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Usually, unless ATC barks about a faulty Mode C trace, most owners wont give the pressure-altitude encoding system a second thought. But if yours acts up in the middle of a flight through Class B airspace, youre 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, its silly to not consider replacing one thats 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.

So-called “blind” altitude encoders were named because earlier models were instruments that looked just like altimeters, but without a face-making them blind. But just like altimeters and other static instruments, blind encoders plumb into the aircraft static system and reference standard pressure of 29.92 inches/Hg.

Encoders feed the transponder altitude information through a series of electronic bit lines, allowing it to send to ATC altitude data in 100-foot increments-whats called Gillham Grey Code.

Serial encoders output Grey code plus RS232 data in more accurate 10-foot increments. Some models are tighter yet, yielding 1-foot accuracy. This RS232 output is called digital serial altitude data. Such serial encoders carry a price premium over basic Grey code models.

Altitude encoders perform multiple tasks. Aside from interfacing with the transponder, they input pressure altitude to IFR GPS systems, traffic alerting systems, moving map displays and TAWS terrain systems and they provide altitude data to autopilot altitude preselect and altitude alerting systems.

Many vintage blind encoders have heater circuits to bring them up to operating temperature, so if you rush the taxi, you might be airborne without Mode C for a few minutes.

Many of the newer encoders solve this by being ready to roll in a minute or so, while others are instant on. To us, this is reason alone to consider proactive replacement. The airspace is too tense these days to be sweating out Mode C warmups.

The Sandia SAE5-35 is a modern encoder which outputs Grey code and dual RS232 serial altitude streams, operating from -1000 feet to a ceiling of 35,000 feet. It has an interesting function called AIM, for altitude inflight monitoring. When connected to an optional panel-mounted pushbutton/annunciator switch, the unit can alert the pilot of altitude excursions. There isn’t much in the way of additional hardware to add this feature, other than the annunciator/switch and extra wiring.

Similar altitude buffering can be achieved with Garmins GTX330 transponder, with the added convenience of an automated voice that chides “leaving altitude” if you wander off target altitude. So if your shop is installing a Sandia/GTX330