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...


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.

ImageOnce 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.

ImageAn 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.

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