Reduced vertical separation minimums

July 1, 1998

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 way these machines are operated. We will need to accommodate twice the number of flights in the same amount of airspace. There will of course be the need to develop additional airports, and therefore a need for additional radio channels to enable precision navigational capabilities.

Many European aircraft operators, as well as US aircraft traveling internationally, have already had first-hand exposure to some of the operational changes. In time, every corner of the globe will be affected.

These new programs include RVSM, B RNAV, FM Immunity and 8.33 kHz spacing. As with most operational changes, maintenance procedures will also be amended. Reduced Vertical Separation Minimums (RSVM) provide a means of increasing air traffic on some existing routes.

The FAA, in conjunction with the International Civil Aviation Organization (ICAO), undertook a study to increase enroute airspace capacity in the heavily traveled North Atlantic and determined that RVSM would be technically feasible without imposing unreasonable demands. It would now allow air traffic control to maintain 1000' vertical separation between aircraft where 2000' used to be required. This would double the number of aircraft in some of the high density air routes in any given time period.

The North Atlantic Minimum Navigation Performance Specifications (NATMNPS) includes the procedures for RVSM operations in the altitude range between 42,000 feet and 28,500 feet. No one is allowed to enter this airspace unless RVSM approval is granted by the country of aircraft registration. Any aircraft not approved can still cross the North Atlantic, but must either fly above FL 420 or below FL 285.

To become eligible for RVSM operations, the aircraft will have some system requirements including:
• Two independent altitude indicating systems
• A transponder with altitude reporting capability - if only one transponder is installed, it will need to be able to obtain altitude data from either altimeter system • An altitude alert system.
• An automatic altitude control system

The approval process begins with the airframe manufacturer developing a data package including the definition of the aircraft group; i.e. aircraft that are identical in design and have identical static systems and like-avionic systems. The data package will also include a definition of the flight envelope, as well as required information to show compliance with RVSM. There is also a procedure to ensure all aircraft submitted for airworthiness approval meets RVSM requirements along with the information to be used to maintain RVSM integrity while in service.

Once the manufacturer has compiled the data package, it is then up to the individual operator to ensure the specific aircraft meets the requirements. Often this includes a detailed inspection of the area around and in front of the static ports. The object of this inspection is to check for skin waviness or deformation, as well as rivet condition and evaluation of any previous damage or repair. In some cases, specialized jigs are used to detect any skin surface anomalies. Some aircraft types will require repeat inspections at predetermined intervals. Static defects need to be determined in varying flight regimes and may require correction.

Most manufacturers of Air Data Computers have programs in place to tighten the tolerance on their units to stay within RVSM standards. Once the aircraft is in compliance, an RVSM operations manual is developed along with a minimum equipment list (MEL) and maintenance procedures. This entire package is then submitted to the local regulating authority such as an FAA Flight Standards District Office (FSDO). After satisfying the regulating agency, authorization is given to conduct a test flight over one of the two height monitoring units (HMU) which are located at stations in Gander, Newfoundland and Strumble, United Kingdom. An option to a ground station flyover is to temporarily install a differential global positioning sensor (GPS) for a test flight. By utilizing satellite positioning, a very exact height measurement can be obtained. Altitude that is displayed in the Flight Deck is recorded and then compared with actual height determined by the HMU or differential GPS.

The purpose of the HMU flight is to verify the autopilot maintains the precise assigned altitude with a 65 foot tolerance. After successful completion of the HMU flight, RVSM approval should be granted.

Once an aircraft is capable of maintaining Reduced Vertical Separation, the maintenance program has to be modified to enable continued operation. This typically includes inspection of the static ports, operational tests of the auto flight system, and very precise testing of the altitude reporting and indicating systems. The testing of altimeters for RVSM will require the use of specially approved air data test equipment.

Aircraft service center personnel should inquire about RVSM status when conducting maintenance operations on the static system and autopilot, or when initiating skin repairs that may affect the airflow over the static ports. Specialized training on test equipment and RVSM procedures will be required for the authority to return them to service.

Most airframe manufacturers whose products are RVSM qualified will provide specific qualifications for each airframe and additional information can be found in FAA Advisory Circular 91 titled, RVSM Interim Guidance Material, or ICAO - NAT DOC 002 titled, Guidance Material for RVSM.

Basic Radio NAVigation (B-RNAV) will accomplish laterally what RVSM does vertically, allowing the present 60 nautical mile lateral spacing for enroute aircraft to be cut in half. Conventional aircraft navigation is based on the use of ground based radio stations called Nav Aids, specifically very high omni directional range (VOR) and distance measuring equipment (DME). Present air traffic routes utilize station to station navigation which lays out a series of highways in the sky.

Area, or radio navigation (RNAV) is a method to allow aircraft to operate on direct routes between two points without the need to fly over ground stations and still maintain a specific degree of position accuracy. The degree of accuracy is outlined in the Manual on Required Navigation Performance (RNP) and is listed in miles. That is, RNP 5 requires the navigation system to have an accuracy within five nautical miles at least 95 percent of the time, and RNP 1 would require a one nautical mile accuracy.

B-RNAV will provide greater flexibility in airspace use, shorter flight distances, and subsequent fuel savings. To be capable, an aircraft must contain navigation systems that use one or a combination of the following sensors: VOR/DME, DME/DME, INS, LORAN C, and GPS to Technical Standard Order, TSO C-129.

The system must also be able to provide continuous indication of aircraft position versus navigation route, as well as display the distance and route to the next waypoint.

There is also a requirement for the system to have at least four waypoints manually inserted. Finally, the system has to be able to recognize when any of the sensors has failed. Some of these sensors do have limitations such as Inertial Navigation Systems (INS). Without the ability for receiving radio updates, INS may be used for two hours maximum from the last alignment.

An aircraft can be considered eligible for B-RNAV operations if the Aircraft Flight Manual shows the navigation system installed has received airworthiness approval in accordance with one of the following FAA Advisory Circulars: AC 90-45A, AC 20-121A, AC 20-130A, AC 20-138 or AC 20-15. A letter of authorization is not required when the eligibility is based on the flight manual. The European Commonwealth implemented B-RNAV to RNP 5 this past April, but will allow exemptions until August 1998. RNP10 went into effect at the same time in the Pacific sector.

The steps for an operator to receive approval is about the same as those for RVSM; that is, application package, certification of equipment, and training programs. U.S. registered aircraft filing flight plans into European B-RNAV designated airspace are expected to meet the European requirements, and operators should indicate their approval for B-RNAV/RNP-5 by placing the letter "R" in block 10 (equipment) of the ICAO flight plan.

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 !