What to Do if Your Transponder Fails

Losing your transponder, or having it inoperative for whatever reason, is something that has happened to me a few times.

The first time it happened I confess I didn’t know what to do. Return to the airport? Make out it wasn’t a big deal? Apologise? The radar controller I was talking to seemed to have a less-than-sympathetic tone to his voice, but he certainly wasn’t asking me to abort the flight, despite still being in Class D airspace. So I just continued and nothing more was said, but it made me wonder what the correct procedure is.


Transponder fails

Can You Fly Without It?

Yes, in the UK at least, it is not a requirement to fly with a transponder, although it makes life safer for everyone if you do since it helps air traffic radar controllers to properly monitor all aircraft in their patch and offer advice on other traffic (depending on the type of service you are receiving). You will likely find, however, that you need one in order to operate in Class D airspace (although you may not have a problem when it is your based airport), or within 30 miles of a major airport or airspace.

In other countries the situation is different. France, for example, also requires a transponder with Mode C to operate within Class D airspace, and Germany has areas of mandatory squawks. For the United States, you can find out transponder requirements here. It is best to make sure you have an operative transponder if you intend to travel long distances just to ensure you are covered and can safely transit control zones.


What Do I Need to Say to ATC?

If your transponder fails in flight it can cause concern from air traffic control since it is in their interest to keep a watchful eye over you and all other aircraft. By losing your tracking code on their radar can in extreme cases lead them to consider that something has happened to your aircraft. At the very least it means they can lose sight of you amongst other aircraft or “noise” which appears on their radar.

If you are aware that your transponder is inoperable when speaking to a controller for the first time, you should include in your communication with them that you have “negative transponder”. If your transponder is working but doesn’t transmit height information (Mode C), simply inform them “negative Mode Charlie”. This gives the controller enough information to effectively plan their coverage of your flight without undue concern.

Primary Radar

What If The Transponder Fails In Flight?

Chances are you won’t know it has failed until air traffic control alerts you to the fact. They may report that your transponder isn’t showing and ask you to Squawk Ident or recycle your transponder (see below). If that fails, try turning your transponder off and on, or from ALT to ON (or vice versa) and report to the controller that you have recycled your transponder.

Remember that your first priority is to safely fly the aircraft. Don’t lose sight of what the aircraft is doing, where you’re flying, and keep a look out for other aircraft.

If your transponder has failed, tell the controller. In most cases this will not be a problem and they will be able to keep track of your movements. They may ask you to confirm your position or height at certain intervals to ensure proper separation from other traffic, but otherwise you may continue your flight as planned.


What Does Recycle Transponder Mean?

Initially, when ATC notices that your transponder has a problem they will ask you to “Recycle/Reset Transponder” or “Recycle/Reset Squawk xxxx”. This means they want to to check out your transponder to make sure it is set correctly. In this instance you should check the correct squawk code is entered, and try turning the equipment to SBY or OFF, and then back to ALT or ON (depending on whether you have Mode C).

Confirm the request and inform the controller when you have done so. Remember, it can take a few minutes for the transponder to correctly communicate once a reset has taken place.

If there is still a problem you may wish to clarify with the controller as to whether they have any information from your aircraft at all, or whether it is missing altitude, or primary contact information. This will help to diagnose the problem.


What Does Squawk Ident Mean?

On your transponder you should see a button labelled “IDENT”. Pressing this will draw attention to your return on the controller’s radar screen, alerting him to your aircraft’s position and allowing him to properly track you.

If you are asked to “Squawk Ident”, simply press the IDENT button. You are often asked to do this when switching to a new controller with a different transponder code.


A Note on Mode S

All new aircraft are now fitted with Mode S, which is a more advanced way of tracking aircraft. It will become mandatory for ALL aircraft to be fitted with it eventually. Countries such as the Netherlands now require all aircraft to have such equipment, so it is best to check before flying to a new area.



Matt Falcus is a private pilot and aviation writer. He has been flying since 2006, taking the opportunity whenever the British weather allows to explore the local area and other airfields. He is author of a number of aviation books.

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3 Responses

  1. arifin says:

    you may find this on ICAO doc. 4444 AIRCRAFT TRANSPONDER FAILURE IN AREAS WHERE THE CARRIAGE OF A FUNCTIONING TRANSPONDER IS MANDATORY When an aircraft experiencing transponder failure after departure is operating or expected to operate in an area where the carriage of a functioning transponder with specified capabilities is mandatory, the ATC units concerned should endeavour to provide for continuation of the flight to the aerodrome of first intended landing in accordance with the flight plan. However, in certain traffic situations, either in terminal areas or en-route, continuation of the flight may not be possible, particularly when failure is detected shortly after take-off. The aircraft may then be required to return to
    the departure aerodrome or to land at the nearest suitable aerodrome acceptable to the operator concerned and to ATC. In case of a transponder failure which is detected before departure from an aerodrome where it is not practicable to effect a repair, the aircraft concerned should be permitted to proceed, as directly as possible, to the nearest suitable aerodrome where repair can be made. When granting clearance to such aircraft, ATC should take into consideration the existing or anticipated traffic situation and may have to modify the time of departure, flight level or
    route of the intended flight. Subsequent adjustments may become necessary during the course of the flight.

  2. Daryl says:

    Good info. Thanks

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