Avionics Report

January 2008 Issue

Altitude Encoders: Transcal, Sandia Prevail

Encoders seem to fail when you need them most. (These days, that’s all the time.) Consider proactive replacement at the first hint of trouble.

Shopping for an altitude encoder is about as exciting as watching paint dry. But in modern avionics, they play a key role in the automation and, the sad truth is, they break and need to be replaced. So when your shop asks which one to buy, you’ll want to have an answer. Usually, unless ATC barks about a faulty Mode C trace, most owners won’t give the pressure-altitude encoding system a second thought. But if yours acts up in the middle of a flight through Class B airspace, you’re not going to be very popular. Also, nav management and autopilot systems depend on these little gadgets. Given the relatively low cost of new encoders, it’s silly to not consider replacing one that’s seen many years of service. In this article, we’ll cover the various options for inputting pressure altitude to your avionics, offer advice on replacing tired encoders and we’ll suggest ways to ensure reliable, accurate Mode C operation.

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