This is an excerpt from a report made to the Aviation Safety Reporting System. The narrative is written by the pilot, rather than FAA or NTSB officials. To maintain anonymity, many details, such as aircraft model or airport, are often scrubbed from the reports.
This event occurred in a Cessna 172 during a lesson with a student.
After rolling wings level from a clearing turn, engine RPM dropped to 1,000. After five seconds the RPM increased to 2,000. The RPM kept cycling in this manner with no change from throttle, mixture or pitch.
I believed a complete engine failure was possible, so I took command of the aircraft from the student, and started flying back to ZZZ since it was the closest airfield.
I pulled the carb heat on, verified the RPM was full and mixture rich, primer locked, master on, fuel on both.
I contacted Approach and requested priority handling to ensure I would receive priority handling and services if an off-field landing would be required. I was able to make it back to the field and performed a straight-in landing for Runway XX.
I contacted ZZZ after safely landing and ensured I no longer required assistance.
The cause of the partial engine failure was a bent push rod.
No pre-flight action nor run up would have caught this condition. It was a good reaction to make an immediate return to the field.
Primary Problem: Ambiguous
ACN: 2011294
GA water detection systems were available via STC in past years. I removed one from a Piper Commanche a few years ago. The fuel sump bowl had been drilled and a probe installed that would activate a warning light in the cockpit. I don’t remember who manufactured it.
Pushrods bend for various reasons- intermittent sticking/bent valve, improper valve train assembly after maintenance, improper pushrod length due to no/improper bled down rocker clearance measurements after cylinder change(s), engine overspeed. Take your pick….it was error on the part of someone.
Run to lean doesn’t pay in the long run
That has nothing to do with lead salts depositing on the exhaust valve stem from burning 100LL.
Had this occur at the flight school I’m at on a 172N w. 180 hp upgrade. Think there may have another one in the last year too.
Wonder if this engine was an O-320-E2D? It’s been almost 50 years since 80/87 gas was phased out, but when that hsppened, it wasn’t long before we experienced bent pushrods in our 172 fleet.
320s bad for bent push rods. Changed many in brantly helicopters. Solved problem mixing marvel mystery oil in fuel.
Cessna aircraft have a flaw in their fuel tanks that do not offer the positive detection of water in the fuel tanks during the pilots preflight.
https://www.wsbtv.com/news/local/pilots-say-being-blamed-plane-crashes-that-arent-their-fault/VGNXKERZANHBDI4JO2H324VVEQ/
I don’t know of any GA aircraft that has a water detection system.
Older Cessnas have 1 quick drain valve in each tank, while later 172R models have 5 valves in each tank, supposedly to allow water to be removed from ‘trap’ areas.
While a bent pushrod is usually caused by a stuck valve. A bend pushrod can also damage the push rod tube, allowing oil to be dumped overboard.
Lycoming has issued a service bulletin on checking for sticking exhaust valves at 400 hour intervals. SB-388C.