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

combination, don’t waste the effort and cost on the remote switch/annunciator-its already there in the GTX330.

The SAE5-35 has a list price of $410, which, given the competition, is right on the mark. Garmin markets the SAE5-35 under the GAE-43 nomenclature, at a price of $475. Navigators such as the Garmin GNS480 require serial altitude input and the SAE5-35 is a worthy choice, with a lower-cost altitude buffering feature as an option.

Transcal Industries has been making encoders for we’ll over 30 years. Original equipment for Cessna and other OEMs, the popular SSD120 encoder is available in several operating altitude flavors, the most popular is the SSD120-30 counting up to 30,000 feet. The SSD120-30A-232 model has serial outputs.

Transcals rugged original blind digitizer, the D120-P2 series, is an instant-on model and a true blind encoder, with the faceless body of an altimeter. At $1178, its pricey so we cant recommend one, unless you need a model to bolt into an existing instrument cutout, which is exactly how the D120 was designed.

The SSD120-series is known for its reliability and is now available in with dual serial outputs (SSD120-30-RS1 in 10-foot and 1-foot increments.) Most shops can attest that the SSD120 delivers solid altitude encoding with little if any periodic adjustments required. From our experience, newer SSD-120s come up ready to operate in less than a minute, offering reliability for immediate departures on cold days. Transcal advertises a mean time between failures of 9.7 to 21.6 years. In our view, this is accurate advertising.

Shadins 8800 series Falcon encoder was one of the first encoders to output serial altitude and is still available in Shadins line. It has dual outputs for Grey code and serial data. The 9200 series high-resolution serial converter takes Grey code input from any encoder and outputs serial data in 1-foot slices, while the 9000 series is low resolution, outputting serial data in 100-foot increments. These converters are a good solution to a complete encoder replacement when serial data is required. As an all-in-one model, the 8800 encoder is an integral component of the Shadin AMS200 altitude management system and a high-quality solution for other devices that require precise altitude data input in a single box.

The ACK A30 model was popular in the late 1980s and early 1990s and is still found in many airplanes, due in part to its small size and low cost. It has a small footprint, measuring only 6 by 2.6 by 1.3 inches. Later units with Mod 8 output both Grey code and serial data. The A30 comes with a pre-made wiring harness.

The no-frills AmeriKing AK350 ($239) is an inexpensive model thats been around for a while. It delivers mixed results in demanding interfaces, in our view. One owner told us he replaced his after several trips to the avionics shop to have it calibrated. It would jump hundreds of feet out of spec after a few hours of operation. In AmeriKings defense, the unit was installed in a Robinson R22 helicopter, which creates enough vibration to drive most electronics out of spec.

Narco Avionics has offered encoders for years, starting with the AR500 dating back as long as we can remember. The logical upgrade from an old AR500 is the AR850. The AR850 will output only Grey code, so it offers little growth potential. Your shop can tell you if a new AR850 will be an easy swap-out from an ancient AR500, based on its configuration and where its mounted in the airframe.

AR850s are available in two types: A base model with a 15-pin interface connector or one with a 25-pin connector configuration for replacing other models with minimum rework. For a while, AR850s were used by new aircraft manufacturers and found in many New Piper models. The AR850 is a reliable unit, but weve seen a fair number that require regular calibration. If a failure symptom includes erratic altitude reporting, it could be the ribbon cable connections inside the box or a faulty pressure transducer. Time for a new one.

The decision to use one model over another is often made by the installing shop based on their inventory. In our estimation, this is fine since an experienced radio shop should know which model is best for a given application, based on track record of reliability. Still, owners should ask if the encoder being installed is factory new or removed “serviceable” from someone elses aircraft. We don’t see many good reasons for spending money on used altitude encoders given their relatively low cost.

If you are replacing an old encoder after a failure, ask if the replacement encoder is strictly a Grey code model or whether it outputs serial data for supporting a future upgrade. Does the model being installed have a lengthy warm-up time or will it come up in under a minute?

And last, when your aircraft is in for its two-year pitot/static system certification, ask the shop if the existing encoder was accurate through the entire range of operating altitudes. If it needed adjustments because it drifted off altitude, that could be a sign of future hassles. Reliable altitude encoders should never hiccup and theyll be right on the money at all altitudes.