All transponder-equipped, both VFR and IFR, must have their transponder tested and certified (IFR must also have their altimeter(s), encoder and static system tested) within the preceding 24 calendar months (see FAR 91.413 and 91.411) .
In addition, FAR 91.217 requires that all transponder-equipped aircraft have their altimeter and encoder checked for data correspondence after initial installation or following any subsequent modification or maintenance.
Note: Part 91 of the Federal Aviation Regulations also apply to Experimental Aircraft.
For VFR operations, the FAA requires all transponder-equipped aircraft to be inspected to verify the proper operation of the transponder.
Transponder systems that don’t meet minimum requirements and/or are out-of-alignment can cause significant problems with ATC radar systems, including incorrect altitude readout, duplicate targets or no target at all.
FAR 43, Appendix F, describes the transponder inspection requirements.
In addition to the transponder tests required for VFR, IFR aircraft are required to have their altimeter, encoder and static system inspected and certified.
- The altimeter is subjected to a series of test to insure it meets the minimum requirements for accuracy and repeatability.
- The altitude encoder is checked against the primary altimeter and adjusted as require to insure data correspondence.
- After the equipment has been inspected and reinstalled in the aircraft, a static system leak check is performed. Leaks in the static system can cause significant errors in the altitude reporting equipment (and airspeed) and are sometimes difficult to locate.
FAR 43, Appendix E, describes the altimeter system inspection requirements. Usually, the altimeter is removed and checked on the bench along with the altitude reporting encoder to insure data correspondence.