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Listening Squawks - What, Why and When

Not every aircraft is transponder equipped, but, for those that are, the introduction of the Listening Squawk (LS), or Frequency Monitoring Codes as they are properly called, provides a useful safety net when near controlled airspace. This article sets out to clarify how to use a LS.

First some basic stuff.
Aircraft transponders automatically broadcast a four number code when asked to by a suitably equipped radar unit. The four number code, which is set by the pilot and known as the “squawk”, appears on the radar screen identifying the shady blob that is your radar primary return. The obvious benefit is that the controller using the screen to monitor air traffic can see that an aircraft is there, without having to rely upon the primary return, the shady blob, which for many small aircraft is sometimes indistinguishable from the return from a flock of birds or may even be too small to show at all. The transponder broadcast state to show your position by emitting a squawk is technically called Mode A. However it is unlikely that the transponder knob or button will be labelled Mode A, but just switching the device from “Standby” to “On” will transmit in Mode A.

Transponders can also be set to show the vertical position of the aircraft. The radar receiver software will display the vertical position as an altitude or flight level, depending upon the level that the aircraft is at. Displaying the aircraft’s vertical position means that the controller now has a three-dimensional understanding of where you are. The broadcast state to show your vertical position is called Mode C. Again, it is unlikely that the setting knob or button will be labelled Mode C, more likely it will be labelled “Alt” for altitude. Unless a controller specifically asks you not to squawk with “Alt” or “Mode C”, you should always use it, because it gives the controller the most information to help look after the safety of his other aircraft and you. Rather like not wearing a seatbelt in a car, not  using  “Alt”, if  it  is available, is viewed as an unnecessary added risk to overall flight safety.

If a pilot requires a service from a controller, perhaps a Traffic Service or wants to enter controlled airspace, the controller will allocate a unique squawk code, known in the ATC trade a discreet code, for the pilot to set on the transponder so that the aircraft can be positively identified. For example you could be allocated any one of the range of discreet codes used by Brize Norton, the UK’s largest RAF base, which are 3740 to 3745. Once an aircraft is positively identified, the controller can issue clearances, instructions or advice to the pilot, so that the flight can be safely coordinated with other aircraft showing on the radar screen.

Of course, a lot of the time the pilot of an aircraft that is not flying near to, or inside, controlled airspace doesn’t need, or want, an air traffic service. However the transponder can still provide both him and others with a valuable safety aid. Setting the transponder to the standard Conspicuity Code of 7000 with Alt will still show your level and position on all radar screens within range as well as being sensed by some on-board traffic alerting and avoidance systems carried in other aircraft. Squawking 7000 with Alt makes your aircraft electronically visible so that others can avoid you. That has to be a safety benefit.

     When you have set the ls, there is no need to make a call; in fact it is best not to

Situational awareness, knowing what is happening around you, is a vital part of a pilot’s flight safety toolkit. We know that lookout for collision avoidance is assisted by listening to radio calls from other aircraft, for instance in the circuit the position reporting at the downwind point and on final  approach  sometimes  helps pilots to locate traffic that otherwise they might not have spotted. Listening on the radio helps build a picture of other traffic and their intentions. When flying close to controlled airspace tuning the radio into the local frequency can help build your situational awareness even if not intending to be part of the radio traffic yourself. You can hear traffic that has chosen to take a service, although it might be outside the controlled airspace, as you are, and this will help you to lookout and avoid, just like in the circuit. So, even if you are not transponder equipped, listening in, or out, can provide you with a safety aid for your benefit.

Getting back to the LS. We have established that your safety is enhanced by using a listening watch on a local frequency to help your situational awareness. It can be enhanced further if the controller knows that you are listening, so that, in the event that he can assist you, he can make the first call. This is where the LS comes in. The LS is a squawk code that is published for use with a particular radio frequency used by a controller. As the name implies, setting the LS on a transponder shows the controller that your radio is tuned to the published frequency and that you are listening out on that frequency. Unlike the squawk that you will be given when taking an air traffic service, the LS is not unique to the aircraft but it is unique to the frequency. When you choose to listen out for local aircraft activity to assist your situational awareness, if you also set the appropriate LS the controller will know that you are listening and that, if he needs to contact you, he could. Squawking the LS doesn’t mean that you will be called, it just lets the controller know that you are listening and that he could contact you if needed. When setting the LS, you have effectively communicated with the controller by just emitting the squawk. When you have set the LS, there is no need to make a call; in fact it is best not to, because that takes up time on the radio frequency which prevents others who may have an important message from communicating with the controller.

Since the introduction of LSs, controllers have helped pilots in a number of ways. For example, aircraft flight paths that have appeared to be random have alerted the controller to the occasional lost pilot. By calling the aircraft, for example “aircraft squawking LS code north of Big Town this is Airport ATC, do you require assistance?” they have been able prevent a possible airspace infringement and help the pilot get to their destination. A similar call could be made when the transponder return shows an aircraft climbing towards controlled airspace to alert the pilot and prevent a potential infringement.

There is a radar replay on the Fly On Track web site that demonstrates this perfectly. Go to and view Replay 8.

If you aircraft is fitted with a Mode S transponder the controller will be able to see your aircraft registration displayed on the screen and so be able to call you by your call sign rather than just by reference to your position. Whether you are Mode S or Mode A/C don’t forget to keep the radio volume high enough so that you can hear a call directed at you as well as listening out for others giving position reports, helping your situational awareness.

The vast majority of flights using a LS will never be contacted by a controller; however having the ability to do so provides the controller with an aid to his management of the airspace for the safety benefit of the aircraft using it, which, of course, includes you.

As your flight progresses and you get further away from the airspace that the LS has been assigned to, you should consider when best to change back to the Conspicuity Code 7000 or, if approaching another airspace block that has a LS, changing to that. In all cases when you decide to change frequency you must cease to squawk the LS. 

It is important to understand that using the LS is not an alternative to asking for an air traffic service. Do not expect the controller to provide you with other information, such as traffic advice or a deconfliction service. If you want to take an air traffic service you must ask for one in the usual way.

For most LSs it will be obvious which squawk to use. When near Birmingham you will choose the Birmingham code 0010. For Manchester you will use theirs, 7366. Further south the choice of frequency may not be so obvious as your flight passes airports that are close together, such as Luton and Stansted, which share the same LS but on different frequencies. However, as long as you are squawking the LS, the controllers will know to find you on one frequency or the other.

Plans are in place to give better delineation of the areas covered by each LS and make them more easily obtainable in- flight on the chart. At the moment they are on the key / legend at the bottom of the chart and so are easily located in the planning stage and written on your flight log for in-flight use.

To summarise, LSs provide an additional safety aid for aircraft flying in the vicinity of controlled airspace but not needing, or wanting, to receive a direct air traffic service. The airspace controller’s situational awareness is significantly improved which means that he can provide a better, safer, service to all his aircraft and he is also in a better position to assist your aircraft if the need arises. Why wouldn’t you use a LS?

Geoff Weighell

Geoff is the Chief  Executive of  the  British   Microlight   Aircraft  Association, a member of the GASCo Board and an industry representative on the CAA’s Airspace Infringement Working Group

For an up to date list of Listening Squawks consult the GASCO website

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