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When should a transponder be turned turn on?

  1. On the ground?
  2. Always in flight?
  3. Only when required: during radar service (flight following), above 10,000ft MSL, inside the Mode C Veil, or on IFR flight?

Should Altitude Mode be used whenever the transpoder is on?

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At least in the United States the FAA recently (I think in 2012?) changed the recommendation for transponder operation in the AIM. It now reads:

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. IN ALL CASES, WHILE IN CONTROLLED AIRSPACE EACH PILOT OPERATING AN AIRCRAFT EQUIPPED WITH AN OPERABLE ATC TRANSPONDER MAINTAINED IN ACCORDANCE WITH 14 CFR SECTION 91.413 MUST OPERATE THE TRANSPONDER, INCLUDING MODE C IF INSTALLED, ON THE APPROPRIATE CODE OR AS ASSIGNED BY ATC. IN CLASS G AIRSPACE, THE TRANSPONDER SHOULD BE OPERATING WHILE AIRBORNE UNLESS OTHERWISE REQUESTED BY ATC.

(Cribbed from Chapter 4-1 of the AIM)

So, basically "Turn it on before you taxi, and turn it off right before you shut down the engine. Always use the Altitude mode (Mode C) unless ATC tells you otherwise."


In practice having the transponder on while on the airport surface doesn't do much unless your field has something like ASDE-X which plots aircraft transponders on a screen in the tower, but it generally doesn't hurt anything and it ensures you don't forget to turn the transponder on when you take off.

Operating your transponder while on the ground may cause "ground clutter" on nearby radar station's displays (possibly setting off the conflict-alert system) - this is why many of us were taught to set our transponders to standby mode while on the ground. In practice this usually isn't an issue, and if it is a problem local ATC will ask you to "squawk standby while taxiing" to resolve the issue.
If anyone gives you a hard time about it copy the number and politely quote the AIM.

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  • $\begingroup$ the other reason why we were taught to turn the xponder off on the ground is because older transponders had a cavity magnetron, which is a kind of high-voltage tube. The idea was to turn them off when not in use, to prevent them from wearing out, and to reduce the load on the electrical systems in old airplanes $\endgroup$ – rbp Jan 12 '15 at 20:40
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    $\begingroup$ @rbp the standby mode runs the cavity tube heater (at least it does in the AT-150 I have), it just doesn't connect it to transmit replies when the transponder gets interrogated. Whether you do more damage cycling the heater on & off or letting it run all the time is debatable, but turning it off on your taxi back to the ramp does take about an amp off the electrical load which is something if your electrical system is marginal at low RPM. $\endgroup$ – voretaq7 Jan 12 '15 at 20:57
  • $\begingroup$ If the above quotation is from the AIM, then it is recommended, not required (as you've noted). Note: I'm not asserting that it's not good practice to use ALT reporting when taxiing! Is there a FAR # that mandates any transponder use outside class A, C and the class B 30NM veil? $\endgroup$ – CJBS Aug 27 '15 at 5:29
  • $\begingroup$ @CJBS FAR 91.215 would be the base regulation: Paragraph C says: While in the airspace as specified in paragraph (b) of this section or in all controlled airspace, each person operating an aircraft equipped with an operable ATC transponder maintained in accordance with §91.413 of this part shall operate the transponder, including Mode C equipment if installed, and shall reply on the appropriate code or as assigned by ATC. (Bolding mine - "required everywhere except Class G airspace" is my read of that regulation.) $\endgroup$ – voretaq7 Aug 27 '15 at 18:47
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    $\begingroup$ @CJBS As far as I can tell it's not a regulatory requirement on the ground (at least under Part 91 - Parts 121 & 135 may impose additional requirements). At larger airports with ground surveillance radar it may be an operational requirement though (i.e. ATC will ask you to turn on your transponder so they can see you on their displays). $\endgroup$ – voretaq7 Aug 27 '15 at 21:47
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For aircraft equipped with ADS-B Out, the following regulation applies (emphasis mine)

14 CFR 91.225(f)Each person operating an aircraft equipped with ADS-B Out must operate this equipment in the transmit mode at all times

This a nuance to the ADS-B regulations that many may miss. So, for ADS-B it is required to be on (ALT) even while on the ground.

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