So I don't know why I did this exactly, maybe the stress of work just made me so avoidant that I leapt at anything else to do, but I got a call today asking if I'd be willing to take up the plane that just got a new engine for a shakedown flight.
The purpose of such a flight is to run the engine at a fairly high power setting for an hour or so so that the brand new piston rings can seat, and so that the mineral oil now filling the engine gets a chance to wear the metal parts smooth. Of course it's also the flight wherein the ability of the new engine to run reliably without blowing a gasket or popping an oil hose is demonstrated, so it's not just the most 100% safe flight. But I did it, and flew 35 minutes in one direction, turned around, and flew 25 minutes back home (there was a tailwind on the way home). I was pretty hypervigilant, though... new engines make little burps now and then as oil leaking into the cylinders past the unseated rings gets into the spark plugs, and every time that happened, I got a little nervous. She did great, though. No vibration, really good performance (never gotten more than 130 knots airspeed out of this plane before, and she was showing 132 most of the way), engine gauges smack in the middle of the green (of course the mechanic had just SET them this way so no surprise there). Eventually I relaxed and just enjoyed the flight.
That was supposed to be it, but when I reported the successful flight to the mechanic, he said all that needed to be done was to calibrate the standby vacuum system, and he'd be ready to sign the plane off as flyable. He had to rig things so that the main vacuum pump could still suck air but not power the instruments, which involved plugging a hose with a spark plug (which conveniently fit exactly), mounting an air filter under the cowling, and tying all of that to the engine mounts with zip ties. Then he sent me up again, with instructions to open the standby vacuum port at a safe altitude, and then proceed to calibrate it at 2000, 4000, 6000, 8000, and 10000 feet.
Now, the standby vacuum system runs off of the intake of one of the cylinders, so it really only generates enough suck to power the gyros at low power settings. That means that while the standby system was off or when I had a high power setting to climb, the main flight instruments weren't working well. In fact, the directional gyro just lolled around meaninglessly, and the artificial horizon tipped over on its side.
It was weird... It was a beautiful clear day today, so I hardly needed these instruments to help me keep the wings level and the plane pointed in the right place, but there was something very ominous about flying with that bright red "VAC FAIL" light, and every time my eyes glanced at the artificial horizon, I had to fight a rising panic of "My God We're In A Graveyard Spiral!" Plus I kept thinking about the rigged up stuff under the cowling, a vacuum hose plugged up with a spark plug for God's sake, and all those plastic zip ties. The result was that I was a regular nervous wreck during the second flight, and I felt rather silly about it.
I guess my confusion here comes from being much MORE apprehensive about, really, the much safer flight.