Brave new world

Change for Good

As an agency of the United Nations, the International Civil Aviation Organisation (ICAO) seeks to develop aviation across the planet in a cooperative manner with safety premium among its goals. Its initiatives are global in scale as it seeks to coordinate change across a diverse range of member states. At face value ‘ICAO Resolution 36-23’ may appear inconspicuous, but within its objectives lay major potential benefits for international aviation and the travelling public.

Resolution 36-23 relates to the implementation of performance based navigation and approaches with vertical guidance (APV). The resolution seeks to continue navigation’s shift from land-locked ground aids and their rigid air routes, to a more flexible routing structure determined by the aircraft’s ability to accurately fly and monitor its own performance along such routes.

Ever-evolving technology is facilitating this new age of navigation which is equally at home en route or in the terminal area for the critical phases of takeoff and landing.

Significantly, the approach phase is integral to the resolution as it also seeks to provide approaches with vertical guidance where previously no ‘slope guidance’ has been available.

The move towards this means of navigation is particularly pertinent to Australia.

Many of Australia’s regional airports are still serviced by terrestrial-based NDBs (non-directional beacons) and VORs (VHF Omnidirectional Range) – technology that has been around for over half a century. These ageing beacons are not only costly to maintain, but provide a relatively low level of precision in this age of GPS.

Critically, NDB and VOR beacons do not provide any vertical approach guidance – they only provide lateral tracking information.

As such, their associated instrument approaches are termed ‘non-precision’.

From safety’s perspective, it has long been recognised that the lack of vertical guidance is a major factor in accidents involving ‘controlled flight into terrain’ (CFIT).

In Australia, the most common form of ‘precision’ instrument approach is the ILS, or instrument landing system. These runway aligned approaches provide both lateral vertical guidance and allow the crews to effectively fly ‘cross-hairs’ down to a relatively low minimum altitude. How-ever, of Australia’s 300 aerodromes with navigation aids, only around 10 per cent are equipped with an ILS. That leaves 90 per cent equipped with no form of direct vertical guidance on approach other than crosschecking a distance and height from a chart. The potential vulnerability and lethal consequences of these approaches can be highlighted by recent fatal CFIT accidents such as Lockhart River and Kokoda, PNG.

The implementation of performance based navigation and approaches with vertical guidance stand to make a tremendous impact upon aviation safety.

So what is Performance Based Navigation?

Previously, aircraft have flown air routes and instrument approaches based upon signals emitted by ground based aids.

These beacons permitted flight over fixed routes and instrument approaches limited to straight line tracks and arcs of distance.

Unfortunately, rigidly defined airways do not always optimise the use of airspace and prevailing winds, and straight line approaches are not always community friendly. PBN seeks to standardise navigation and its terminology across the planet In an era of aviation where efficiency is a prime consideration, the ability to tailor the tracks along which an aircraft travels can directly translate into cost savings, lower carbon emissions and fulfilling an obligation to be a considerate neighbour. This is where performance based navigation comes to the fore. PBN calls for a defined level of performance when the aircraft navigates along a prescribed airway or instrument approach. It is not measured, or limited by, the performance of a ground beacon, but is an aircraft’s ‘self-contained’ ability to comply. This ability has been in a large part facilitated by the growth of precision Satellite technology and systems such as GPS.

The first major evolution from ground based aids came with the emergence of the likes of INS (inertial navigation systems) and LORAN (long range navigation).

However, the next major leap came with the advent of satellite navigation and this technology has evolved at a tremendous rate since the turn of this century. The associated evolution of technology and avionics has opened up a whole new world of precision navigation to all areas of aviation, not merely the big boys in the flight levels.

Flowing on from this comes the growth of PBN.

PBN permits an aircraft to fly along its own desired, pre-planned flightpath defined by the performance limitations of its own self-contained navigation system.

The onboard systems have the ability to maintain lateral tracking to a very high degree of precision of up to 0. 1 nautical miles, if needed. This is not only beyond the scope of guidance on offer from air traffic control and radar, but facilitates a high degree of flexibility in route planning. Through PBN, aircraft can fly more direct routes or take the fullest advantage of prevailing winds which equates to far friendlier flight times, lower fuel burn and less by-products pumped into the atmosphere. According to the International Air Transport Association (IATA), more efficient air routes have already cut CO2 emissions by six million tonnes per year.

Performance based navigation can be broken down into two primary categories, area navigation (RNAV) and required navigation performance (RNP). In a broad sense, the defining difference is that RNAV requires the older technology INS andRNP is supported by the higher precision global navigation satellite system (or GNSS). Additionally, RNP requires an onboard monitoring and alerting system to warn of faults and approaching limits, while RNAV does not. Both are selfcontained systems onboard the aircraft and provide far more accuracy than traditional terrestrial based navaids, but the GNSS supported RNP provides the higher degree of precision. As a consequence, the procedural separation standards applied by air traffic control to RNAV involve greater separation than for the satellite based RNP.

Both can co-exist, but the flexibility on offer from RNP is more substantial.

Whether crossing the globe, or arriving and departing within terminal airspace, PBN offers far more flexible and harmonious airspace than has previously existed.

Winding arrival paths in the sky can sidestep noise sensitive areas and permit constant, fuel-efficient ‘power-off ’ descents which link up with runway approaches.

And departure tracks can give both households and angry terrain a wider berth.

These are just a sample of the benefits to be gained from precise navigation and the implementation of performance based navigation.

A Safer Slope

Having used the technology of PBN to cross oceans and smoothly transition into the terminal area, a challenging phase of flight still confronts the crew – approach and landing. It is in this phase, down among the hills and weather, that the potential threats are at their most prevalent and the crew’s workload is at its highest.

Contributing to the intensity even further are the limitations of land based navaids.

NDB and VOR approaches offer lateral tracking to a relatively high minimum altitude before visual reference is called for to fly the aircraft to the runway. Often these approaches are not runway aligned and many are so far offset that they require significant manoeuvring, or ‘circling’, to position the aircraft to land.

Beyond the lateral limitations, the critical factor is that these ‘non-precision’ approaches, by definition, offer no vertical path guidance. They offer tracking guidance, but no indication of the aircraft’s progress in the vertical sense. This is very worthwhile information when one is descending towards the earth. As previously mentioned, approaches that lack vertical guidance are more likely to result in CFIT and virtually always, CFIT proves to be fatal for all onboard. ICAO rates the risk of a straight-in approach without vertical guidance to be eight times more likely to result in a CFIT accident. Unfortunately, it rates the risk for a circling approach as 25 times greater.

Sadly, a high percentage of CFIT accidents can be traced back to human error as the absence of vertical guidance can introduce all manner of potential human factor issues. Non-precision approaches call for the single pilot or crew to cross check their vertical progress with a distance from the airfield. This calls for the interpretation and processing of information from at least three separate sources – the altimeter, DME and the approach chart. The application of standard operating procedures (SOPs) can go a long way towards circumventing omissions and misinterpretation.

However, in the high workload, dynamic environment of a flightdeck void of visual reference, occasionally an error can slip through a hole in the Swiss cheese.

Unfortunately, CFIT is not the only unwanted potential by-product of landing off non-precision approaches. Vertical guidance also stands to position the aircraft in the ideal, stable position to execute the landing manoeuvre. Without this, the chance of undershooting the landing threshold, or running off the far end are both increased.

The existing precision approaches offering vertical guidance include ILS, global positioning landing system (GLS) and microwave landing system (MLS). They have a far greater safety record than non-precision approaches by virtue of their vertical path guidance. Of these forms of precision approach, the most prevalent is the ILS, but less than 10 per cent of Australian airports with navigation aids are equipped with this device.

The implementation of approaches with vertical guidance will seek to propagate three forms of RNP approach with APV and gain far greater coverage of Australian airports. Importantly, all three will provide direct lateral and vertical path guidance, while additionally requiring system performance monitoring and alerting to warn the crew when errors within the system exceed the prescribed tolerances. Also, obstacle clearance is not based upon a ‘splay’ emanating from the runway threshold, but lateral and vertical tolerances based upon the defined approach path.

The first variant is the RNP approach – LNAV/VNAV, offering vertical guidance based on traditional ‘barometric’ (altimeter) information, augmented by GNSS technology.

Secondly, the RNP approach – LPV (localiser performance vertical guidance) is a satellite based augmented system (SBAS).

It can most easily be thought of as a GNSS supported ILS, hence the absence of any ground based equipment.

Finally, at the top of the tree are the RNP-AR (authorisation required) approaches.

Like the other RNP approaches, they also offer vertical path guidance irrespective of ground based equipment.

However, they need not be aligned with the runway until the very final stages of the approach, allowing it to weave a path clear of towns and terrain. If an ILS can be seen as a rigid rod, then the RNP-AR approach is effectively a flexible hose. RNP is also able to provide guidance to extract an aircraft safely in the case of a non-normal event such as an engine failure.

While each of these three forms of RNP approach offer a differing level of precision, all of them still greatly outperform traditional non-precision approaches. Furthermore, as the equipment is self-contained aboard the aircraft, the need for expensive infrastructure to be built and maintained is bypassed. Once the approach procedures are surveyed, tested and promulgated, the monitoring of approach performance reverts to the aircraft and its crew. Significantly, the potential of vertical approachguidance being provided at all ‘instrument’ airfields is much closer to reality.

Getting the Job Done

ICAO wants to make the entire world a safer, more efficient place with the introduction of PBN and APVs. As with any global task, it is both a massive and ambitious undertaking. Here in Australia, we are in the fortunate position of already being well versed in the ways of PBN.

ICAO’s recommendation is to implement PBN for area navigation and APV for all instrument runway ends, or as a backup for precision approaches, by 2016.

For more than 20 years Australia has been using area navigation, virtually necessitated by our vast airspace, low surveillance coverage and relatively sparse ground based navaids. In our busy terminal areas, standard instrument departures (SIDs) and standard terminal arrival routes (STARs) are relatively commonplace.

Even at the ‘sharp end’ of the implementation, Australia is ahead of the game with nearly 20 aerodromes already utilising RNP-AR procedures. A number of these also boast RNP SIDs for departures. While many are already at major airports that are equipped with precision approaches, the likes of Ayers Rock and Karratha have also joined the list of RNP-AR airfields.

Even at those airfields serviced by an ILS, the routing along the RNP-AR can be significantly shorter and more efficient, as evidenced by Brisbane’s early operational trial where over nearly 5000 approaches an average saving of 11. 4 track miles, 143kg of fuel and 486kg of CO2 emissions were achieved on each approach.

The implementation strategy is a far reaching process, although there are some key milestones to note. By 2012, CASA is seeking to avail RNAV in all classes of airspace and for RNP to become available in all airspace in that period. The move towards APV will initially see LNAV/VNAV approaches that are dependent upon barometric vertical navigation (Baro-VNAV).

Accordingly, the provision of approved, accurate and timely QNH information will be required for the airfields seeking to utilise Baro-VNAV via an expansion of the aerodrome weather information service (AWIS).

In the medium term, towards 2017 and encompassing ICAO’s 2016 target, the implementation of PBN will see the parallel availability of RNAV and the more precise RNP in area navigation. In the terminal area, the provision of APV to all instrument runway ends by 2016 will be the goal. Depending on availability, SBAS will determine the means by which this is achieved and the type of approaches provided.

In either case, CASA has a strategy in place. In the far term, the approach augmentation will feature GNSS. The first stage of implementing Baro-VNAV will provide APV protection to 15 per cent of IFR aircraft, but 97 per cent of fare paying passengers. Once the global satellite systems can be incorporated, that protection will see virtually 100 per cent of aircraft and fare paying passengers fall within the safety net.

CASA’s implementation strategy cannot take place overnight, but it is well positioned to make it a reality in a tangible timeframe and meet ICAO’s target. For the travelling public and the industry in general, the successful implementation of PBN and APV will mark a major milestone in Australian aviation safety.

Brave New World

The NDB and VOR approach have been the backbone of the majority of Australian airports for over half a century. While they have performed honourably, the advancement of technology has marked PBN and APVs as the way of the future. The need for costly upkeep of beacons will be gone and the precision afforded to aircraft and crews will be far beyond the 5 degrees or half-scale deflection offered by flickering beacons. Most importantly, the provision of vertical path guidance to all instrument approach runways will hopefully see a massive reduction in the occurrence of the tragic outcomes afforded by CFIT.

Through CASA, Australia has embraced the move towards precision navigation and its implementation. Already, nearly two-thirds of Australia’s domestic jet fleet is certified to 0. 3 RNP or better. PBN will lead to more efficient airspace and the harmonised transition of aircraft from en route to the terminal area. The efficiencies will come in the form of safety, costs, and the environment. The equipping of approaches with vertical guidance will do the same, although the benefits to safety are potentially of landmark significance.

Through PBN and APVs, Australian aviation is set to transit the skies with the best the 21st century can offer. It is truly a brave new world.