Review policy on Frequency Blocking

  • Home 2006 Review policy on Frequency Blo....

Review policy on Frequency Blocking

45TH ANNUAL CONFERENCE, Kaohsiung, Taiwan, 27-31 March 2006

WP No. 92

Review policy on Frequency Blocking

Presented by TOC


1.1.  At the 44th Annual IFATCA Conference in Melbourne, Technical policy on RTF Frequency usage was accepted. At that time, it was believed that the new policy could have resulted in duplications in the IFATCA Policy Manual, thereby referring to the policy on Frequency Blocking.

1.2.  IFATCA Policy on Frequency Blocking dates back to 1984.

1.3.  This paper is prepared as part of the review of existing policies by the Technical and Operations Committee (TOC), which are conducted on a regular basis.

1.4.  This paper will review existing policy, and will also address the possible duplications in the IFATCA Policy Manual.


2.1.  IFATCA Policy on Frequency Blocking is as follows;

“Any device designed to prevent inadvertent transmissions from blocking RTF frequencies, should be installed in all stations capable of transmitting on aeronautical frequencies.”


2.2.  IFATCA Policy on RTF Frequency Usage is as follows;

“If a controller is providing ATS for two or more areas, the relevant channels must be located on the Controller Working Position being used.

If more than one RTF channel is being used, then suitable ‘retransmit’ facilities must be provided to enable all users to receive all transmissions. The ability to enable or disable ‘retransmit’ facilities should be provided.

Future systems should include technology that warns the controller in the event of a crossed transmission.

Independent backup equipment should be provided.

Communications with aircraft should only be undertaken within the Designated Operational Coverage (DOC) for the frequency being used.

Voice switch systems must include facilities to:

  •  mute individual frequencies (due to open microphone, etc. problems) which will also cancel the retransmit for that frequency;
  •  present equipment failure alarms and provide the ability to isolate equipment which has failed;
  •  select secondary equipment (i.e. receivers, transmitters and paths) at the Controller Working Position;
  •  indicate the frequency on which the last incoming call was made.”


2.3.  While both policies have relevant statements dealing with the same situation, being a blocked frequency, it can be concluded that there is no duplication. The policy in para 2.2 clearly describes requirements for ATC systems when it comes to air to ground voice communications. When drafting these requirements, TOC also considered the most common situations that occur, one of these being a blocked frequency. The policy that is reviewed with this paper clearly indicates the need for a system that should be installed in “all stations capable of transmitting on aeronautical frequencies”. This includes other systems than the ATC system.

2.4.  The original paper from 1984 concluded with a number of issues that are worth revisiting.


“Frequency jamming remains a very real operational problem”.

This statement seems to remain valid, since occurrences of frequency blocking are still reported today.


“No procedural solution acceptable for world-wide applications is practicable”.

Since the problems are of a technical nature, it is logical to search for technical more than procedural solutions.


“A technical solution on the flight deck should continue to be sought, but that such a solution must be acceptable to both pilots and airline operators”.

Currently, there are systems installed in aircraft that warn a pilot (by an audible alert) in case of 30 seconds of continuous transmission. These systems automatically stop the transmission after another five seconds. It is however not known what percentage of aircraft in service are equipped with such a system.


“Communications failure procedures should provide in a standard form, indication of the action to be taken by pilots in any particular phase of flight or any particular airspace, if R/T communications failure occurs”

Agenda item B.5.19. also addresses this issue. These procedures are developed and implemented, but are not always easily accessible to aircrews. The information and procedures on communications failure procedures is spread out over a number of ICAO Documents and Annexes, and also National AIPs.


“Communications failure procedures should provide a reliable basis upon which controllers can provide uninterrupted ATC service to those aircraft retaining R/T communications with the ground in a safe, orderly and expeditious manner, without impairing the freedom of action of the pilot experiencing R/T failure”

This seems to be a standard design requirement for current and new communications failure procedures and remains valid.


“Communications failure procedures should provide freedom from interference by inadvertent aircraft transmissions which cause temporary, but total, loss of R/T communications between air and ground to all other aircraft on the R/T channel affected.”

Same as above in 2.4.5, statement remains valid as a basic design requirement.


“A visual/audio warning be included in the specification for aircraft radio transmitters to indicate to pilots when transmission is actually taking place.”

New systems have been developed and installed, as indicated in 2.4.3. With this the additional remark that an aural alert seems to be more effective than any visual signal. One could argue if hearing another transmission is also a form of indication to aircrews.

2.5.  IFALPA Policy related to this subject is as follows;


Continuous Transmission

Continuous transmission on an R/T frequency due to:

a)  internal circuit malfunction of the transmitter; or

b)  stuck microphone; or

c)  inadvertent activation of the microphone; or

d)  inadvertent activation of the PTT line from other causes is responsible for situations that jeopardise the safety and economy of airline operations.

Text should be inserted, therefore, requiring that the transmitter should activate a visual display in the cockpit whenever a transmission has exceeded a pre-set time period and be cut off after a further period.


There is a requirement for the installation of the means for precluding inadvertent simultaneous transmissions on a given frequency.

Such development should consider the following factors:

i)  An override feature must be available to the pilot to allow emergency transmissions;

ii)  The design should preclude gaining priority by another user who maintains a continuous PTT action during periods of channel activity.”

2.6.  Normally spoken, one would expect that it would technically be feasible to create a feature in transceivers that would detect that the frequency is already used and then block any transmission from that transceiver. With this feature, the ‘interrupting’ sender of a message would immediately note that the transceiver is not transmitting, because the other transmission would still be heard. This feature has not yet been installed, the reason of this is unknown.


3.1.  Although substantial progress has been made with systems and procedures, the problems associated with frequency blocking have not yet been solved. Especially the feature that would prevent transceivers from transmitting while another transmission is in progress has not yet been installed throughout the aviation community.

3.2.  After review and discussions, TOC has decided that current IFATCA policy on frequency blocking remains valid and should therefore be retained.


It is recommended that;

4.1. This paper is accepted as information material.

Last Update: September 29, 2020  

March 29, 2020   443   Jean-Francois Lepage    2006    

Comments are closed.

  • Search Knowledgebase