Tackling the risks of ComLoss – a community effort

SWISS civil and military aircraft

Communication Loss (ComLoss) is a very sensitive security problem, since controllers are unable to distinguish between communication failure and potentially sinister intent.

As a consequence, the number of interceptions in Europe has increased. A large part of them can be attributed to ComLoss situations.

An unnecessary interception is a disruption to the air traffic management (ATM) system and a waste of scarce national resources.

At EUROCONTROL, we work closely with our partners at the North Atlantic Treaty Organisation (NATO) to mitigate the negative impact of ComLoss on safety, security and finances for the military, civil aviation and airlines, including the pilot in command.

~ 90%

ComLoss-attributed interceptions in Europe

Best practice

To help mitigate ComLoss here are some simple, yet effective measures air traffic controllers and pilots alike can take.

Sufficed to say, always wear a headset and make sure that the volume of loudspeakers is sufficient. In addition, you should always be alert to the possibility of loss of communication and follow standard procedures for copying, setting and cross-checking the radio telephony frequency (RTF).

Pilots should:

  • Suspect that there may be an issue (abnormal saturation) if there is a prolonged period without communication between ATC and pilots should.
  • Expect possible frequency changes when crossing borders/changing ATC areas of responsibility.
  • Always read back assigned frequencies and use standard radiotelephony phraseology.
  • Check radio equipment settings, including volume.
  • Carry out a radio check if you have the impression that you have lost contact.
  • Continuously monitor 121.5 MHz.
  • If you are in doubt about possible loss of communication, call ATC on 121.5 and/or squawk 7600.
  • If you are not successful, use other frequencies appropriate to the position.
  • Use other aircraft in your vicinity as a relay station.
  • Monitor ACARS.
  • In the event of radio communication failure, follow standard ICAO procedures and squawk A7600.

Controllers should:

  • Provide clear instructions for frequency changes to pilots using standard radiotelephony phraseology.
  • Listen carefully to the read-back.
  • Follow published data link procedures if applicable.
  • In the event of ComLoss, try to get in contact with the company to instruct/advise the pilot via ACARS or other means of compliance.


Awareness-raising and information campaigns targeting civil and military stakeholders are just one part of the great work done through the NATO/EUROCONTROL ATM Security Coordinating Group (NEASCOG). The group published its first new definition of ComLoss including implementation guidelines in 2005. As a follow-up NEASCOG created a taskforce to evaluate solutions for ComLoss this task force launched a campaign to promote positive awareness patterns that reduce risk and enhance readiness.

Have you experienced ComLoss?

Report it through the EUROCONTROL voluntary ATM incident reporting (EVAIR).

Regulations to consult

For more information, regulations concerning 121.5 MHz can be found below

  • Commission Implementing Regulation (EU) No 923/2012.
  • SERA.8035 Communications - An aircraft operated as a controlled flight shall maintain continuous air-ground voice communication watch on the appropriate communication channel of, and establish two-way communication as necessary with, the appropriate air traffic control unit, except as may be prescribed by the relevant ANSP in respect of aircraft forming part of aerodrome traffic at a controlled aerodrome.
  • ICAO Annex 10 Volume II - 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. The user of the air-to-air VHF communications channel shall ensure that adequate watch is maintained on designated ATS frequencies, the frequency of the aeronautical emergency channel, and any other mandatory watch frequencies. The requirement for an aircraft to maintain an airground voice communication watch shall remain in effect when CPDLC has been established.