NTSB Warns of Sticking Exhaust Valves in Piston Aircraft Engines

The National Transportation Safety Board (NTSB) is urging aircraft owners, pilots, and maintenance personnel to pay closer attention to exhaust valve condition after investigating dozens of accidents involving stuck valves and engine power losses.

In a new safety alert issued Oct. 29, the NTSB said it investigated 42 accidents between 2015 and 2025 in which a stuck valve contributed to a loss of engine power and an accident. Those events resulted in five fatalities and 12 serious injuries.

Sticking valves can disrupt the combustion cycle in reciprocating piston engines, producing severe vibration and partial power loss. Depending on where a valve becomes stuck in its guide, the condition can also bend pushrods. Deposits accumulating on the valve stem or inside the valve guide are the primary cause.

The NTSB identified several conditions that can contribute to deposit buildup, including localized engine overheating, infrequent flying and rapid engine cooling. Incorrect fuel-air mixture settings, high ambient temperatures, low-airspeed operations with reduced cooling and fuel containing high lead concentrations can also contribute.

Piston-powered helicopters and flight school aircraft may be particularly susceptible. The NTSB noted that helicopters often operate at higher engine temperatures, while training aircraft frequently spend extended periods in the traffic pattern and may operate with rich mixture settings.

The alert noted that recognizing the early symptoms can be difficult. A sticking valve may produce a rough-running engine, particularly during cold starts, along with unusually low exhaust gas temperatures (EGTs) or cylinder head temperatures (CHTs) on an individual cylinder. The symptoms can be mistaken for other engine problems, and an unexpected power loss during takeoff or landing can leave little time to diagnose the issue.

The NTSB recommends that owners follow manufacturer-recommended oil and air filter change intervals, use proper leaning procedures, and become familiar with engine-monitoring indications that could reveal valve problems.

Mechanics should also inspect engine cooling baffles and baffle strips for deterioration, use engine-monitoring data when troubleshooting rough-running engines and follow manufacturer service instructions for diagnosing and correcting stuck valves.

The complete safety alert, SA-109, is available from the NTSB website.