Altitude encoder allowable deviation?

It occurred to me earlier today that I don't have a clue what the allowable deviation is for altitude encoder reporting when checked during bi-annual transponder service. Can somebody enlighten me?

Thx.

SB

I don't know either, except that mine is off by a hundred feet, and the guy who does the certification can adjust it rather quickly if he knows what the error is. I will get it done next year. It is embarrassing to be perceived to be so sloppy in maintaining assigned altitudes.

FAR43 Appendix E

That appendix pertains to altimeters. Does it apply to blind encoders? Appendix F is for transponders but it's limited to electronic standards for the transponder itself, not the encoded information. Thus the question.

SB

(c) Automatic Pressure Altitude Reporting Equipment and ATC Transponder System Integration Test. The test must be conducted by an appropriately rated person under the conditions specified in paragraph (a). Measure the automatic pressure altitude at the output of the installed ATC transponder when interrogated on Mode C at a sufficient number of test points to ensure that the altitude reporting equipment, altimeters, and ATC transponders perform their intended functions as installed in the aircraft. The difference between the automatic reporting output and the altitude displayed at the altimeter shall not exceed 125 feet.

I missed that. Thanks for the help.

Stewart

I routinely test my encoder readout with a local class D tower.I set my altimeter to the ATIS pressure and ask the tower what my encoder is telling them.If they both agree I'm happy.

Bill