Grounded, Effectively

It's been going on two months since I last flew. Weather was crappy on some weekends, the Thanksgiving weekend was lost to travel.

So a few weeks ago, I was home on a nice weekend day. It was a little cold; I loaded the pre-heater and its gear into the car along with my flight stuff. I had to pump up the tires and then began my preflight.

Now for a necessary detour into a little discourse on engines:

Other than experimental airplanes, piston-engined airplanes have their spark plugs fired by magnetos, which is a very old technology not found in cars since fuel injection and advanced emission controls came into wide use. Magnetos don't require an external electrical supply; they are little generators powered by the rotation of the engine's crankshaft (that's why you can push-start an old car). Magnetos feed electricity to the distributors and the rotor of the distributor routes power to the spark plugs. Airplanes have two spark plugs per cylinder, as well as two distributors and two magnetos. That is so that there can be no "single-point failure" of the ignition system.

Magnetos are always "on." What they have is a wire that shorts out the generator part of the magneto. When you turn an engine with magnetos "off", what you really are doing is closing a switch that is wired into that grounding wire. That wire is known as the "P-lead" and if it breaks, the magneto cannot be shut off.

You can hand-start an airplane by pulling smartly on the propeller, a method known as "hand-propping."



Now imagine this scenario: Some passer-by is walking along a line of airplanes and comes upon a taildragger. He says: "This is how you used to start these things," and he gives the prop a yank. If a P-lead is broken, the engine can start. It will run until someone either breaks into the cabin and shuts the fuel off or pulls the mixture control all the way off. Even if the mixture control is all the way off, enough fuel might slowly creep into the carburetor to allow the engine to fire a few revolutions.

Which is enough to injure or kill a careless fool who didn't expect the engine to start. I shut off the fuel valve from the tanks to the engine and, when I shut down, I do that and let the engine run to drain the line to the carburetor.

In doing the pre-flight, I turned the fuel selector switch back to a tank so I could take a fuel sample. I came around the landing gear to walk to the right side of the airplane (it's easier for me to take a sample from that side) and I smelled aviation gasoline. I looked down and I saw that gas was dripping from the cowling. I shut the fuel control off, secured the airplane, left a phone message on the answering machine of my mechanic and went to lunch.

It might have been something that I could have discovered by looking at the carburetor or maybe lightly tapping on it in case the float had stuck. It was 25degF out with a slight wind, pre-flighting it was miserable enough and the mechanic works in a heated hangar. Easy choice.

It took him about two weeks to get to it, as his priority is to fix airplanes that people use to make money, which I fully understand. He called to say that it wasn't leaking when he looked at it, it started OK, but did I know that there was a pretty good oil leak from the back of the engine?

Um, no. The oil level was were it should be when I checked it.

So the airplane is in his hangar and I'm on the ground.

(Before you comment, read the first word of advice just below the title of this blog.)