NTSB officials have issued a safety alert warning pilots, aircraft owners, and mechanics about the hazards of sticking exhaust valves in reciprocating piston engines.
Between 2015 and 2025, the NTSB investigated 42 accidents involving stuck valves that caused engine power loss, resulting in five fatalities and 12 serious injuries.
According to NTSB officials, a sticking or stuck valve disrupts the combustion cycle, causing severe vibration, partial loss of engine power, and potentially bent pushrods. The condition stems from deposit accumulation on valve stems and guides caused by localized overheating inside engine cowlings, infrequent flying, sudden engine cooling, incorrect fuel-air mixtures, high ambient temperatures, and high-lead fuel.
Piston-powered helicopters and flight school trainers are particularly vulnerable due to higher operating temperatures, rich mixture settings, and frequent traffic pattern operations, according to the safety alert. Because symptoms often mirror other mechanical issues and are rarely covered in primary training, pilots may misdiagnose the problem in flight.
Recommendations
NTSB officials advise aircraft owners to take preventive measures before takeoff:
- Schedule oil and air filter changes at intervals recommended by original equipment manufacturers.
- Monitor for warning signs, including cold engine roughness, low exhaust gas temperatures, and abnormal cylinder head temperatures in individual cylinders.
- Apply proper leaning techniques as outlined in the pilot operating handbook or engine operator manual.
Maintenance Guidance
Mechanics can prevent valve-related failures through targeted inspections and operational awareness:
- Educate aircraft owners regarding the safety implications of timely oil changes.
- Analyze engine-monitoring device data to troubleshoot rough-running engines.
- Inspect cooling air baffles and baffle seals to prevent cylinder hotspots.
- Adhere strictly to engine manufacturer service bulletins and instructions to measure, diagnose, and clear valve guide deposits.
Recent Investigations
The safety alert highlights three representative accidents demonstrating the dangers of unresolved valve issues:
- On June 29, 2019, a Zenair CH750 STOL sustained substantial damage during a forced landing near Paducah, Kentucky, after its No. 3 exhaust valve stuck open during descent, causing serious injury to the pilot.
- On May 25, 2022, two occupants died when a Cessna 172F crashed during an attempted takeoff at Show Low Regional Airport (KSOW) in Show Low, Arizona, following an initial aborted attempt; investigators found the No. 4 exhaust valve stuck from heavy deposits.
- On May 8, 2024, a Piper PA-32 experienced a partial power loss on departure and made a forced landing after carbon buildup fouled exhaust guides across five cylinders following repeated unresolved power loss events.
For more information: NTSB.gov, AOPA.org, Lycoming.com
It’s not carbon deposits, it is lead, specifically, lead-bromide deposits on the exhaust valve guide.
Lycoming engines are more likely since the exhaust valve is sodium filled allowing it to run cooler, so the lead will condense on the stem and guide.
Continental engines are not immune. My GO-300 stuck a #3 exhaust valve wide open.
I now use TCP, in the fuel , which converts the lead-bromide to lead-phosphate, which is much softer and remains as a gas at much lower temps, so most goes out the exhaust.
The TCP will also scavenge the existing lead from the guides.!!
So, lean aggressively during taxi and at all cruise altitudes.[ below 75% power ]