Report on the Introduction of 8.33 kHz Spacing

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Report on the Introduction of 8.33 kHz Spacing

38TH ANNUAL CONFERENCE, Santiago, Chile, 15-19 March 1999

WP No. 99

Report on the Introduction of 8.33 kHz Spacing


On 30th April 1998 together with Dave Grace I attended a meeting at Eurocontrol with Mr Christophe Hamel, chairman of the 8.33kHz Project Management Cell (PMC), to discuss the introduction of 8.33kHz spacing. Dave Grace also attended a meeting to consider the term of reference of the Safety Validation Group and the Hazard Analysis requirements. Chris Gilgen attended the SVG meeting on 4th August 1998 in Brussels.


Briefing by Christophe Hamel 30th April 1998

Mr Hamel was very keen to explain the progress of the project and take on board our concerns both at the meeting and in subsequent telephone conversations. We are grateful to him and also to his colleagues Antonio Astorino and Gillian Heath for providing detailed information and replies to our questions. Mr Hamel explained that the early priorities of the project had been the technical issues. The priority now is the development of the operational procedures and attention to the remaining operational issues.

The plan for converting to 8.33kHz involves identifying the sectors where existing frequencies will be transferred to 8.33kHz spacing. These frequencies will initially be changed from 25kHz to 8.33kHz operation while remaining on the same actual frequency. E.g. should 132.0 be identified, it will be changed to 132.005, the equivalent 8.33kHz channel. An initial period of operation after this change (probably around 2 to 3 months) will take place to ensure that there are no problems such as interference.

Some of the existing frequencies to be changed to 8.33kHz will be within the 8.33kHz sub-band, but many of the frequencies to be changed to 8.33kHz will currently be using 25kHz frequencies outside the 8.33kHz sub-band. A program of frequency swaps will take place to move the 8.33kHz frequencies inside the sub-band. Each 8.33kHz frequency outside the 8.33kHz sub- band will be swapped with a 25kHz frequency inside the sub-band. This will gradually free up additional frequencies in the 8.33kHz sub-band and in the 25kHz band to provide the additional capacity required.

The actual introduction and swapping of the frequencies will be done by each State in co- ordination with Eurocontrol. The initial 2 to 3 months period is intended to ensure that there are no fundamental technical problems with the system. Once the swaps into the 8.33kHz sub-band start to take place, it must be realised that some of the 8.33kHz frequencies inside the sub-band may be allocated to the new channels being created (i.e. to those which are not exactly equivalent to 25kHz frequencies). This will be the first point that the new closely spaced channels will come into use.

The swap process will populate the 8.33kHz sub-band with the new frequencies but Eurocontrol advise that, due to frequency protection considerations, it will not be possible to remove all 25kHz frequencies from the 8.33kHz sub-band.

Timescales and Equipage rate

The introduction of 8.33kHz has been the delayed until 7th October 1999. The delay is a technical delay to allow the airlines time to equip and also due to some certification problems. This delay is felt to be to our (ATC) advantage as it will ensure a high equipage level and time to put the operational procedures in place. The 8.33 PMC intend that all the operational measures will be in place by 31st Dec 1998 so that it will be possible to start the operators are equipped.

The indication of 8.33 on the flight plan will still start in October 1998. The 8.33 PMC reports that there is a good feeling that the airlines are taking it seriously and will be equipped for the 7th October 1999 deadline. It is hoped to achieve equipage of 98% by then.

There is a need to have this very high aircraft equipage level for the system to work. It was acknowledged that there had been problems with the introduction of BRNAV due to the low level of equipage and a repeat of this situation would not be acceptable for 8.33kHz as it would result in a serious loss of capacity in the lower airspace.

A simulation tool called SAAM will be used to assess the impact of 8.33kHz operations. Each sector/area can be assessed according to the actual percentage of equipped aircraft operating or planned to operate through this area. An assessment can then be made on the amount of re- routing or flow restrictions that will be required and acceptable equipage trigger values will be set for each area.

Quality and Interference

We were pleased to report that our concerns over the quality have generally been answered. We expressed concern over the lack of realistic tests with operational traffic. A test with live traffic is planned for Maastricht in June/July 1998.

We expressed concern over frequency blocking. It was agreed that further investigation is required as the present problems and implications are not fully understood. A safety analysis of the 8.33kHz system is ongoing and this will be an important aspect of this analysis. We pointed out that this is rather late in the day and the opportunity to make changes if unacceptable problems are identified is extremely limited.

UHF coverage for military aircraft will generally be provided for all sectors but the details of the implementation are not yet clear as to if a separate UHF frequency will be provided for each sector (a one to one pairing with VHF frequencies), or in some cases a shared frequency may be provided for use by several sectors. This will depend on the demand for UHF in each area.

Flight Planning and Operational Issues

The news that France will initially implement 8.33kHz only above FL245 initially is very helpful in simplifying some of the operational problems but there remain many important issues to be considered. The principle means for managing the situation is through the flight plan. Non- equipped aircraft will not be permitted to file repetitive flight plans above FL245. Other flight plans will be checked and either rejected or warnings sent as detailed in the user guide. We again raised the issue of late notice aircraft changes from an equipped to a non-equipped aircraft. If a flight plan change message is not filed, we believe there remains a serious risk of this aircraft infringing the 8.33kHz area. The only remaining chance to recognise the problem is when the pilot is asked to contact “Channel…” at which point he must recognise that this is not an acceptable frequency and advise the controller. This may well be too late, as the aircraft may have been cleared above FL245 or into an 8.33kHz sector. The workload in re-routing this aircraft would be unacceptable (see para on re-routing below). (Should this fail, the aircraft will enter the 8.33kHz area unable to communicate!)

The handling of aircraft within the EUR region is still of great concern. Firstly, for flights wholly within the EUR region, the general principle is that a non-equipped aircraft may not fly above FL245. For aircraft flying entirely within the non-8.33kHz areas, (e.g. Edinburgh to Stockholm or a Spanish domestic flight) an exception is permitted to allow these flights above FL245. The exemption will be indicated on the flight plan and these flights do not appear to cause any problems. The more contentious issue will be flights crossing into and out of the 8.33kHz region. As an example a flight from Glasgow to Naples routing via France. The intention of the ICAO policy is that a non-equipped aircraft on this route will not be permitted to fly above FL245 THE WHOLE WAY. The problem is that both the UK and Italy are initially non-8.33kHz States and either or both could exempt the flight. In which case it would be permissible for the flight to climb to FL310 after take-off, descend to FL240 over Southern UK to cross France and then climb back to FL310 for the remainder of the journey over Italy. While this is not intended, it will be entirely up to the States concerned to authorise exemptions for flights like this and we must ensure that they are aware that we consider this to be unacceptable.

The next issue concerns flights entering the EUR region. Equipped flights will not cause a problem as they will maintain their cruising levels until the descent to land as at present. The issue concerns non-equipped flights entering the EUR region. The Eurocontrol position is that the EUR region will define the carriage of radios it requires for this region and that it will be up to other surrounding regions to send us (EUR region) aircraft at an acceptable level. This will require States surrounding the EUR region to identify and descend non-equipped aircraft (except for exemptions). The arrangements for doing this will be made individually by the states concerned by Letters of Agreement and it is essential that these are in place before 8.33 is implemented. It must be emphasised that these surrounding States must make allowance in sector capacity figures for this extra workload and that operational sector controllers must be actively involved in the development of these plans.

When these non-equipped flights flight plan into the EUR region, the correct way to submit the flight plan will be to indicate the requested flight level (above FL245) on the route outside the EUR region and then to indicate a level change to a level below FL245 to get under the 8.33kHz area. In this case the flight plan will be accepted and no warning issued. It is of concern that many will not do this, but will flight plan above FL245 all the way to destination. Should this involve entering the 8.33kHz area in error, a warning will be issued that this flight plan does not comply with 8.33kHz requirements. While this warning will be distributed to all centres in the EUR region, there remains concern that it will not reliably reach the controllers who actually need it. Also of great concern is a flight which plans above FL245 all the way to destination and which incorrectly indicates 8.33kHz equipped (e.g. last minute aircraft change or far distant aircraft operator who does not understand the system). In this case the aircraft will approach the 8.33kHz boundary and require last minute re-routing.

The UK has recently advised of some concerns about last minute re-routing. It appears that this may be the most serious operational consequence, and in some cases may be as disruptive as the frequency blocking problem. To take the UK example, should a non-equipped aircraft enter the UK from the Atlantic and not be recognised (last minute aircraft change/mistake on flight plan/warning message not indicated), it will only be when the aircraft approaches the French boundary that the problem is recognised. In this case the aircraft must either divert round the 8.33kHz area or descend below FL245. Either of these options will require an immediate change to the flight plan.

This raises many questions. The sectors below FL245 or any re-route sectors will not have the flight details. This flight will not have been allowed for in the flow measures so this re-route/level change potentially overloads these sectors and the sector could potentially refuse to take it as it does not have a slot for that route. The huge co-ordination effort required for this operation at short notice does not bear thinking about and would quite likely overload the last 25kHz sector handling this flight, especially if holding or circling to loose height before the 8.33kHz boundary was involved.

While the above is a specific case, it will be similar for all States on the periphery of the EUR region. In addition, a very similar problem occurs within the 8.33kHz area when an aircraft takes off and because of the reasons above attempts to climb above FL245 when not equipped. Very similar co-ordination problems exist. It has been suggested that these sectors should confirm the 8.33kHz capability of aircraft before clearing them into the 8.33kHz area. The idea the every aircraft entering the EUR region and every aircraft taking-off within the region should be checked on RTF has been rejected due to the workload involved. There are also issues concerning weather avoidance and diversions to non-planned airfields (e.g. for operational or medical reasons) which must be planned for. With all these operational problems, the 8.33 PMC consider that there will not be many aircraft non-equipped, therefore these problems will be small-scale.

Regarding the issue of “cheating” (non-equipped aircraft filing as equipped). The 8.33 PMC consider that this should rare due to the high equipage and each state must have strong measures to combat this (e.g. insist any aircraft causing a problem must land immediately and not proceed to destination). This is a self defeating situation which overloads the system and delays everybody and the other airlines should not allow it to continue. This should be the reason to put pressure on the offending company not to do it again. Again, this is up to each state to do this and it is easier said than done. Also, the problem has already happened so it does not immediately help ATC.

It was agreed that non-8.33kHz capable designation on strips/data displays/radar screens as appropriate was desirable so that when all aircraft were equipped it would die a natural death! Our concerns about overloading of other items e.g. RVSM, Datalink etc. shown on radar were passed on, although this is probably only a transitional issue until everyone is equipped.

These operational issues are the main cause for concern over the introduction of 8.33 kHz and they are the main subject of the considerations by the Safety Validation Group (SVG).

Concern exists that when there is the possibility of sub-version that CFMU initiates flow measures to prevent overloads.

Other issues

On the question of Datalink, 8.33kHz radios are designed to be Datalink (VDL Mode 3) compatible. Most 8.33kHz radios are being purchased brand new to replace existing sets. These should be upgradable at a reasonably low cost. One company is upgrading existing 25kHz radios to 8.33kHz and these may be more expensive to upgrade to Datalink.

The ANT has reviewed the phraseology and passed it to ICAO for approval. We were advised that this does comply with ICAO requirements.

The Safety Validation Group (SVG)

The SVG has been tasked with identifying the safety issues associated with the introduction of 8.33 kHz and proposing mitigation measures that will allow 8.33 kHz to be introduced with an acceptable level of safety. Chris Gilgen attended the SVG meeting on 4th August 1998 at Eurocontrol, Brussels.

I was concerned by the fact of how far these office people in Brussels and aviation consultants are away from the operational reality of an ATCO or a pilot working and flying the summer traffic peaks of 1998! Good participation and support of our views by UK NATS (operational people) and the representatives from Belgium, Switzerland and also Maastricht. Pity that the pilot side was not present at this meeting, as I believe they would have supported our interventions and comments.

The SVG identified a large number of operational issues which could present a risk to safety if no action was taken to limit the occurrence of these problems. Mitigation measures will be designed to limit these problems where possible.

There was a recent meeting of the Project management Cell which unfortunately IFATCA could not attend. However the UK representative who did attend provided IFATCA with the following comments:

The PMC had some problems with the the SVG report ,issues that they did not agree with and some items which they considered factually incorrect. As a result the consultants have been asked to redraft the final report. It is anticipated that IFATCA will receive a copy of this report.

The IFATCA policy as prepared at the ERM in Oslo was presented to the meeting. This created a great problem as the policy was presented with no written rationale. With the inability to have a direct discussion with IFATCA at the meeting, the report was noted.

A new situation has been brought up by the SVG report. During the British Airways test at Bournemouth it was found that 8.33 and 25kHz equipment operating on the same frequency could be heard and understood OK. The report has proposed that, where an aircraft approaches the 8.33 boundary and at the last moment it is found to be non-equipped, it would be possible to transfer it to the 8.33 frequency in any case. tHe proposal is that the aircraft is transferred to the 8.33 frequency. This means that although it will enter the 8.33 area, it will still be able to communicate. The first 8.33 controller will then have the responsibility to descend the aircraft or turn it back. This is an interesting proposal. It does mean that the aircraft will not become a radio failure , but will be able to talk. On the down side, it gives the first 8.33 controller an increased workload.

The current plan is to go ahead in Oct 1999 with the introduction of 8.33. At this time it would not be possible to have a backup 25kHz frequency for the situation above. In this case this proposal might be helpful. In Jan 2000 it is planned to move the 8.33 frequencies into the sub- band. This will free up 25kHz frequencies and it would allow the possibility that a spare 25kHz could then become available for a backup in the above situation. The UK has proposed that a safety study should be done to ensure that the 25kHz and 8.33 equipment can communicate safely in the above situation and this is being done.

While there are a number of issues still to be resolved, the main action now seems to have moved from the PMC on to LOA (Letter of Agreement) meetings. All 8.33 States must have LOA’s with surrounding states to determine how non-8.33 aircraft will be handled. These meeting are now underway and the LOA’s will contain details of the procedures to be adopted. It will be up to the States to ensure that these agreements are satisfactory from an operational point of view and that the procedures are safe.


It would appear that there are still many issues of concern regarding the 8.33kHz project. It is possible that many of these issues can be dealt with provided that suitable mitigation solutions are implemented in a timely manner. Of particular concern are the operational issues highlighted above, the safety analysis including the frequency blocking issue, and that an adequate level of equipage will be achieved.

IFATCA should define those items and procedures that must be in place before 8,33kHz spacing is implemented.

IFATCA must now decide what to do next. Our inputs so far have been rather fragmented. The next 8.33 PMC is 8/9 March. This will discuss any remaining points from the SVG report and implementation issues. It is debatable if IFATCA can have any significant input here, as it seems as if it might be too late for much to be changed. It would seem perhaps that the main area of interest has moved to the LOA meetings where the specific procedures are being sorted out. Perhaps it might be better to step back from the PMC and recommend that MA’s get involved in the LOA discussions to ensure that the procedures being agreed are safe and workable.

It is recommended that:

Although it is recognised that 100% equipage will not be achieved and in line with the Safety Validation Groups conclusions based on the assumption of an equipage rate of 95%, IFATCA believes that prior to the introduction of 8.33kHz the following conditions must be met:

  1. Appropriate “filtering/gate keeping” procedures must be in place in the ECAC area and surrounding states.
  2. Detection of 8.33 KHz carriage by IFPS and the display of non-equipped status to the controller must be in place.
  3. Education programmes for pilots and controllers must be completed. This is particularly important in states surrounding 8.33KHz airspace which will perform the “filtering/gate keeping” procedures.
  4. Procedures which consider a controller as the principle means of mitigation are unacceptable.
  5. Last minute diversions and sub-versions of non-equipped aircraft directly affect the capacity of the sectors involved therefore such re-routings must be kept at the absolute minima in order that the safe operations of the sectors involved are not degraded.
  6. CFMU shall not re-route automatically non-equipped aircraft into 8.33kHz sectors.
  7. 121.5 cannot be considered as a contingency frequency for non equipped aircraft nor can it be used to re-route or divert aircraft that are not equipped.
  8. A 25kHz independent contingency frequency must be available to re-route non-equipped aircraft.
  9. Non-equipped medical flights will only be accommodated in the case of an in flight emergency.
  10. The introduction and use of 8.33 kHz spacing must be proven to meet at least the current target levels of safety (i.e. 25 kHz).

Last Update: September 28, 2020  

March 10, 2020   586   Jean-Francois Lepage    1999    

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