Plane & Pilot
Tuesday, July 26, 2011

Should We Rely On GPS?

Is GPS too good to be true?

Ido some post-maintenance test flying for a Cessna 300/400 shop in Long Beach, and a month or so back, I got a call to fly a 421 just coming out of an annual inspection. The day was hazy fog, not totally opaque but on the lower edge of VFR, no ceiling, four miles' visibility and haze to the moon.

The chief mechanic and I lifted off into marginal yuck, and headed out toward Catalina Island, 26 miles across the sea. A pressurization check is standard procedure following an annual inspection, and accordingly, I dialed the pressurization system down to -2,000 feet, and we climbed up to 10,500 feet to check pressurization to max differential.

The coast quickly disappeared in the mist as we headed for the island. That was about the time I noticed the Garmin 530 had apparently gone inop sometime after takeoff. I tried a cold start, and there was still no joy.

Not a problem, I thought. I'll just whip out my trusty backup Garmin 496, position it on top of the panel and proceed.

That plan probably would have worked fine if I hadn't left my 496 in the backseat of my car. Okay, no problem, I knew I had a very tired but perfectly adequate Garmin 196 in my headset bag.

Of course, I did, and its power supply of AA batteries was almost totally dead with no backups at the bottom of the bag. Hmmm. I tried the standard trick of rubbing the AAs' positive terminals vigorously against my jeans to excite some static charge, but it was no joy.

Oh well, have to go back to VHF. I tuned the number-two box to 115.7 mHz for Seal Beach VOR, despite a vague memory of someone on the Long Beach ATIS saying it was out of service. There was no ADF in the airplane, and the ATIS was still advertising four miles. In addition to that, the CDI on number-two VHF box was doing weird things.

What a great preflight I'd done. Well, perhaps I could be forgiven, at least a little, as the big Cessna was destined for the avionics shop as soon as the annual was signed off.

Fortunately, I could see the mountains northeast of Los Angeles from 10,500 feet, and the weather began to lift as I approached the coast. I managed to sneak back into Long Beach, legally but somewhat chagrined.


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