Every two years
As mentioned earlier, FAR 91-413 specifies that not only is testing required every two years, but also when there is a chance of correspondence error. This could be a result of component replacement such as an encoding altimeter or air data computer (the device that supplies altitude data to the transponder). In addition, a radio tuning unit can supply the "ident" code to the transponder so anytime it is disconnected, proper system operation needs to be verified.
In today’s digital world, with computerized components incorporating Built-In Test Equipment (BITE), many assume that initiation of internal tests and absence of faults will meet the testing requirements. Every FAA Maintenance Inspector that I have talked to says, "There is nothing like seeing a transmitted signal on a test set to verify proper transponder operation." Although with new technology, testing is still somewhat of a gray area. It would be beneficial to communicate with a local airworthiness agent to verify that any test procedure not directly in compliance with the FAR or manufacturers data, is acceptable.
Jim Sparks is Manager of Technical Information Support Services for Dassault Falcon Jet. He is an A&P and an Electrician, who began his aviation career as a technician in General and Business Aviation. Later, he became a Technical Instructor on Falcon aircraft and then a Field Representative.