Sep 11, 2011

Aircraft Emergency Frequencies

The aircraft emergency frequency (also known as guard) is a frequency used on the aircraft radio band reserved for emergency communications for aircraft in distress. The frequencies are 121.5 MHz for civilian, also known as International Air Distress (IAD) and 243.0 MHz for military use, also known as Military Air Distress (MAD). Both are in use at the international level.

The choice of 121.5 MHz was made by the ICAO in conjunction with ARINC and the ITU as a result of its third harmonic frequency relationship with the 40.5 MHz military tactical low band ground-to-air channel. Similarly 121.5 MHz is itself a sub harmonic of the military UHF distress frequency at 243 MHz. This choice gave a number of technical and operational compatibility and efficiency gains in the context of design and proximity interference.

121.5 MHz is monitored by most air traffic control towers, national air traffic control centers, and other flight and emergency services, as well as by many airliners. Separate frequencies exist for military and other government emergency frequencies. If an aircraft violates or is on a trajectory that will violate Restricted or Prohibited airspace, it will be warned of military interception on 121.5 MHz.

The frequency may also be used by ATC to establish contact with an aircraft that has inadvertently switched to an incorrect frequency. As pilots are strongly recommended to monitor 121.5 MHz at all times, a common practice is to set a secondary communications radio in the aircraft (often COMM 2) to 121.5 MHz in order to monitor, but not transmit on, 121.5 MHz. A pilot accidentally transmitting on 121.5 MHz will often hear a reply stating that they are "on guard", i.e., that they are on the guard frequency and should switch to the appropriate frequency instead.


  1. Vipin - i still listen a lot on vhf and hf aircraft communications. if anybody is interested Chennai approach radar is 127.9, cruise control is 118.9 [main control] ground control is 118.1 etc. couple of months back i had the chance to monitor a Qatar airways emergency communications as a pregnant flier needed some medical attention
    hf oceanic controls are on 4675,5670,10018 and 11285 depending the time of the day. my little icom icq7a covers from 30khz to 1250mhz and pulls in everything in between and vhf/uhf transmission with 400 watts, more than enough to access the local repeaters. hope u r fine.

    73s de vu3mes

  2. one of your well wisherOctober 24, 2011 at 9:20 AM

    VIII. Secrecy of Correspondence

    If any message which the licensee is not entitled to receive is, nevertheless received, the licencee shall into make known or allow to be made known its contents, its origin or destination, its existence or the fact of its receipt to any person (Other than duly authorized officer of the Central Government or a competent of legal tribunal) and shall not reproduce in writing, copy or make any use of such message or allow the same to be reproduced in writing, copied or made use of.

  3. dear well wisher - agree with you 100%, i just said i happened to listen this communication, and what i heard word by word is recorded in my mind, it was not exposed to anybody. this is where we are ages behind the western world, we still follow the ages old rules formed by the colonial rulers, while they themselves have changed according to the times. you can get a ham radio license by passing a simple test in those countries in five days flat and here you have to dream and dream and dream.

  4. useful post do post relative to aviation field, I am in Chennai and doing pilot training