Any type of navigation system can be used to provide RNP, provided that it meets the required navigation performance accuracy. RNAV equipment operates by automatically determining the aircraft position using inputs from one, or a combination, of sources such as VOR, DME, Inertial Navigation Systems (INS), and global positioning systems (GPS). The primary means for achieving RNP is by the use of radio navigation equipment.
Currently, a large percentage of commercial as well as private aircraft are outfitted using varying combinations of the above equipment leading to varying degrees of performance and RNP-type approvals. In many cases the aircraft flight manual is the only document needed to prove aircraft compatibility to the local airworthiness authority. The term RNP is most often followed by a number such as “1, 5, or 10”. This implies the statute mile distance for the respective RNP level. Requirements for aircraft operating in RNP-10 airspace include the navigation capability to maintain strict lateral position sensing with an error factor of less than 10 nautical miles 95 percent of the time. Stand alone Inertial Reference Sensors tend to have a certain amount of drift that will make position sensing less accurate as flight time increases.
In European BRNAV/RNP-5 airspace, limitations based on the requirement of ground-based navigation aids may not be an issue for an INS or an Inertial Reference System (IRS) equipped aircraft to gain approval. However that same aircraft would not be approved to operate on the westbound Pacific Organized Track System (PACOTS), which is designated as RNP-10.
Does that mean if you are approved to operate in RNP-4 airspace, you are automatically approved to operate in RNP-10 airspace? An INS-equipped aircraft approved for RNP-4 operation with a time restriction of two hours between navigation systems updates would not be able to operate in the North Pacific Region (NOPAC) designated as RNP-10 without updates. RNP and RNAV operators have the responsibility to ensure that their equipment meets the required “navigation performance accuracy” and that the flight crews will follow prescribed procedures for the respective RNP type environment.
RNP-20 was an early minimum considered acceptable to support various route operations. But it may not be needed any longer as systems currently in use already support minimum separations.
RNP-10 supports reduced lateral and longitudinal separation and enhanced operational efficiency in oceanic and remote areas where the availability of navigation aids is limited. On April 23, 1998 Anchorage Air Route Traffic Control Center (ARTCC) implemented lateral separation standards within the North Pacific route structure from FL 310 to FL 390.
As of Dec. 3, 1998, a 50-nautical mile lateral separation standard was applied in the Central Pacific only to all aircraft that are RNP-10 approved. RNP approval is required from 31,000 feet through 39,000 feet altitude inclusive. A monitoring group was formed to collect, coordinate, and review data to reach an agreement on when to expand 50-nautical mile spacing to additional tracks.
RVSM is another key component to the free flight environment. The International Civil Aviation Organization (ICAO) is making this happen on a worldwide basis. The premise is to space aircraft operating within certain optimum altitudes at 1,000 feet vertical spacing rather than 2,000 feet. This can be achieved once the aircraft is determined to have a static system with minimal error and an auto flight system that can hold the machine at constant flight level with minimal deviation. RVSM is presently scheduled for implementation over the continental United States in late 2004.
What about maintenance?
How do the results of RNP and RVSM affect those of us in the aviation maintenance business? Well for one, a greater awareness has to result. An example would be to open the nose compartment of the aircraft. This may often be considered an everyday task. What if the aircraft static ports were located just aft of this nose compartment and one day during the closing operation a slight offset occurs in the way the nose fits to the fuselage? The resulting step may have an influence on static port airflow inducing what may be enough static source error to cause the aircraft to become recognized as not being within the prescribed tolerance to allow continued flights in RVSM airspace.
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