Guarding 121.5 MHz

Guarding 121.5 MHz

The Problem

A European Air Navigation Service Provider (ANSP), within the context of a significant number of “loss of communication” events, shared the findings made locally:

  • There is no firm requirement for aircraft to guard 121.5 MHz except for designated areas, long over-water flights and areas where there is a risk for interception; however, there is a recommendation to do so.
  • As regards aeronautical stations, they are required to guard 121.5 MHz.

ICAO Provisions in Annex 10 Volume II Aircraft on long over-water flights, or on flights over designated areas over which the carriage of an emergency locator transmitter is required, shall continuously guard the VHF emergency frequency 121.5 MHz, except for those periods when aircraft are carrying out communications on other VHF channels or when airborne equipment limitations or cockpit duties do not permit simultaneous guarding of two channels. Aircraft shall continuously guard the VHF emergency frequency 121.5 MHz in areas or over routes where the possibility of interception of aircraft or other hazardous situations exist, and a requirement has been established by the appropriate authority. Recommendation.— Aircraft on flights other than those specified in and should guard the emergency frequency 121.5 MHz to the extent possible. Aeronautical stations shall maintain a continuous listening watch on VHF emergency channel 121.5 MHz during the hours of service of the units at which it is installed.

Support Requested

Readers were requested to share their national and company provisions regarding monitoring of 121.5 MHz.


Responses were received from a wide range of operators and ANSPs:

  • Two operators stated that it was company policy to monitor 121.5 MHz continuously;
  • 12 operators stated that company policy was to guard 121.5 MHz whenever feasible;
  • Only one operator reported that it was company policy not to monitor 121.5 MHz;
  • Several operators pointed out that it was often necessary to reduce the volume on the emergency frequency below the normal audible threshold due to interference or inappropriate use of the frequency. Sometimes, pilots forget to restore the volume to the normal level when the distraction has ceased and on one occasion, this contributed to loss of communication and subsequent interception of an aircraft;

Note: Interference or inappropriate use of the emergency VHF frequency should always be reported to the appropriate national authorities.

  • Some operators also monitor other frequencies when possible, e.g. 123.45 in remote geographic areas;
  • All aircraft operating in U.S. airspace are required to maintain a listening watch on frequency 121.5;
  • Some operators reported that monitoring of 121.5 or 123.45 had enabled them to assist, or be assisted by other aircraft following loss of communication;
  • The use of Aircraft Communications, Addressing and Reporting System often enables the emergency frequency to be guarded continuously because the second VHF radio is not required for weather broadcast, etc.;
  • One Air Navigation Service Provider stated that in their region, emergency calls are responded to by the Flight Information Centre, who co-ordinate response with other stations if necessary. For operational reasons, the monitoring of 121.5 MHz is not compulsory for other national Aeronautical Stations;
  • One responder commented that, if a significant number of losses of communication occur, efforts should be focussed on finding the reasons for the losses of communication and appropriate remedial solutions.


© European Organisation for Safety of Air Navigation (EUROCONTROL) March 2007. This alert is published by EUROCONTROL for information purposes. It may be copied in whole or in part, provided that EUROCONTROL is mentioned as the source and to the extent justified by the non-commercial use (not for sale). The information in this document may not be modified without prior written permission from EUROCONTROL. The use of the document is at the user’s sole risk and responsibility. EUROCONTROL expressly disclaim any and all warranties with respect to any content within the alert, express or implied.


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