This month’s MX Quick Hitter feature is based on a problem an owner is having with the Mode C altitude reporting system in his legacy Cirrus. The problem: At some altitudes (not all) controllers are calling out the aircraft’s reported altitude 200 feet higher than assigned. In other words, the pilot is dialing in the correct altimeter setting and flying the assigned indicated altitude, but the altitude reporting system is reporting a 200-foot discrepancy. This smells like an altitude encoder that needs calibration, and a trip to the avionics shop is in order.
This tech is old hat and these encoders have been in existence from the beginning of Mode C veil requirements (I installed one of the first back in 1990 or thereabouts while working at a shop on the fringe of the New York TCA). Using a pressure sensor, the encoder (digitizer) converts pressure altitude data to a digital stream the transponder can interpret and passes it along electronically to ATC, whose computer applies the local baro setting to accurately compute MSL altitude to the nearest 100 feet. The transponder in this panel is a solid-state Garmin GTX 327, mated with a Transcal SSD120 altitude digitizer. Like other digital transponders, the Garmin shows pressure altitude on its display so you’ll always know what the reported Mode C altitude is. In this case with a discrepancy this small, it’s likely that a calibration can fix the issue. Worth mentioning is that the maximum allowed error between the primary flight altimeter and the altitude digitizer is +/- 125 feet as required by TSO C88a, so this one isn’t that far out of spec, but enough for a controller to call out in the busy Class B where you’re expected to be right on the money all the time every time.
It’s also one of the things that your shop is testing when it performs FAR 91.411 biennial IFR inspections, which is yet another reason not to blow off these checks—even if you only fly the airplane VFR. As the tech runs the altimeter up to its certified altitude, he’s also noting the altitude encoder’s readout every 1,000 feet along the way. Most standalone altitude digitizers have adjustment pots for tweaking out any error that’s out of limit. Modern altitude encoders with serial data (RS232) capability can output high-res serial altitude in 10-foot increments, while plain-vanilla encoders output Gray/Gillham code in 100-foot increments.
Also worth mentioning is that not all aircraft have standalone altitude digitizers. Many integrated avionics suites have air data computers, which like altitude encoders are plumbed into the aircraft’s static system and generate high-resolution Mode C data to the transponder and to other components in the system.
Funny you mention this. Earlier this year my airplane started reporting inaccurate altitudes intermittently. The fix was to replace the encoder, which was original from 1979, a mechanical-electric gizmo the size & shape of a soup can. Incidentally, we looked up its errors in the manual and found that it was always a problem with pins C4 and B2. The replacement is a Transcal SSD120-30N, similar to the one in the photo. I only hope it lasts 45 years like the original did 😉
Good one, Larry. Here’s something worth mentioning while we’re on the topic. I used several GTX 327 units in my trainers, probably starting in the early 2000s, and students would occasionally ask about the “Pressure ALT” line and the arrow next to it. So I read the manual and gave them my best explanation.
The display shows the pressure altitude being transmitted to ATC, based on 29.92 inches, not whatever’s set in the altimeter’s Kollsman window. That number comes from the encoder via the static system. The arrow next to it is a simple trend indicator: up for climb, down for descent, nothing if stable.
Add in normal altimeter and static system error, temperature effects, and panel leveling, and ATC could see an aircraft 200 to 300 feet high or low, even with everything technically working within parameters.
It’s a handy way to verify what you’re actually squawking to ATC. If the number looks off or the arrow doesn’t match what’s happening, it might be the first sign something’s off in the encoder or static system.