Off-track error Avidyne STEC55X and GNS430

Hi folks,

since last week, i have the following problem :

when in NAV GPSS mode between 2 RNAV points, or event between a VOR and a RNAV point, the aircraft always stays 0.3 NM left off-track.

Do you know where the problem could come from ? The radio repair shop told me that I coud do following procedure :

  1. AP-ROLL NULL adjustment (CDI needle wentered, GNS off, heading bug on the lubber line, with a tiny screwdriver, and small increments)
  2. perform the PFD AP calibrations to get the HDG bug centered
    I know how to do that, but did anybody encounter such a problem before (it is the first time I have the case…)


Found this for you: - Eric
Flight testing the STEC GPSS after installation I typically saw cross track error in the range of .01 to .07 NM. The lower number is 60 ft. The roll steering ARINC 429 label from the GPS is “bank angle” and it is measured in degrees in increments of a 15 bit binary value plus a one bit sign where each increment is 180/16384 of a degree. So there is plenty of precision in the basic data. But there isn’t a bank angle input to the autopilot, so the GPSS converter develops a heading error signal on the theory that the heading error (how far the heading bug is from the desired heading) is somewhat proportional to bank angle. That is when the heading error is small, the autopilot commands a small bank and when it is larger, it commands a larger bank angle. So a 5 degree heading change might command the autopilot to at most a 5 degree bank, and a 10 degree change might command a 10 degree bank, and so on until the maximum bank value for the autopilot is reached (typically close to standard rate or limited by 25 degrees). The point of this is that the autopilot doesn’t respond rapidly to very low heading error, for example one which would require .01 degree of bank.

John D. Collins CFI, CFII, MEI
68 V35A N7083N Home Base KUZA"

thank you very much for this extensive answer, that seems to confirm that if I do the AP ROLL null adjustement again + heading sync, the cross-track error might disappear. Though, if I take carefully care, there is a 2 degrees error in the heading bug (when I select HDG300 with the bug the plane flies HDG302). Thank you


Michel, you should join COPA for even more helpful info… You might have to calibrate your Avidyne compass. This is a avionics shop task and you need to run it up while carefully calibrating against a reference compass. For 2 degrees of error I probably would not do it