Imposition of radio silence is a procedure that is sometimes used in case of aircraft emergency.
The purpose of radio silence is to ensure the frequency channel is available exclusively to the aircraft in distress so that better service is provided:
- Exchange of information (pilots elaborating on the situation and controllers providing useful information) is easier
- Navigational assistance (which is sometimes necessary during emergencies) requires considerable time spent on communication
- Distraction of both the flight crew in distress and the air traffic controllers is reduced so that they may focus on the emergency situation
There are two ways to ensure radio silence:
- Suppressing the calls from other aircraft on the frequency. Aircraft requested to maintain radio silence are expected to do so until advised that the emergency situation has been resolved.
- Transferring the communicaiton with the rest of the aircraft to other frequency (or frequencies). Transferring the distress traffic is considered inappropriate because (a) the stress and workload levels in the cockpit are already high and (b) in such situations the possibility for human error is greater and may lead to loss of communicaiton. The transfer may be done using a single "all stations" call (see Phraseology section below as well as the Notes) or by transferring each aircraft using a separate instruction.
Whether radio silence will be imposed and if so, which method will be used, is decided by the controllers involved based on the specific circumstances.
This section contains phraseology to be used for imposing and terminating radio silence.
- If the controller needs to impose radio silence, the following general call is used:
All stations [ATS unit] stop transmitting. Mayday.
- If appropriate, the controller may also address a specific traffic:
[Aircraft callsign] stop transmitting. Mayday.
- When the emergency is resolved and radio silence is no longer necessary, the controller cancels it using a general call:
All stations [ATS unit] distress traffic ended.
- The controller may use the following phrase to transfer all aircraft except the one in distress to another frequency:
Mayday, [callsign of aircraft in distress], remain this frequency, break break, all other aircraft contact [appropriate ATS unit] [frequency], out
- The last example is taken from UK CAA's CAP 413 (Radiotelephony Manual) and not from an ICAO document
- Using an "all stations" call (as opposed to transferring aircraft one by one) has higher potential for aircraft changing to a wrong frequency
- Using an "all stations" call for this purpose may be contrary to local procedures