The subject of Required Navigational Performance (RNP) in the approach environment was not placed on the SC 1 work programme by conference. It is being considered at ICAO by the All Weather Operations panel (AWOP) and it is an important subject when considering the introduction of new approach aids such as satellite based systems (GNSS). SC1 has therefore decided to consider the subject this year. The concept of RNP has been developed by ICAO for use in en-route navigation. It lays down the accuracy required of the aircraft navigation system. For example, RNP 4 would require the aircraft to be within + or – 4nm of the required track for 95% of the time, RNP standards will be introduced for en-route navigation from 1995. The important feature of the RNP principle is that it only defines a standard of equipment performance. It does not lay down the equipment required to achieve it. This is decided by the aircraft operator and the certification authorities. In our example, the required accuracy could be achieved by e.g. VOR/DME, INS or GPS. As far as the controller is concerned, it is of no interest how the navigation is being achieved, only that the aircraft is equipped to the standard required to fly in this particular piece of airspace. This is only an extension of the Minimum Navigation Performance Specifications (MNPS) concept already used in some areas such as the North Atlantic.

RNP For Approach

ICAO has asked the All Weather Operations Panel to consider the possibility of extending the RNP concept to approach and landing aids. AWOP has determined that the concept for approach use is much more complex than en-route, and they have established the criteria that must be met This consists of accuracy (En-route currently only specifies horizontal accuracy, but approach use will require vertical accuracy as well); integrity; continuity; and availability of the system. RNP must meet, or exceed the standards already laid down for ILS and MLS equipment.

The practical application of RNP in approach has not yet been determined, but a proposal being considered by AWOP is known as the tunnel concept. The approach path is defined as a series of ever smaller windows across the approach track producing an ever narrowing corridor or tunnel down to the runway. The closer to the runway the aircraft is guided , the more accurate the navigation system must be. Ignoring any other factors, the decision height would come when the navigation equipment was no longer accurate enough to contain the aircraft within the corridor. this accuracy will be the RNP value for that particular combination of aircraft and ground equipment. As the RNP values used for en-route only describe the accuracy of the system. This will probably have to be extended for approaches to provide RNP values similar to the current Cat II / Cat II / Cat III standards which also specify the integrity of the system.

It is envisaged by those designing the system that the controller will not need to know what aids the aircraft is using when flying the approach. It is theoretically possible that a Cat 1 curved approach could be flown with; Precision RNAV to an ILS straight final segment; an MLS curved approach; or a GPS curved approach. In this case the pilot will use whatever aid his aircraft is equipped with (he may have a choice ! ). Assuming all the ground equipment is serviceable there is no problem . ATC must inform the pilot of deficiencies in any of the equipment to enable the pilot to choose which aid is suitable for the approach.

Practical Effects of RNP

The concept of RNP for approach appears to have some advantages and some disadvantages for ATC.

Advantages

If the concept of RNP is accepted then the controller does not need to know which aid the aircraft is using. The aircraft will have an RNP category which will determine the weather minima and decision height, The same as the situation today. All the aids which are declared for this runway will be available and protected as appropriate at all times. The aircraft will fly the approach to decision height and the pilot will take the appropriate steps to ensure that he has the appropriate equipment for this approach. The RNP concept potentially improves the situation regarding the MLS transition enormously. SC1 has been very concerned that a long transition, with the possibility of ILS, MLS and GPS being used at the same time, could result in considerable increase in controller workload. the RNP concept could reduce this extra workload considerably.

Disadvantages

Either the controller will need to know which aid each aircraft is using, and just pass the information appropriate to that aircraft, or else the controller will have to pass a large amount of information about all the aids, most of which may not be required. In our example above , there are many factors affecting the design of the procedure, and it is difficult to imagine that the procedures for all the aids on the runway could be made into one single procedure. The aircraft must be within MLS coverage before intercepting the approach. This is unlikely to be relevant to GPS. The protected range of the ILS glidepath is normally only about half that for the MLS. There will be many other considerations, and designing a single approach procedure meeting the criteria for all the approach aids will be a significant challenge. It is implicit in the RNP concept that the procedures for the different aids must be identical.

As all the aids that could possibly be used will need to be protected at all times, the holding points providing the maximum protection (usually ILS ) will have to be used all day even if an aircraft never actually uses the ILS. This will reduce capacity.

To Conclude

The extension of the RNP concept to the approach and landing phases of flight appears to offer advantages to ATC. It could reduce workload and eliminate some of the complications faced by controllers as a result of the proliferation of landing aids which is almost certain to occur over the next few years. RNP could also offer advantages to the airlines and administrations designing approach procedures by introducing a single set of standards to be used by all the new types of approach equipment. The RNP concept also presents ATC with some problems. It could mean unnecessary transmissions about aids that are not being used. In high traffic situations the use of an ATIS broadcast or new datalink capabilities could relieve this problem.

The most serious problems concern ILS. It is unlikely that it can be included in the RNP concept, as it is about to be withdrawn. This would mean that the full RNP concept could only be introduced after ILS has been removed. Even if ILS were to be included in the RNP concept, its many limitations would reduce capacity and seriously limit the ability to introduce advanced procedures. In light of the development of a new approach aid , IFATCA should support the introduction of RNP procedures for approach and landing. ILS should not be included in these procedures as it will restrict the introduction of advanced procedures and reduced capacity. As ILS is excluded , this should support our already stated objective of a short transition period from ILS to a full RNP environment using MLS and / or GPS.

The RNP procedures should be developed as soon as possible so that they can be applied to the new equipment currently under development, notably MLS and GPS. If possible, RNP operations should commence as soon as these new aids are introduced in order to achieve the desired aim of simplifying the transition period and achieving the benefits of RNP as soon as possible.

It is recommended that:

The RNP concept should be extended to cover the approach and landing phases of flight, but should exclude ILS.

RNP standards should be developed in parallel with new approach aids such as MLS and GPS and should, ideally, be introduced at the same time as these new aids. In order to achieve the benefits of RNP, the transition period from ILS to the new aids must be as short as possible.

A significant factor in the determination of RNP procedures should be the aim of limiting the increase in ATC workload due to the proliferation in landing aids expected in the future.