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Transponder On During Taxi?

Featuring John Krug

Subscriber Question:

"I have heard the statement on some ATIS broadcasts to leave the transponder on during ground operations. I thought they were supposed to be turned OFF or to STANDBY." - Nicholas B.


"The FAA recently changed the guidance in AIM paragraph 4-1-20 to state:

Civil and military transponders should be turned to the on or normal altitude reporting position prior to moving on the airport surface to ensure the aircraft is visible to ATC surveillance systems.

The reason for this is twofold. The FAA has installed ground radar surveillance systems at many large air carrier airports. These systems rely on information from the aircraft's transponder and discrete beacon code to identify the aircraft as it is moving on the surface.

Controllers in the tower see this information presented as a color display of aircraft and vehicle positions overlaid on a map of the airport's runways/taxiways and approach corridors. The system essentially creates a continuously updated map of all airport surface operations that controllers can use to spot potential collisions.

The other reason is that with older radar systems, transponder operation on the ground could cause interference and multiple targets on the radar display. Modern digital systems can filter out transponders close to the antenna.

Since you may not know whether you are operating at an airport with a ground surveillance system, good operating practice is to leave the transponder on at all times."

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