The student pilot and flight instructor were conducting a personal flight in the Piper J-3 Cub.
The CFI reported that the pretakeoff checks were “normal” but that, during the takeoff, the engine felt “weak” and that it lost power during the initial climb.
The flight instructor conducted a forced landing to a field near Brenham, Texas, during which the plane hit the top of a tree and then the ground.
Examination of the airplane revealed that the exhaust valve for the No. 4 cylinder was stuck in the “open” position, possibly due to excessive deposits from the combustion process. It is likely that the stuck exhaust valve resulted in the partial loss of engine power reported by the flight instructor.
Probable cause: The reciprocating engine’s exhaust valve being stuck in the “open” position, which resulted in a partial loss of engine power during initial climb.
NTSB Identification: CEN17LA093
This January 2017 accident report is provided by the National Transportation Safety Board. Published as an educational tool, it is intended to help pilots learn from the misfortunes of others.
Running 100LL in these engines is just asking for stuck valves.
BTDT.
It’s Non-Alcohol Mogas for me.
I wonder why the flight instructor didn’t abort the take off since he said that the engine felt weak during take off ???????????????
The loss of power caused the need to land. The accident was caused by the landing.
There is no accident unless there is damage or injury.
Greg, I find that the majority of the time the engine that is shown in the aircraft’s registration info is the model it came with even if it has had a different engine installed. I know our Champ has had an 85 Continental for at least 10 years, done with a Field Approval, but the registration info on the aircraft still shows a Continental 65. During WWll, the Cubs, L-5’s, etc. all used the same gas out of drums that the tanks & jeeps, etc. used, with no ill effects. However, when people wanted to start using unleaded auto fuel, the FAA made a big deal out of getting approval and requiring an STC to burn gasoline that worked well for years during the war in the old 72 and 80 octane Continentals and Lycomings.
Good chance it was due to exhaust deposits adhering to the valve stem, which is more common on Lyc’s. This could have been more easily diagnosed with an engine data monitor during start. Lose a cylinder on a six, not a biggy. Lose one on a four, and this is the result. Having an EDM is actually more important on a four.
A preventive measure would be to lean aggressively on the ground after start and/or borescope the engine during the annual or 100 hour.
The “why” here might be useful information for those with similar powerplants. Did a spring break, push rod break, be useful to know.
The registration, N7039H, shows it is still powered with an A65 Continental, therefore it is likely that the valve stem was fouled with lead deposits.
The A65 was originally certified with 73 Army avgas which was a no lead fuel. Depending on the carburetor and airframe installation others are certified with 80/87 avgas,(grade 80), that fuel was also an extremely low lead fuel, most of it was actually no lead and ASTM D910 had a maximum of .5 grams per gallon as opposed to the 2.0 grams per gal. allowable in 100LL.