Frequency Congestion

Frequency Congestion


A single radio frequency is capable of handling a limited number of radio messages in a specified time. The maximum number of messages will be determined by the length of each message and its response. Ideally, a pilot should be able to transmit a message at any time of his/her choosing, and receive an immediate reply. As radio traffic increases above the ideal, the frequency becomes congested. The pilot must wait for a break in transmissions to pass a message and may have to wait for a response from the ATCO, who has to judge different priorities.

Persistent (rather than transient) frequency congestion is a frequent contributory factor in communication breakdown, usually in association with other factors.


  • Pilot or ATCO is unable to pass an important message at the desired time.
  • blocked transmission from another aircraft which is trying to communicate is lost or misheard.


ATCUs attempt to reduce the impact of frequency congestion by Sectorisation and the use of multiple frequencies. However, too frequent frequency change can also contribute to communication breakdown.

Typical Scenarios

  • The pilot, who is awaiting a clearance, takes a clearance intended for another aircraft. The ATCO does not notice the error on read back.
  • In his/her attempt to pass the message quickly, the ATCO does not use standard phraseology; this increases the chance of communication breakdown.
  • The pilot transmits at the same time as another, who is conducting communications in the local language. This results in a blocked transmission.
  • Radio Interference makes it difficult to read the call sign in a transmission, which is mistakenly taken by the wrong pilot.


Contributory Factors



  • Pilots:
    • Always use standard phraseology;
    • Listen in on the new frequency before transmitting;
    • Call ‘blocked’ when you detect a blocked transmission;
    • Use English language, to assist other pilots to determine when communication with ATC is finished.
    • Check that the PTT (Press to Transmit) switch releases freely at the end of each transmission and if any functional fault is identified, alternative transmit options are used.
  • ATCOs:
    • Always use standard phraseology;
    • Use short and concise messages;
    • Use English language, to assist other pilots to determine when communication with ATC is finished.
    • Do not talk continuously but give pilots the opportunity to check in on the frequency by building in rest blocks; In practical terms, it may be best to transfer several aircraft to their next sectors before checking in new aircraft - this is a matter of judgement for the ATCO.
    • Reduce the number of aircraft on a frequency or extend the number of controllers, so that the workload per frequency is lower.
  • Technology
    • Blocked transmissions can be partly overcome by anti-blocking devices.
    • Design a warning if the mike is pressed too long, for instance by a beep indicting the microphone is still transmitting. (Some aircraft already have such a feature).


Related Articles

Further Reading

HindSight Articles:

AGC Safety Letters:

EUROCONTROL Action Plan for Air-Ground Communications Safety, including:


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