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Swing your compass

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Recently, I was going a little crazy chasing what I would’ve figured was excessive precession in my 235’s HSI system.

The bird has a PAI-700 vertical card compass, and an old NSD360a slaved HSI. The HSI recently started dropping heading flags, but Porter Strait in Tuksa, OK was able to clean it up for $200 and pronounced it fit to churn on a bit more. Yes, I’m aware that these things are end-of-life (though the office manager at Porter Strait opined that a belt replacement solution for these seems likely in the near future).

When operating the NSD in slaved mode, I’d see nasty variations between the DG and my compass, sometimes after my first turn-out or during taxi. Like, 20 degree variations. This led me to suspect the HSI was on its last legs.

During a recent ADS-B validation flight, I was flying some ATC vectors in the Phoenix class B. I noticed the HSI heading had much better agreement with my GPS track (and it wasn’t a particularly windy day) than did my compass. Hmmmmm.

Last weekend, I went out to the closest compass rose and performed a swing with my idependent mechanic. The old correction card was present, but unreadable. Who knows when it was last swung.

Wow. Just wow. My initial corrections for N and S were nearly 15 degrees. Both eventually lined up to dead-on, with 0 to 1 degree of deviation. My compass doesn’t like east very much, with a -6 to -7 deviation ...but I was later surprised to find out that up to 10 degrees is allowable by FARs.

Short version...if it’s been a long time, a compass swing might be worthwhile. It’s likely often ignored, but can be an eye-opener.
 

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