Thursday, January 17, 2013

Instrument Failures in VMC

I have a feeling the mechanics at our flight club are going to learn to fear my squawks and the other pilots are going to love them, especially the IFR pilots. Since starting training on my instrument rating I am not very tolerant of instrument failures, even when they are not required for VFR flight.

I took my boss from out of town up for a nice, nighttime Bay Tour last night. Not a cloud in the sky, great visibility above the low haze and calm air. The plane I wanted was in its 100 hour annual. So I grabbed another 172 that I like for the flight. The engine on this particular 172 just purrs… its the smoothest running engine in the fleet I'm pretty sure. I think I'll fly it more often. Once its issues are fixed. I digress...

We take off with flight following and get cleared into San Francisco's Class Bravo quickly. As we fly along in the post sunset red glow of twilight I notice the Attitude Indicator (AI) indicating a left turn, when the turn coordinator (TC) indicated straight and level and the heading indicator (HI) didn't indicate any turning going on. This was night VFR conditions and I had good reference to the horizon at all times, so we weren't in danger. However, I have to admit, having one of instrument that I just spent months learning to trust indicating one condition and another instrument indicating another condition was disorienting, even in VFR flight. I knew by looking out the window I was not turning, but I needed to determine if this was a full vacuum failure or a failure of the AI only.

I did a couple turns left and right and the HI and TC both agreed with the direction I was turning, the AI stayed stuck. OK, AI is bad. Don't trust it. As a final check to see if I could actually trust the TC I put wings level on the basis of the TC and watched the compass and track information on the GPS. If I really was wings level, the compass and track would remain consistent (calm winds last night). If I wasn't wings level, I'd be turning. The HI, compass and GPS all agreed on track/heading while the TC showed wings level. So I could probably trust the TC.  Somewhere in the same 10 minutes I noticed the NAV2 CDI would go full deflection every time I used the pilots PTT to respond to ATC directions. I was glad I didn't take this plane into IFR conditions!

I actually enjoyed figuring out which instruments were working and which were not on that flight. Its very good to exercise aeronautical troubleshooting skill in VFR conditions. It reminds me instruments do not always fail the way I expect them to and to always cross check and be vigilant, in VFR or IFR flight.

Once I was done identifying which instrument(s) weren't trustworthy I returned my mind to enjoying the flight and the beautiful nighttime lights in downtown San Francisco, the Golden Gate bridge, Oakland and the rest of the bay. We had a great flight.

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