Last Friday, a friend drove me over to High River airport so that I could pick up my club's Pawnee towplane, which had been there for maintenance. After a careful pre-flight and an even more careful downwind taxy and backtrack - the wind was 25G30, luckily straight down the runway - I took off for the 20 minute upwind flight to Black Diamond. I was cruising at a leisurely 70 kts and being overtaken by westbound cars below me! There was a spectacular lee-wave Chinook Arch to the west:
After landing and taxying into the lee of the hangar, I did the post-landing checks and pulled the mixture to Idle Cutoff. The engine kept running!
I cycled the mixture control several times with the same result. So my shutdown option was Mags Off. Nothing happened - the engine was still running! I had to pull the Fuel Shut-off valve to stop the engine. The engine continued to run for nearly a minute.
When my friend arrived back from High River in his car, I explained what had happened and he hopped in and started the engine. The mixture control behaviour was still the same, but this time, he was able to stop the engine by turning off the mags.
Inspection under the cowling revealed that the mixture cable had fatigue-fractured where it joined the mixture-control lever on the carburettor. Here is a view of the underside of the engine (O-540) looking forward - the gold-coloured mixture-control lever is in the centre and you can see the fractured end of the wire:
So the reason why the mixture control failed to shut down the engine was obvious, but the intermittent mag switch problem was initially a mystery. However, when I talked to our AME to tell him what had happened, he said that the P-leads were showing their age (it's a 1962 Pawnee) and he was planning to replace them this winter during the off-season.
I learned some interesting lessons from this experience.
1. Shutting off the fuel doesn't stop the engine immediately.
2. When one P-lead has an open circuit, you can't tell which one it is by doing a L-R mag check.
3. I shall continue an old habit of always treating a prop as live, even if the switches are off.
4. Early in my flying career, I was told that carburetor controls were spring-loaded to "fail safe". I now know this is not the case.
I don't know when the cable broke, but if it was in flight, the mixture-control lever could have easily vibrated to a position that would have shut down the engine. I might have had the conversation with our engineer while standing in a field! However he did say that this was the best time of year to have an engine failure - lots of half-mile-long flat stubble fields, as is obvious from my picture.