Queenstown RNP—An Air New Zealand First

By Graeme Mollison.

A traditional instrument approach into Queenstown in New Zealand’s rugged South Island involves a long straight run towards the VOR at Slope Hill, with the descent limited by the height of the ridges that an aircraft must clear on the way in. At the conclusion of the instrument approach, one must still manoeuvre tightly to descend visually, thousands of feet to the runway below. On a clear VFR day, it is possible to maintain a smooth, continuous descent profile, skirting the “rocks” as one curves around narrow mountainous valleys to the runway. Imagine if it were possible to fly a similar continuous descent while remaining cocooned in cloud. As Graeme Mollison explains, that is exactly what an RNP approach enables one to do!

Stand in the “cab” of Queenstown’s control tower on a stunningly clear southern day and you would have to agree that the guys and girls manning this tower have one of the best “office window” views in the world.

Surrounded by spectacular mountains and narrow steep valleys, Queenstown has grown into a tourist Mecca. In days gone by, its airport’s “rush hour” involved a gaggle of tourist aircraft returning from Milford Sound with their loads of wide-eyed customers. Nowadays, Queenstown airport is a hive of activity at almost any time of the day as tourist and aerial work operators ply their trade alongside visiting business jets and scheduled airline operators who run a network of domestic and trans-Tasman services to and from the tourist town.

“Spectacular” brings constraints and Queenstown has its fair share. The 1,839 m (6,034 ft) Runway 05/23 lies in a valley 1,171 ft above sea level and is surrounded by imposing terrain. The Remarkables mountain range rises to 7,625 feet less than five nautical miles to the southeast of the runway; within a 15 nm radius of the airport, one would be hard pressed to find peaks lower than 6,500 ft in any direction.

The local weather also brings its own set of challenges. A northerly wind aloft can result in a tailwind at both ends of the runway, and squally winter weather can make for challenging approaches and landings. Combine all this with the plethora of parachutists, paragliders, hangliders, light aircraft, helicopters and the odd aerobatic aircraft, and one can quickly understand that Queenstown’s airspace can be a “minefield” in which it is not always fun to manoeuvre a 50-tonne jet.

Until recently, Air New Zealand’s arriving Boeing 737-300s were forced to fly the traditional VOR/DME approach utilising the Slope Hill VOR located a few miles northeast of the runway. If the aircraft were being operated at their maximum landing weight of a little under 53,000 kg, the minimum altitude to which they could descend on the instrument approach was a rather limiting 4,900 feet (3,729 ft above aerodrome level). In such mountainous terrain, it is little wonder that weather conditions could easily disrupt operations.

At most airports, the arrival is typically the most restrictive with regard to the minimum cloud base and visibility requirements. Queenstown differs in that it is often the departure that is the most restrictive. The weight of the aircraft directly affects its performance capabilities, which can be significantly degraded with respect to an engine failure scenario. Such “worst case” scenarios must be considered when designing approach and departure procedures; an aircraft must always be capable of either returning visually for a landing, or, if IMC, be capable of climbing safely away while clearing any obstacles or terrain in the vicinity.

Once fuel is uplifted for the 550 nm return flight to Auckland, Air New Zealand’s aircraft often depart Queenstown heavier than when they arrive, so this is where these departure restrictions can arise.

It is not easy to explain to a terminal full of passengers that an aircraft that has just landed cannot depart because of the weather, which may well not look at all bad from the terminal window. On average, Air New Zealand was losing 36 days of operations into Queenstown each year as a result of the weather, and the limitations posed by the existing approach and departure procedures.

A disruption at Queenstown would often cause a ripple effect across the airline’s domestic network as flights were diverted to nearby Invercargill or perhaps Christchurch, leaving aircraft and crews at stops that weren’t planned. Such disruptions force the airline’s operations staff into “damage control” mode and involve a great deal of work behind the scenes to get the aircraft and crews back into the network as soon as possible.

Ground-based approach aids were always going to have operational restrictions at Queenstown, so in the late 1990s, Air New Zealand’s Chief Technical Captain at the time, Ian Varcoe, began investigating alternatives.

His research took him to an area of the world that has more pilot licences per head of population than anywhere else—Alaska, a region of BIG terrain and challenging weather. There, Alaska Airlines had been using a new approach and departure procedure that did not rely on traditional ground based aids; the procedure was known as RNP.

Alaska, the Home of RNP Approaches

RNP approaches and departures started with Steve Fulton, Alaska Airlines’ Technical Pilot in the mid 1990s. He recognised that recent developments in navigation technology, RNP—which was being used for transoceanic en route flying at the time—could be taken a step further to a far more precise level, and used for “precision” approach and departure procedures. This promised to relieve some of Alaska Airlines’ operational problems, particularly at Juneau.

Operations into Juneau—located in mountainous terrain in Alaska’s southeast—were often disrupted by bad weather. There, the conventional approach, which used a ground-based radio aid, had a high MDA (minimum descent altitude), from which aircraft flying the approach were required to manoeuvre visually to land (much like Queenstown).

Fulton set about designing a procedure that would allow Alaska Airlines’ aircraft to fly the approach to a much lower altitude, utilising GPS (Global Positioning System) technology to fly a path between the mountains with absolute accuracy whilst still in IMC (instrument meteorological conditions).

Alaska Airlines carried out the first RNP approach into Juneau in 1996 and over the next few years, developed a number of other similar approaches that improved the reliability and efficiency of operations across its network.

In 2003, Fulton and another Alaska Airlines pilot, Hal Anderson, left the airline and joined forces with Dan Gerrity to form a company specialising in developing RNP approach and departure procedures. They named the company Naverus, derived from two Latin words: Navigare, which means to sail or navigate, and Verus, an adjective meaning true; very apt for the business the company is in.

Naverus’ area of expertise has not been limited to designing approach and departure procedures; the company also works with customer airlines to assist with procedures, training and certification—remembering that in most cases, RNP is so new that the regulators often lack experience and knowledge as well.

The Seattle-based company has grown from small beginnings to a staff of 35, and apart from having worked with Air New Zealand and Qantas Australia (not to be confused with Qantas’ New Zealand domestic operation, JetConnect) in developing RNP operations into Queenstown, it has recently won large contracts to develop procedures for such airlines as Southwest in the United States. Naverus is also heavily involved in the rapidly developing Chinese market.

From Juneau to Queenstown

Just as Fulton had done in Alaska, Varcoe could see what his own airline was achieving with its en route RNP operations across the South Pacific; he saw the same potential for Air New Zealand at Queenstown as Fulton had seen for Alaska Airlines at Juneau.

In light of the well-publicised success of Alaska Airlines’ RNP operations, Varcoe set about building a “business case” to support the development of similar approaches and departures into and out of New Zealand’s adventure capital.

The arrival of B737-300 aircraft to replace Air New Zealand’s aging -200s gave the project momentum, as it gave the airline aircraft that, with modification, could be made RNP-capable.

It was a long process; in 2004, the project finally won airline approval for development to commence. Varcoe devoted many hours to the project before vacating his Chief Technical Captain’s role last year and handing the project over to Phil Hickman (Manager Fleet Support—Technical) to complete.

RNP Approaches and Departures—What and Why?

RNP approaches and departures take advantage of the increased accuracy of modern aircraft’s GPS-based onboard navigation systems, which are now capable of flying far more accurate arrival and departure profiles than ever before, increasing safety and often simplifying the procedures to be flown.

Despite the significant advancements in modern aircraft systems, the increased navigational accuracy they allow has yet to be recognised and incorporated into procedures at most of the world’s airports, which still utilise ground-based aids such as the ILS (instrument landing system) for precision approach guidance—a system first developed in 1938. The terrain at Queenstown prevents the use of an ILS and—in the absence of RNP—limits Queenstown to a non-precision VOR and co-sited DME navigation aid.

The use of ground-based aids incurs significant operational limitations, particularly when installed in mountainous terrain. The containment area, or protected airspace either side of the centreline (often referred to as a fan) that is necessary to allow for any signal inaccuracy, expands the further an aircraft is from the approach aid.

A standard instrument approach or departure procedure utilising such an aid requires a straight segment of approximately 10 nm. More complex procedures often require the use of more than one aid to be successfully flown. This makes designing suitable procedures challenging—particularly in mountainous areas. Stepped descents and/or steep descents are often unavoidable, and require sound energy management skills from airliner crews. These same approaches are often forced to terminate with the aircraft still at significant altitude, and too high to achieve visual reference with the ground in the presence of any substantial cloud cover. If visual reference is achieved, a complex visual manoeuvring segment may still be required to position the aircraft on a stable approach profile for the runway.

RNP procedures differ in that they do not rely upon ground-based navigation facilities, but instead, recognise the accuracy of modern onboard navigation systems and make use of highly predictable satellite information.

Modern aircraft flight management systems (FMSs) are capable of being programmed with what are known as procedure waypoints, which require no reference to a ground station. Multiple legs can be created, which are not limited to straight lines as they are with ground-based aids. Great Circle tracks, specified tracks to a fix, and constant radius turns to a fix can all be created. These allow for the design of approach or departure procedures that can be tailored to fly around terrain or restricted areas, or even designed for the purposes of noise abatement.

RNP Diagram 1 relates to paragraph above.

Vertical profiles are added to procedures, which can then be stored in the aircraft’s FMC (flight management computer) and coded as RNAV (from the FMC’s perspective, RNP is similar to RNAV) arrival or departure procedures.

The accuracy of the flight path is assured by GPS satellite information and the aircraft’s IRS (inertial reference system—dual IRS in the case of the B737-300). This means there is no requirement for a “fan” type of expanding lateral containment area (protected airspace) as there is with a traditional ground-based navigation aid. Instead, the lateral containment area is a corridor of constant width.

Diagram 2, depicting comparison of arrival corridors, relates to paragraph above.

This is where required navigation performance, or RNP, comes in. RNP is the navigation performance required to operate within the defined airspace or on the published RNP approach/departure procedure. It is defined by a value in nautical miles. For example, en-route airspace may require a value of 10 nm, while an RNP approach may have a value of 0.3 nm or even as little as 0.1 nm.

RNP approaches and departures state the width of the containment area corridor in terms of an RNP value. The size of the lateral containment area either side of the centreline is twice that of the RNP value, which creates a corridor with a total width of four times the value. Using RNP 0.3 as an example, the containment area is 0.6 nm either side of the centreline, giving a total width of 1.2 nm; picture it as a monorail weaving around the mountainous terrain with all the obstacles cleared away 0.6 nm either side of the track.

Diagram 3, depicting RNP containment, relates to paragraphs above.

An aircraft can operate in this corridor provided that its ANP (actual navigation performance—also expressed in nautical miles, and which represents the radius of a circle centred on the computed FMC position) is equal to or less than the RNP. Therefore, in order to fly the RNP 0.3 approach into Queenstown, the ANP value must never be greater than 0.3 nm (ANP predictions are provided to crew at the flight planning stage). Information on the current ANP value is displayed on the aircraft flight management system’s control display unit (CDU). Just as they would tune and monitor a ground-based navigational aid on a traditional approach, crews constantly monitor the ANP information throughout the approach.

Vertical path is the next issue to be tackled when designing and flying RNP approach procedures. The coded path is defined by three parameters: waypoint barometric altitude, waypoint speed and vertical angle.

Vertical deviation is sensed relative to the aircraft’s barometric altitude, so it is imperative that the correct QNH is set. For those operating in the Queenstown region, this may explain why one often hears requests from Air New Zealand’s arriving Boeings for confirmation of the airfield QNH.

Vertical Obstacle Clearance

As well as the lateral containment, a vertical obstacle clearance requirement is applied to the approach procedures; however, the vertical component is measured in feet.

On the initial segment, the required obstacle clearance (ROC) is 1,000 feet in non-mountainous (5,000 ft or less) and 2,000 feet in mountainous (above 5,000 ft) areas. This changes once the aircraft enters the intermediate segment, where a 500-ft obstacle clearance is applied. Inside the final approach fix (FAF), it gets a little more complex where a variable ROC applies that takes into account such factors as aircraft angle of bank (body geometry), ISA deviation (cold weather altimetry), lateral ANP inaccuracies, allowable FTE (flight technical error), QNH reporting areas (i.e. local pressure variations due to orographic effects) and static source errors.

The result is that RNP allows the construction of less complex procedures using continuous gradient profiles that take the aircraft to lower minima whilst avoiding obstacles such as terrain and conflicting airspace. These procedures can also be constructed to keep the aircraft clear of the worst areas of expected mechanical turbulence.

Aircraft Modification

Not all commercial airliners can carry out these procedures. Air New Zealand has had to make modifications to the Boeing 737-300 to allow it to carry out RNP operations. It is an expensive process, which is why only six of the company’s current fleet of fourteen B737-300s—deemed a sufficient number to service Queenstown—have been approved for modification.

The modification process requires the installation of a dual GPS System—essential to ensure accurate navigation—the dual configuration provides redundancy. GPS operation is automatic and geographic position information is supplied to the aircraft’s flight management computer (FMC). FMC logic (which can be set by crew) combines the position from both GPS sensor units and the IRS (inertial reference system) to update the FMC position. Should the GPS data become unavailable, the FMC position can be determined by the IRS alone; thus, there is always redundancy.

Changes had to be made to some system power supplies to ensure that equipment essential to an RNP procedure was less likely to be lost in the event of a malfunction.

Refinements to the aircraft’s EGPWS (enhanced ground proximity warning system) were also made in an effort to reduce nuisance warnings in aircraft weaving their way around the terrain.

Phased Approval and Implementation

Designing an approach and modifying aircraft did not immediately give Air New Zealand the authority to operate RNP procedures into Queenstown. First, they had to train crews and validate the procedures.

A “control group” was formed, comprising 22 specially trained B737 pilots (11 crews). They were tasked with carrying out 90 approaches and departures in visual conditions.

Project pilots Captain Grant Fausett and First Officer Hugh Pearce flew Air New Zealand’s first RNP approach and departure at Queenstown on 20 October 2006. This was the start of Phase 1, which required a cloud base no lower than 10,000 feet (amsl).

During this period, data was retrieved from the aircraft’s recorders to monitor and verify that the procedures could be consistently flown within the parameters prescribed. This meant working closely with the New Zealand Civil Aviation Authority.

Air New Zealand regularly forwarded information from the aircraft’s data downloads and flight crew written reports (which were required to be completed after every flight) to the CAA. CAA representatives also accompanied a number of RNP flights as observers and monitored the airline’s flight simulator exercises.

The implementation of Phase 2 allowed operations with a cloud base down to 5,000 ft (amsl).

Finally, in late March this year, the airline received its RNP approval from the New Zealand CAA, allowing operations to RNP 0.3, which has a decision altitude of just 2,423 ft (1,253 ft agl). This signalled the start of Phase 3 of the programme, which runs for 12 months, during which time, the airline continues to collect data from the aircraft recorders and the flight crew reports. At the end of this period, a decision will be made as to whether to enter Phase 4, which involves going below RNP 0.3. The aircraft actually has the capability to fly to RNP 0.11 minima, allowing a decision altitude of 1,421 ft (250 ft agl)!

Experience and Training are Keys to any Successful Operation

Air New Zealand takes its Queenstown operations very seriously. Not all of the airline’s 151 B737-300 pilots operate Queenstown services; the company prefers to have fewer pilots trained and approved so that they can be rostered to operate more regularly into the airport, thus maintaining high levels of currency and local experience.

At present, the company keeps 30 crews (30 captains and 30 first officers) trained and current for Queenstown operations and, by July, all should be qualified in RNP operations.

The airline requires that the captain must be the “pilot flying” for all Queenstown arrivals and departures (RNP, VOR or visual approaches or departures) and before a pilot can be released to operate into Queenstown, he or she must have been operating as a captain on the B737 for at least 6 months and have completed the simulator and line training package. First officers are also required to have completed at least 6 months flying on the B737 (with Air New Zealand) and must also complete the simulator training package.

RNP Qualification

The RNP qualification requires each pilot to complete a training programme, which involves a self study package, a short ground course, a computer-based examination and simulator training, followed by line-training with a company training captain. It doesn’t end there either, as each pilot must undergo an RNP recurrent/check simulator ride every 6 months (180 days).

Experience is also important, and an Air New Zealand B737-300 captain will typically have more than 10,000 flying hours in his or her logbook and will probably have been flying a jet aircraft for 10 years or more.

Where to from here?

It has taken some three years and many man-hours for Air New Zealand to achieve RNP certification for the Boeing 737-300 operation into Queenstown. It is the first airline in the world to have the 737-300 certified (Alaska Airlines was using the -400).

The project hasn’t finished yet. Naverus is working on a new RNP approach for Runway 23, which tracks down the Kawarau Gorge, and as Phase 3 for the B737-300 continues, work is being carried out with regard to the airline’s Airbus A320 operation.

While Air New Zealand’s domestic B737 operation may now have a clear operating advantage over its non-RNP-capable rival, JetConnect, which operates Qantas’ services within New Zealand, this not the case on trans-Tasman routes. Qantas Australia operates its RNP 0.1-capable B737NGs across the Tasman in competition with Air New Zealand’s A320s, which are not yet certified for RNP approaches/departures; the airline plans to have completed this process in time for the 2008 ski season.