One of the many reasons to keep up with FAR 91.411 and 91.413 inspections is to check on the health of the altitude encoder/digitizer. An unappreciated remote box, it may be the main source of altitude for the ADS-B, GPS and autopilot. If it hiccups in controlled airspace, as aging ones do, you’ll hear about it from controllers and maybe from the FAA with a follow-up letter from the FAA looking for proof the system has been repaired.
While the tech has gotten better because even entry-level EFIS can source high-resolution pressure altitude through the air data computer, there are plenty of legacy analog panels still flying with aging standalone altitude encoders. That means replacement is imminent. Here are some tech tips for dealing with the chore.
Mystery box
The earliest altitude digitizers were called blind altimeters because they had the chassis of standard altimeters, but no face. OEMs put them on the backside of the panel because there was nothing to see. They’ve long since evolved into compact remote boxes that can mount anyplace there is room—behind interior side panels, under the glove box, to the firewall and in the nose/baggage area on twins.
You’ve heard the theory in ground school. Using a pressure sensor, the Mode C encoder converts pressure altitude data to a digital stream the transponder can interpret and pass along electronically to ATC. Blind encoders plumb into the aircraft static system (another reason to keep up pitot static checks) and reference standard pressure of 29.92 inches Hg. The ATC computer applies the local baro setting to accurately compute MSL altitude to the nearest 100 feet, though most modern encoders have tighter reporting resolution thanks to RS232 serial data circuitry.
There’s also the encoding altimeter. This is an altimeter with a Mode C digitizer built in. Because of their pricey cost of replacement, many of these get replaced with standard altimeters and external encoders—or all-in-one EFIS upgrades. While blind encoders are generally reliable, they live hard lives and are subjected to heat, cold and vibration. They’re also relatively inexpensive as things go in the avionics world. But troubleshooting a Mode C problem can generate impressive invoices, especially if the troubleshooting goes off the rails. Vintage analog transponders can complicate matters, since altitude encoding problems could be related to the transponder and not the encoder. Find a shop that takes a logical approach to chasing down Mode C issues, and replace aging transponders with modern digital units. At the same time, invest in a new encoder. Most new units use the latest solid-state altitude sensors and familiar wiring. For many encoder swaps, the big job is accessing it and changing the mounting bracket. As you’d expect, altitude encoders have gotten smaller and lighter over the years—a good thing.
Gray code or serial?
Serial encoders output Gray 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. Older encoders might only output Gray or Gillham code, which is a binary numeric code that makes for a big wiring bundle from the transponder to the encoder.
During Mode C troubleshooting, you’ll hear your shop talk about checking the bit lines, which is a combination of wires that correspond to a given altitude block. These Gray-code-only encoders are still out there, but models with RS232 output rule and we can’t find any reason to do an upgrade without installing one with serial output. Eventually, the avionics will need the data, and serial encoders are flexible enough to program for matching the data format requirements of the avionics it’s communicating with, plus there are only a few wires to connect.
Again, serial altitude data is already generated from most EFIS units, including Garmin’s G5 and Dynon displays, eliminating the encoder altogether.
When they fail
An inconvenient trait of older designs is a lengthy warmup period. It can get worse as the heater circuit ages, and with a quick runup in cold-weather ops you could find yourself departing without valid Mode C. Remember, valid and accurate altitude reporting is integral to ADS-B and especially autopilots that rely on the data for altitude preselect and alerting functions.
One simple tool for keeping tabs on the data that the encoder is outputting might be found in a pressure altitude window displayed on most digital transponders or on other systems that display the current baro altitude, including GPS units. If the encoder is connected with a GPS unit (it’s been long required for IFR certification) you should get a failure alert message when the data is invalid.
Another failure mode might be
inaccurate (or inoperative) Mode C reporting at certain altitudes. But that could also be a problem in the 12-wire connection between the encoder and the transponder. If one or more of the wires are connected the encoder could report accurate at some altitudes but faulty at others. It could also be a problem in the transponder.
The other bugaboo is that altitude encoders are susceptible to static system leakage. The leak can be at the external static source connector or at a component inside the chassis. If that’s the case, it’s time for replacement. Here’s a look at just some of the models that shops might put in, and the chart above shows how select models compare.
Altitude problems can be a game of cat and mouse. One scenario is you set the altimeter before takeoff and when you check in with the controller, you’re told your altitude is off a couple hundred feet. They give you a new altimeter setting, you dial it in—changing the indicated altitude a couple of hundred feet. Then, the controller calls and says you’re now off 700 feet. Take good notes before showing up at the shop.
Trans-cal
The TSO’d SSD120 series is known for its reliability and is available with dual serial outputs (from 10-foot to 1-foot increments). Most shops attest that the SSD120 delivers solid altitude encoding with little if any periodic adjustments required. From our experience, newer SSD120s come up ready to operate in less than a minute, offering reliability for immediate departures on cold days. Trans-Cal has advertised a mean time between failures of 9.7 to 21.6 years, an accurate estimate, in our view.
Company principal John Ferrero told us that Trans-Cal offers the longest warranty in the market at 42 months—which includes a direct replacement. If it’s determined the encoder failed (when installed and used properly), you get a new unit with a reset warranty period.
Acknowledging that competitor ACK (with the A-30 series) is a popular encoder, the company offers an adapter plate to ease the transition to a Trans-Cal C-series unit. Street priced around $265, this solid-state SSD120-35C (it works up to 35,000 feet) has two asynchronous RS232 serial outputs for sending high-res (selectable 100- or 10-foot resolution) pressure altitude to multiple systems on the panel—from autopilots to ADS-B systems to GPS navigators. The unit can be configured (with internal jumpers) for six serial data protocols. Baud rates can be set from 1200 bps to 9600 bps.
There’s also the SSD120-30/35/42N models and the last numbers in the nomenclature represent the maximum operating altitude, while an “E” suffix in the part number stands for extended operating temperature range—from -67 to +158 degrees F.
Catering to the installer, we like that Trans-Cal has perhaps the most diverse encoder lineup in the market, with models designed for high-altitude ops and one with a 36-degree adjustable swivel hose barb static port fitting. It even still has one of the original so-called blind encoders, the electromechanical D120-P2-T, in service since 1971.
Sandia, ACK
The $550 Sandia SAE 5-35 outputs Gray 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 push-button/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.
The ACK A-30 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. ACK says it has produced over 248,000 units. It has a small footprint, measuring only 6 by 2.6 by 1.3 inches. Later units with Mod 9 output both Gray code and serial data. The A-30 comes with a premade wiring harness. The model A-30.5 outputs Gray code only to 30,750 feet and the model A-30.9 has Gray code and dual RS232 serial outputs to 42,000 feet. Street price for the A-30.9 is around $265 and the A-30.5 is slightly less. We say buy the one with serial output, and we give the company high marks for support.
Garmin
The $310 GAE 12—more of a pressure sensor than an encoder—smartly connects to the rear of the transponder tray and the static line connects directly to it. The advantage is that if the transponder has to be removed for repair or replacement, the GAE 12 stays put and the static system remains undisturbed. That could mean less paperwork compared to legacy designs that have to be recertified (leak checked) if they’re removed from the aircraft’s static source. That avoids the hassle and expense of recertifying the static system for a minor repair.
Think of the little GAE 12 as the altitude-sensing portion of an air data computer LRU. It works only with Garmin’s GTX 335/345 ADS-B Out transponders—perhaps among the most common for retrofit. But it’s worth mentioning that these transponders can also accept Gray code and serial input from other encoders, including Trans-Cal, ACK and Sandia.
In the end, it’s the installer that will likely make the altitude encoder buying decision for you, and we don’t think there are bad choices on the market. All are proven designs with good support behind them.
If we had to choose, it would likely be a Trans-Cal SSD120-C model, especially with an adapter plate when replacing an ACK, or Garmin’s GAE 12 to complement the GTX 335/345-series ADS-B transponders.
Contacts
ACK Technologies
408-287-8021
www.ackavionics.com
Aircraft Spruce and Specialty
877-477-7823
www.aircraftspruce.com
Garmin International
800-800-1020
www.garmin.com
Sandia Aerospace
505-341-2930
www.sandia.aero
Trans-Cal Industries
818-787-1221
www.trans-cal.com
