RNAV: What’s In A Name?

Featuring the first contribution from Simon Kelsey, newest member of Avsim's Editorial Staff .

An instrument approach procedure, or IAP, is designed to safely bring an aircraft in instrument meteorological conditions from the en-route portion of the flight to a height and position sufficiently close to the aerodrome that the remainder of the approach and landing may be completed visually. Traditionally, this has been accomplished with the use of a radio-based tracking aid such as a VOR, NDB or Instrument Landing System (ILS).

But with the advent of highly accurate Area Navigation (RNAV) systems, the traditional ground-beacon-based IAP is increasingly being supplemented with GPS or Required Navigation Performance (RNP) procedures. These procedures make use of on-board navigation equipment, which in modern aircraft is capable of flying extremely accurate lateral and vertical paths in space -- to within a matter of a few metres laterally and a few tens of feet vertically -- with no reference to traditional ground-based navigation aids, provided the system is properly set up and operating at the correct level of accuracy.

This approach has a number of advantages -- for instance, there is no need for the costly installation, maintenance and checking of ground-based navigation aids, whilst accuracy is often many orders of magnitude greater than, for instance, that afforded by an NDB or VOR. In addition, many of the limitations associated with radio navigation aids are removed -- terrain and obstacles which may otherwise mask or distort the signal from a radio aid are irrelevant if the path is simply being generated by the aircraft’s on board computer, and with no requirement to track in a straight line to or from a beacon the approach designer may introduce curved approach segments. This has enabled instrument approach procedures to be developed for runways and aerodromes situated in areas of challenging terrain where installation of a radio aid, or development of a radio aid-based procedure, would have been impractical or impossible in the past.

RNAV and RNP are the products of a gradual shift in worldwide aviation away from traditional ‘sensor-based navigation’ -- where routes, procedures and waypoints are defined in terms of radio navigation aids -- and towards so-called ‘performance-based navigation’ or PBN. In simple terms, PBN means that instead of requiring an aircraft to use a specific type of sensor -- say, a VOR or NDB -- to navigate to a particular point, what is more important is that the aeroplane is capable of navigating accurately to that point, whether that is achieved with raw-data beacon tracking or via a magenta line and a GPS. In other words -- the emphasis is shifted from the method of navigation to the outcome.

In the early days of RNAV, the main method employed (primarily in larger aircraft) was to use an Inertial Navigation System (INS) combined with radio (usually dual DME) updating in order to provide a corrected position for use by the Flight Management Computer (FMC). Whilst this system works well in areas of good radio beacon coverage, on routes over sparsely-populated terrain where radio aids are not readily available the FMC position tends to drift away from the aircraft’s real position until radio coverage is available to correct the position once again. Those who are familiar with the Level-D 767 will likely have experienced a “map shift” of this type after a long oceanic segment!

In the last twenty years, however, GPS technology has become ubiquitous within consumer devices such as smartphones and car satnavs, and it is a natural progression that the aviation industry should take advantage of the system as well. GPS offers a number of advantages over the INS/DME-based system in terms of accuracy, reliability and availability, and not only is GPS technology now installed in almost every commercial airliner flying, the relatively cheap and lightweight technology is fast becoming a standard in light aircraft as well -- whether installed as part of the aircraft’s avionics suite or in the form of portable hand-held devices. GPS also offers a further benefit in that it incorporates a measure of self-monitoring -- a requirement for Required Navigation Performance (RNP) operations where the aircraft is obligated to alert the crew if it is not capable of remaining on track within tightly-specified limits.

RNP is undoubtedly the future of high-level enroute navigation as well as terminal procedures: with the ability for aircraft to track given routes with a high degree of accuracy and certainty, airways and procedures can be designed in such a way that reduces the amount of separation required between aircraft and thus increases airspace capacity. Already the RNP4 specification is being introduced over the North Atlantic, enabling tracks to be spaced at half the distance they previously had been (about 25 nm laterally instead of 50 nm), and it is likely that in time RNP will be introduced on to congested European upper routes as well.

One of the greatest areas of confusion for pilots is the naming of RNAV approach procedures, not least because of the number of different standards and requirements and a worldwide inconsistency in naming conventions.

The ‘standard’ RNAV approach specification is referred to by ICAO as RNP APCH. This is a source of much confusion, particularly for US-based pilots because the FAA title these approaches RNAV (GPS)! To make things even more confusing, the old ICAO standard referred to these as RNAV (GNSS), because GPS refers to a specific satellite-based navigation system controlled by the US military: GNSS is a generic term for satellite-based navigation systems. Although many procedures are being re-titled in line with the new ICAO standard, there is still a long way to go and so you can still expect to see various titles.

The basic RNP APCH specification involves straight legs only at this time, and with an RNP value of no less than 0.3 in the final segment and 1.0 in all other segments. As such, almost all aircraft (provided they are equipped with both a GPS and an RNAV-capable FMS) can fly this type of approach.

The more stringent specification is known in ICAO as RNP AR APCH, where ‘AR’ stands for ‘Authorisation Required’. These are the ‘exciting’ approaches to places like Queenstown, New Zealand and Palm Springs, California, and often involve curved ‘Radius to Fix’ (RF) legs and extremely low RNP values, as low as 0.15 in some cases. Special equipment, approval and training is required to fly these types of approach, hence why they are ‘AR’. Again, there is an FAA difference in naming convention: in the USA, these approaches are titled RNAV (RNP).

A further development is RNP 0.3. This is yet another (!) new standard which aims to provide something of a ‘halfway house’ between the extremely stringent requirements for RNP (AR) procedures and the more relaxed RNP APCH specification which can be flown by most aircraft but does not offer the benefits of, for example, curved approaches. Work is still ongoing to develop this standard but it is likely that we will see many approaches of this type start to appear as the years go on.

As flight simmers we are blessed by the extraordinary work of add-on developers producing ever more advanced simulations of ever more advanced navigation equipment. Most of today’s complex addons are capable of flying at least a standard RNP approach both accurately and realistically, though I believe we are still waiting for a truly ‘RNP AR’ capable model: PMDG’s stable is not currently capable of flying true radius-to-fix legs, whilst the FSLabs A320 lacks the lateral and vertical deviation scales required to fly RNP AR procedures. In practice, however, both will fly procedures such as the Queenstown RNP Y 05 approach quite adequately as far as the simulator is concerned and if you’ve never tried, give it a go: the future is here, and the days of the ground-based navigation aid are numbered.

Simon Kelsey

Contributing Editor

Avsim.com