lead in these higher grade fuels has increased the lead build up and fouling of spark plugs." The SB noted that, when this type of fuel is used, "exhaust valve sticking could result from lead salt accumulation in the lubricating oil," and it recommended that "regular 50 hour oil changes be implemented to reduce such accumulation." The SB also recommended that the spark plugs be rotated every 50 hours of operation and cleaned and rotated every 100 hours. A review of the engine maintenance logbooks revealed that the airplane's most recent annual inspection was completed about 60 hours before the accident. No record was found indicating that the oil was changed after the annual inspection. Based on the condition of the exhaust valve and the spark plugs and the maintenance records, it is likely that the pilot did not follow the SB-recommended procedures, which resulted in the engine carbon buildup and the exhaust valve being stuck open.

Text from the NTSB. Open the NTSB docket for ERA13LA406 for the full record.