Review of 8.33 kHz Phraseology

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Review of 8.33 kHz Phraseology

41TH ANNUAL CONFERENCE, Cancun, Mexico, 15-19 April 2002

WP No. 86

Review of 8.33 kHz Phraseology

Presented by SC1


1.1 At the 40th IFATCA conference 2001 in Geneva, SC1 accepted the work item concerning the number of digits used in transmitting a frequency. This item was put on the agenda following the discussion that arose on the presented paper on multiple instructions in a single transmission.

1.2 8.33 kHz Phraseology was introduced on 7 October 1999, when 8.33 kHz spacing was implemented in the ICAO EUR region (8.33 kHz spacing was implemented in the following 7 states: Austria, Belgium, France, Germany, Luxembourg, the Netherlands and Switzerland. Via the Horizontal Expansion Programme (HEP), an additional 21 states will be added to the 8.33 kHz area by 31 October 2002). Several difficulties were reported since the implementation, and research is being done at this moment to improve 8.33 related phraseology.

1.3 This paper will address the impact of 8.33 kHz phraseology and the intended changes. Most recent developments within the European region demand IFATCA policy on this proposal.


2.1 The implementation and operational application of the 8.33 kHz procedures have raised several difficulties to the operational personnel. During and after its implementation, concerns were raised by both air traffic controllers and pilots. Confusion was reported and the following is a summary of the main concerns that have been raised:

  • loss of communication;
  • blocking of frequencies;
  • two different types of phraseology in the same airspace;
  • difference between frequency and channel values;
  • confusion for controllers in the number of digits to pronounce and which phraseology to use;
  • additional numbers to display, pronounce, hear, remember, read back and select for the pilots; and finally
  • confusion for pilots in when to add “5” or when to add “0”. The following examples illustrate this confusion:
    • Frequency 125.025 (25 kHz) is transmitted as “One Two Five decimal Zero Two”(125.02).
    • A 25 kHz pilot selects exactly as transmitted, while a 8.33 kHz pilot has to add “5”, and
    • Frequency 125.050 (25 kHz) is transmitted as “One Two Five decimal Zero Five” (125.05)
    • A 25 kHz pilot again selects exactly as transmitted, while a 8.33 kHz pilot has to add “0”. This confusion is due to equipment not in accordance with ICAO specification, where pilots are able to select frequencies not specified in the ICAO table of 8.33 frequencies. It leads to additional head down time, with its associated safety implications.

In relation to the operational problems with radio telephony (RTF) procedures Eurocontrol states:

“There was not enough help from ICAO with 7030 amendments, which meant that the programme staff had to negotiate themselves with the States. The 8.33 Team are not experts in RTF Procedures, and the experts in-house were not available to help. This also resulted in a lack of follow-up on the approval status of the phraseology. As a result there are some inaccuracies in the 7030 document.”

2.2 The operational difficulties were acknowledged and in order to simplify procedures Eurocontrol ATM Procedure Developments Sub-Group (APDSG) was tasked to review the existing 8.33 radio telephony phraseology in light of its operational exposure since its implementation. The APDSG proposed the following procedures: spelling out six digits when transferring an aircraft to another frequency or channel and dropping the word “channel” from radio telephony. Besides this, the deletion of the word “decimal” or replacing it by the term “point” was also considered. At a later stage, the proposal was accepted to reinsert the word “decimal”, since the requirement was acknowledged to assess the impact on ATC, both operationally and from the human factors point of view. The Air Navigation Team (ANT) tasked the APDSG to review deletion of the word “decimal”, as well as the possibility of using “point” instead of “decimal”. These changes would imply that transfer to the 8.33 kHz channel 132,205 should be transmitted as: “Contact (unit call sign) ONE THREE TWO Decimal TWO ZERO FIVE”. The amendments have been drawn up for implementation in ICAO Doc 7030, European Supplementary Procedures. IFATCA had the reservations recorded in the minutes, together with France, IFALPA and UK. This was mainly raised because of the dismissal of the word “decimal”, but also because of the dismissal of the word “channel”. IFATCA requested research on dismissing “decimal”, and also on the dismissal of the word “channel”, since the impact was not addressed at all. Finally, a review of the safety case was also requested by IFATCA. The proposal for amendments in Doc 7030 was discussed at the 43rd meeting of the European Air Navigation Planning Group (EANPG) in December 2001. During the discussion at this meeting, IFATCA, IATA and Switzerland called for further review and consequently the EUROCONTROL proposals were not accepted fully as presented, even though there was acknowledgement that there is a need to amend the ICAO provisions. As a result of this discussion, the requirement was set for the EANPG Secretariat, in co-operation with IFATCA, IFALPA, and IATA, to develop an agreed amendment to the ICAO Regional SUPPS for submission to the COG in Spring 2002.

2.3 Within the 24th meeting of the ANT, ICAO expressed concern with regards to the fact that two types of phraseology exist in the same airspace and that therefore the need exists to restore simplicity and uniformity. The fact that specific phraseology was developed and is being used to indicate equipment is also unacceptable to ICAO.

2.4 IFALPA believes that the problems encountered in the day-to-day use of (and compliance with) the Regional VHF 8.33 procedures confirm the initial concerns by IFALPA with regard to the channel designation scheme which have led to the suggestion of a complete review of these conventions. IFALPA does support the review and removal of superfluous phraseologies, but the use of the word “channel” should be retained as a required phraseology to indicate that three trailing numbers will follow the word “decimal”. IFALPA also states: “Pilots should not be required to use the word “channel” in readback to alleviate RTF congestion”, and “It should be noted that the ATS Committee was unable to find the regulatory basis for the European practice that frequencies are always read back by pilots”. IFALPA policy has been developed, and it has been concluded that it would be beneficial to retain the word “decimal” or possibly change it to “point”. Regarding the use of all six digits, IFALPA rather referred to the use of “significant digits”. An alphanumeric solution for the transmission of frequencies has been developed by IFALPA, as described in Volume V of Annex 10.

2.5 The use of the word “channel” as mitigation was included in the safety validation conducted by the Safety Regulation Committee (SRC). Controllers were instructed to use the word “channel” and transmit all six digits only for 8.33 kHz separated frequencies from the moment 8.33 kHz spacing was introduced. The numeral designation is used to identify an 8.33 kHz channel, with always three digits after the decimal. The word “channel” is used in all cases to emphasise the difference between a 25 and a 8.33 kHz spaced frequency.

The separate indication of an 8.33 kHz channel was also used for two reasons:

  1. as a mitigation factor to stop unequipped aircraft entering 8.33 airspace, since the display of non-equipped status is not always available to the controller. It is also possible that incorrect information is available, because of last minute changes in a flightplan (e.g. change of aircraft from equipped to non-equipped); and
  2. to avoid communications of 25 kHz radios in 8.33 kHz airspace.

2.6 The use of six digits in transmitting a frequency will have two problems:

  1. an increase in frequency congestion due to longer transmission time. This will result in greater frequency occupation time and a higher workload resulting in a reduction in sector capacity (Eurocontrol Capan methodology), and
  2. an impact on the short-term memory of the flightcrews.

Several tests on human capability to deal with lengthy figure groups have been performed in the past. The “digit span test” (By Miller (1956)) was developed to assess one’s capability to remember information from one presentation, by instructing a person to read back a number of digits in the correct sequence. The number of digits is increased by one, until the person can no longer recall the entire sequence correctly. Subjects were able to repeat no more than five to nine items, with an average of seven. These results were obtained under optimal conditions with a clear signal and when remembering the digits was the only task to be performed; performance on the same task with other concurrent duties, as in an ATM environment, would be expected to be poorer.

2.7 With present day RTF procedures, readback errors involving radio frequencies are the most common type. This is very understandable, since a frequency is not as crucial to the flight-crew as, for example, a flight-level or a heading-instruction. The number of errors in readbacks of frequencies results in increased frequency congestion and increased controller workload, as more communications are necessary to correct the misunderstanding. Depending on the nature and the timing of a readback error, miscommunications have the potential of narrowing the margin of safety to an unacceptable level.

2.8 ICAO Annex 10 Volume II clearly states:

“In all communications, the consequences of human performance which could affect the accurate reception and comprehension of messages should be taken into consideration.”


3.1 New radio telephony procedures should maintain or improve current safety levels.

3.2 Research is needed on the human capability of dealing with lengthy figure groups in an ATM-environment with, or without a pause created by the term “decimal” or “point”, but also on the increase of controller workload caused by longer transmission times.

3.3 The deletion of the term “channel” should be addressed by review of the 8.33 Safety validation study. The need for pilots to read back “channel”, as long as all six digits are transmitted to identify 8.33 compatibility, should be investigated as a possible alleviation for radio telephony congestion.

3.4 The impact of dismissing the word ”decimal” should be addressed. The deletion of either “decimal” or “point” while transmitting a frequency would lead to a group of six digits without a pause, again adding to the pressure on the short term memory. Replacing the term “decimal” by “point” might be confusing, since the term “point” is already used in aviation terminology for defining a geographical location, but also several terms as “missed approach point” and “holding point”. On the other hand, the term “decimal” is unique in its use and can only be related to the transmission of a frequency or channel. The need for pilots to read back “decimal” should be investigated as a possible alleviation for radio telephony.

3.5 Flightcrews entering, or leaving the ICAO EUR region will be confronted with two different types of phraseologies during their flight. Since the different phraseology is no longer being used to indicate a type of equipment, there will a potential risk of aircraft entering an 8.33 area without the required equipment. With two different types of equipment in one airspace it could be necessary to have two different types of phraseology, to ensure that the right type of equipment is being used.

3.6 All 8.33 kHz related procedures and superfluous phraseologies will have to be reviewed in order to solve the concerns expressed by the operational personnel. Transmitting six digits in all cases and the removal or replacement of the word “channel” might be one of the most effective and desirable methods to solve these concerns. Other potential solutions as an alphanumeric system and the removal of the first digit in transmitting a VHF frequency or channel have been left out of consideration and should be reviewed in this light.

3.7 There is a demand for IFATCA to act rapidly to these proposals and co-operate in developing an agreed amendment to the ICAO Regional SUPPS.

3.8 As a long term, and preferably global, solution, IFATCA should develop joint policy with IFALPA, and other involved parties, on identification of communication channels.

3.9 These fundamental issues should be taken into account, when developing new phraseology:

  • New RTF procedures should not have a negative impact on controller workload;
  • Any change in RTF procedures must be based on proper research in human capabilities and limitations; and
  • Any future procedure on number of digits used in transmitting a frequency must address safety implications and minimise the influence on the Human Machine Interface (HMI).


It is recommended that:

4.1 The use of the word “channel” by ATC should not be removed from radio telephony without an independent safety review. However, in readback, the pilot may omit the word “channel”.

4.2 The use of the word “decimal” should be retained in transmission of frequencies and channels.

4.3 Six digits shall only be used in transmitting an 8.33 kHz spaced channel.


Aeronautical Telecommunications, Annex 10, Fifth Edition of Volume II (Communications Procedures), Chapter 5 (Aeronautical Mobile Service); International Civil Aviation Organisation; July 1995.

Regional Supplementary Procedures, Doc 7030/4, European Regional Supplementary Procedures, Part 1 (Rules of the Air, Air Traffic Services and Search and Rescue); International Civil Aviation Organisation; Fourth Edition; 1987.

Eurocontrol 8,33 user guide version 4.0, 19 April 1999.

8.33 kHz Programme, Successes and Opportunities for Improvement; Edition 1.1; 8 May 2000.

Program Steering Group meeting 2, 8.33 Horizontal Expansion Programme; draft meeting report.

APDSG/26 WP 26.4, 8.33 kHz Channel Spacing.

APDSG/27 WP 27.12, 8.33 kHz ATM Procedures, Revision of ICAO Doc 7030/4 relevant parts.

ANT/24 meeting report.

ANT/24 WP 24, R/T Phraseology, 8.33 kHz Channel Spacing.

ANT/25 draft meeting report.

ANT/25 WP 6, Review of 8.33 Related Phraseology – Proposal for Amendment of ICAO Doc 7030/4 – Eur.

IFALPA statement 8.33, 1 November 2000.

IFALPA Annex 10 (Com), Volume III Part 2, Chapter 2 (Aeronautical Mobile Service).

IFALPA Annex 10(Com), Volume V, Chapter 4 (Utilisation of frequencies above 30 MHz), July 1998.

Cognitive Psychology, Karl Haberland (Trinity College) second edition 1997.

An Analysis of TRACON (Terminal Radar Approach Control) Controller – Pilot Voice Communications, Kim M. Cardosi, Bryan Bett and Sherwin Han; Report number DOT/FAA/AR-96/66, June 1996.

An Analysis of En Route Controller – Pilot Voice Communications, Kim M. Cardosi; Report number DOT/FAA/RD-93/11, March 1993.

EANPG/43 – WP/17, Radio telephony phraseologies (RTF) related to 8.33 kHz channel spacing.

Last Update: September 29, 2020  

March 16, 2020   656   Jean-Francois Lepage    2002    

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