Reduced vertical separation minimums

Reduced Vertical Separation Minimums Change is coming By Jim Sparks July / August 1998 It has been forecast that by the year 2015, there will be twice the number of aircraft flying as today. This will have a dramatic effect on the...


The 1979 World Administrative Radio Conference of the International Telecommunication Union extended the VHF FM broadcasting bands from 100 MHz to 108 MHz. This change was implemented in Europe, Africa, Russia, and the Middle East. The use of these upper range frequencies will increase the risk of interference to the VHF Aeronautical Navigation systems specifically, the ILS Localizer and VOR.

There is also some concern with VHF Communica-tion Receivers. The (ICAO) International Civil Aviation Organization, in association with the aviation industry, has developed FM Immunity standards for affected equipment. Recommendations outlined in ICAO Annex 10 are designed to improve receiver selectivity and dynamic range in order to limit levels of VHF FM broadcast interference. Technically, the changes are in hardware only. For communication receivers, a discrete filter may be added.

Navigation receivers are more complex and may require internal circuitry changes. Most manufacturers of VHF Airborne receivers have developed modification programs for their equipment and, as of January 1995, no manufacturer could sell or install noncompliant equipment. Even though January 1, 1998 was set as the date where all aircraft operating in ICAO jurisdiction should be in compliance, some member states may choose to adopt a more relaxed compliance schedule.

The next few years will bring about changes in the VHF Communication band that will affect all European operators as well as those international operators who fly into Europe.

European Air Traffic Control is currently handled by independent countries and is not integrated as is the case with the FAA in the United States. This lack of integration of the EATC creates areas of extreme congestion and frequency saturation. It is predicted that frequency saturation will not occur in the United States until the year 2005. Since 1958, the required number of communication frequencies double every 16 years.

This need has been satisfied by extending the range of the frequency and decreasing the channel spacing. The original broadcast frequency spectrum was from 118 MHz to 136 MHz, with channel spacing at 200 kHz intervals. In 1958, channel spacing was cut in half to 100 kHz, and a year later, the frequency range was extended up to 136 MHz. Five years later, it was necessary to reduce channel spacing down to 50 kHz. Our present 25 kHz spacing was implemented in 1974 and 1979 saw a second range extension to 137 MHz. Today's action plan calls for tripling the number of available channels by further reducing the channel spacing to 8.33 kHz. This takes the present 760 active frequencies to 2280 and with a long term solution of using Digital VHF transmission, it would make the 8.33 obsolete.

The transition to 8.33 kHz will begin with the new spacing being implemented at altitudes over 20,000 feet, but levels will come down as congestion goes up. Equipment manufacturers have developed plans to upgrade their units and in most cases, compliant equipment will have new part numbers and in-service units will be modified by service bulletins. Unfortunately, some older units will not be economically feasible to modify and will require replacement. In addition to receiver/transmitters being modified, the cockpit control heads may also require an update. Flight management systems (FMS) frequently have the ability to tune in communication radios, so they too will be affected by this change. Operators incorporating 8.33 kHz spacing will enter a "Y" in field 10 of the IACO flight plan to advise air traffic control of compliance.

Even though the changes discussed have an immediate affect on European and international operators, it is only a matter of time before RVSM, B-RNAV and FM Immunity will engulf the world-wide aviation community. FAA Advisory circulars and JAA information leaflets provide a good source of new information as does communication with airframe and avionic manufacturers. Change is coming — Count on it !

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