8.33kHz Voice Channel Spacing communications

07-09-2017 09:45:40 - Radio equipment

The fundamental axiom in aviation is: “Aviate, Navigate, Communicate.”  The first priority is to “aviate” - fly the aircraft and keep it safely in the air; secondly, pilots must navigate - fly the aircraft towards the destination.  The final step involves communicating.  Whether this is for talking with other aircraft that share the same airspace, with the relevant Air Traffic Services Units along the way or just for receiving important information about weather or conditions at the destination airport, all these involve using the radio equipment on board. These series of articles will try to elaborate on the principles of radio communication, the need for channel spacing, correct usage of the equipment as well as on the changes to be expected when transitioning to an 8.33kHz voice channel spacing environment.

Chapter 3 – Safety hazards related to 8.33kHz Voice Channel Spacing

A series of typical hazardous scenarios

Following the introduction of 8.33kHz channel spacing communications in the lower airspace several hazardous scenarios may develop in day to day operations. These may be especially of importance for the General Aviation airspace users and are presented here as awareness for all pilots operating in an environment where radio communications are used:

  • Unintentional or forced deviation (due to technical or weather reasons) entering in a controlled airspace by a non-8.33kHz capable equipped aircraft operating from an uncontrolled airspace.
  • Unintentional planning of a flight by non-8.33kHz capable equipped aircraft into an area in which 8.33kHz radio carriage is mandated
  • Incorrect frequency change due to misunderstanding of the ATS clearance/information

It has to be noted that the last scenario is valid even in today 25kHz voice channel spacing environment, but it was included here due to the increased probability of occurrence in an environment requiring 8.33kHz capable radio equipment and new communication procedures

Associated Hazards

Safety related hazardous situations in the context of the deployment of 8.33kHz channel spaced communications are mainly related to: lack/loss of communications, impossibility to contact air traffic services when required (including the reception by pilots of  essential information regarding the flight conditions, traffic, weather), increased controller workload due to accommodation of non-equipped aircraft, etc.

Basically such incidents usually occur in the case of radio Interference, mismanagement of communications equipment, wrong application of prescribed procedures and requirements or, in most unlikely cases due to a malfunction of communications equipment.

A non-exhaustive list of hazards associated with or impacted by the introduction of 8.33kHz channel spaced based communications is provided below for the awareness of the pilots operating in such environment. Depending on the local factors (e.g. traffic characteristics, airspace definition, procedures etc.) some of these hazards may not be applicable (or could have a low probability of occurrence) while other new hazards may be identified.

  • Non-8.33kHz equipped flight entering an area where 8.33 kHz communications are provided.

Depending on the traffic density and airspace characteristics, such an event may create severe problems and disruptions in a controlled airspace environment. It has to be noted that this hazard may occur as well for non-equipped flights trying to land on controlled aerodrome in which communications are handled in 8.33kHz channel spacing. The inability to tune to a 6-digit 8.33 kHz channel means that ultimately the pilot may not be able to communicate or receive instructions or information from an air traffic service. Moreover, if the pilot attempts to communicate on a 25 kHz channel, then there is a risk of interference on the adjacent 8.33 kHz channels

  • Selection of wrong frequency by a pilot.

Mistuning can be considered also a hazard for 25kHz spaced communications in today’s environment. However the addition of an extra digit and sometimes uncertainty about phraseology, may increase the probability of occurrence of this hazard which could lead to communication problems, including additional pilot and controller workload

  • Unplanned flight diversion due to equipage requirements.

This hazard occurs when a flight with a non-8.33kHz capable quipped aircraft is erroneously planned through an area in which 8.33kHz equipage is required. This hazard is applicable as well for flights planned to land to controlled aerodromes where communications is exclusively performed in 8.33kHz channel spacing; in such cases the flight may be required to divert to an alternate aerodrome compatible with the equipage on board. This hazard may result in additional controller workload, and the diversion itself may present a risk to the affected the flight and other aircraft

Potential Safety Effects

Depending on the environment and traffic density and characteristics, as well as the type of air traffic services affected by the 8.33kHz induced communication hazards the following consequences could occur:

  • Inability for the pilots to receive (and therefore to follow) an ATC clearance, leading to loss of separation and possibly an AIRPROX;
  • Inability to receive essential information regarding the flight (e.g. traffic information, weather, etc.) which may lead sometimes to loss of safe separation between aircraft, late diversion, unsafe flying conditions for the type of aircraft and the flight rules
  • Impossibility for a pilot to pass essential information to the air traffic services, with effect on the safe ATC provision
  • Misuse (or excessive use) of the emergency dedicated frequency for routine communications (remember that 121.5 MHz emergency frequency remains in the 25kHz spacing)

Risk Mitigation

Several means can be put into place by flight crews, pilots or service providers to mitigate for the effects of the above mentioned hazards. A non-exhaustive list of generic risk mitigation measures (also referred to as safety requirements in safety assessments) is provided for illustration below.

  • Refresher training for pilots shall contain new procedures and specificities of 8.33kHz channel spacing communications
  • New pilots shall be trained in relationship with the implementation of 8.33kHz channel spacing communications, regarding especially RMZ carriage requirements, contingency procedures, phraseology, etc.
  • Better flight planning, including consultation of AIP and AIC and correlation of the communication requirements with the data contained on navigation charts

A few recommendations for actions mitigating against VHF communication problems are provided below as good practice guidelines:

  • Good radio discipline is a key defence against communication problems (for a extended description of radio discipline practices please consult
  • Flight Crew or pilots, the radio system on board allowing,  should not alter the previous frequency on the pre-select position on a COM radio used for primary ATC communications until two way communications have been established on the new frequency/channel.
  • Flight crew awareness of the likely frequency sequence can be helpful. In the event that a loss communication or impossibility to receive information on a specific frequency is discovered, reference to radio navigation charts, and procedures can provide contingency or back-up solutions


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