From Pete:
While flying my Sundowner today, I was told by ATC that my transponder’s Mode C was showing I was off by 600 feet (when I was at 5,500 feet). When I descended to 4,500 feet, I inquired what ATC was seeing, and they were seeing 4,500. I played around a little, and it seems that at/above 5,500 feet my Mode C shows 600 feet high, but when I’m at 4,500 feet or lower it’s accurate. I’m going to fly again tomorrow and see what happens when I go above 5,500 in 500 foot increments. Any ideas on what could be causing this? I just had the transponder checked during my IFR cert in June and all was fine. Is this an encoder problem? What is a blind encoder?
Bob Steward, A&P-IA:
Your transponder does not generate the altitude signal; the signal originates in the “Blind Encoder”. The “blind” in “blind encoder” means that it always references 29.92″ as the altimeter setting; the pilot can’t see the encoder’s altimeter setting, and can’t change it. The blind encoder sends an altitude signal to the transponder, which in turn sends that altitude code to ATC when the transponder is “interrogated” by ground radar (and is set to “Mode C”). When the REAL (current local) altimeter setting begins to get away from a standard day of 29.92, the ATC system reports an “error”, meaning an apparent discrepancy between the encoded altitude displayed on the ATC “scope”, versus the pilot’s assigned and radio-voice confirmed altitude. The controller can ask you for your indicated altitude and then punch in a correction on his scope. Your encoded altitude is OK at some altitudes, and then has a fixed amount of error at higher altitudes. This is not consistent with the constant offset caused by the difference between standard day altimeter and actual readings, so something else is wrong.
The encoder output sent to the transponder for Mode C is in a digital format; missing 1 bit of the data will cause an erroneous reading. I’d pull the transponder and clean the edge connector with a pencil eraser, and inspect the harness to the encoder for a nicked wire. After that it’s bench check time.
Editor’s note:
If it turns out that you need an encoder repair, it might pay you to check around. Many planes came with the old Trans-Cal 120, which is typically mounted in a large instrument hole, sometimes with a blank cover over it. The 120 is a near-bulletproof encoder, but everything needs work eventually. There are now several fairly inexpensive newer-technology encoders on the market. If you are facing a high repair estimate, talk to your avionics shop about alternatives before sinking a lot of dough in an older unit.
Also keep in mind that, legally speaking, only someone with an avionics license with a radar endorsement can reinstall and test a transponder. This is due to the very high frequencies involved. Very small and unnoticed changes can detune reception or transmission, so the FAA requires installation and testing by trained people.