Down To The Minima

A precision approach

Low cloud bases and poor visibility have long been the enemy of aviation. In a commercial sense, they can mean the difference between an on-time arrival and a costly diversion to a secondary airport. From a safety perspective, these two meteorological phenomena deny the pilot the required visual cues needed to fly the aircraft proximate to the terrain. Consequently, whether en route or in the final approach to land, aircraft that are being flown with reference to instruments are restricted to minimum operating altitudes to provide a buffer between man and Mother Earth.

For some time, the ILS has been the most effective means of safely reducing that buffer on the approach to land. Its ability to offer precise guidance in both vertical and lateral planes along a descending radio beam has held a great advantage over non-precision approaches such as the VOR and NDB. Still, even as the ILS gradually offered lower minimum altitudes that could be flown on instruments, there still came the point at which the pilot had to transition into visual flight and land the aeroplane. The ability for the aircraft to ‘autoland’, improved technology, approach lighting systems and greater levels of redundancy have allowed the ILS to evolve through a range of categories from CAT I to CAT II and even down to CAT III. Each category reduced the required cloud base and visibility to increasingly lower levels and consequently, facilitated a greater proportion of safe arrivals at the planned destination. Now Sydney Airport is set to benefit from the lower minimums on offer via a CAT II ILS.

A new set of numbers

Sydney Airport has commissioned CAT II ILS approaches to runways 16 Right and its reciprocal, 34 Left. In terms of numbers, the minima permits a decision height (DH) of 100ft above the runway threshold (HAT) and a ‘runway visual range’ or RVR of 350m for both runways. For 16R, this is a reduction in cloud base by 102ft and 200m less visibility and for 34L, 106ft and a very substantial reduction of 1,150m forward visibility. In fact, on runway 34L there is a substantial benefit for aircraft only equipped for CAT I operations as well. By virtue of the new high intensity approach lighting (HIAL) on offer, all aircraft capable of flying the ILS have their required visibility almost halved from 1,500m down to 800m.

Airservices was required to upgrade the instrument landing systems for both Runway 34L and 16R and designed new operating procedures to facilitate CAT II approaches to facilitate the lower minima. This new set of minimums will have a particular operational impact on those Sydney mornings when fog loiters about the skyline. And while there will be no change to the alternate minima for inbound aircraft, those conducting an approach now have a greater chance of achieving a successful arrival.

Airservices Sydney Tower line manager Wayne Lewis relates that there are around 25 hours per annum that Sydney Airport experiences low visibility procedures (LVP), of which six hours would be CATII. And while these may not sound like huge numbers, one need only consider the movements per hour that may need to divert to realise that resultant costs that flow on are potentially huge. Conversely, CAT II brings to the table significant savings and schedule integrity.

The CAT II ILS does bring new operational considerations for Lewis and his team in the control tower at Sydney Airport. A significant aspect is the additional protection of sensitive areas during low visibility procedures so that aircraft do not interfere with the signal of the ILS. For aircraft on the ground this means holding well clear of the threshold and allowing time for landing aircraft to exit the runway and move clear of the sensitive areas. This ‘box’ needs to be protected from the time an aircraft is on a two mile final approach, and/or a departing aircraft is 30 seconds past the upwind localiser. Consequently, increased spacing between arriving aircraft needs to be managed accordingly.

Melbourne Airport already conducts CAT II operations, but the situation at Sydney is somewhat different. Environmentally, Lewis relates how the visibility at Sydney Airport can be unpredictable and vary significantly from runway to runway. “It is not unusual to have 400m visibility on one parallel runway and 1,000m on the other. ” Also, in conducting CAT II operations, there is a requirement for the ILS on the reciprocal runway to be switched off. Melbourne’s reciprocal runway is not ILS-equipped, but Sydney will be required to turn off the ILS to the reciprocal runway during CAT II operations. This will be achieved by means of an interlock system that is activated from the control tower.

Lewis and his fellow controllers have had to undergo further training in systems such as the interlock andthe stringent protocols that come with CAT II operations and low visibility procedures. The training encompassed the new technology as well as the management of the sensitive areas and all that goes with the new operations. All of this training is set to the backdrop of an international airport that manages a complex mix of traffic spanning everything from Piper Chieftains to Airbus A380s.

Making it happen

Before the first aircraft ever descended to the lower CAT II minima, a great deal of work was done by the team at Sydney Airport’s airfield planning team, headed up by Ken Allcott. Planning commenced in July 2012 and construction got underway in 2013 and involved a number of components. The newly constructed Runway 34L HIAL and touchdown zone (TDZ) lights entered service to an operating minima of 800m (CAT I) in December 2013. Subsequently, the runway 16R HIAL upgrade and installation of TDZ lights was completed in March/April 2014. This was followed by CASA approval for both Runway 34L and 16R to operate to CAT II minima in April 2014.

The approach lighting provided a particular challenge given the fact that the threshold of 34L extends into the waters of Botany Bay. HIAL systems are an essential component in the design of a CAT II ILS as they are a means by which the pilot can begin assessing the forward visibility prior to physically reaching, or seeing, the runway threshold. While both runways will ultimately have an approach lighting system with sequenced flashing strobe lights (ALSF) – a ‘running Rabbit’ – only 16R possesses a 900m approach lighting array.

By necessity, the Runway 34L HIAL is a reduced length system consisting of an ICAO-compliant

The approach lighting provided a challenge given the threshold of 34L extends into Botany Bay.

420m layout as found at a number of European airports, notably Barcelona. As a consequence, an even higher degree of scrutiny was applied during certification.

The flight check process was more rigorous than usual as the newly installed HIAL systems are ALSF2, the first of this type in civil operations in Australia. For both 34L and 16R, flight checks were conducted by an authorised CASA flight check pilot in day and night conditions, in addition to checks by internationally experienced pilots and an internationally qualified CASA pilot. The HIAL and TDZ lights were used during Runway 34L operations to enable flightcrews to familiarise themselves with and provide feedback on the system layout and length. A spokesperson for Sydney Airport reaffirmed that also: “Risk assessments – attended by majorAustralian and some international carriers, AIPA and Airservices – were subsequently undertaken. ”

Down to the minima

A great deal of resources and expertise by a number of stakeholders were required to lower the approach minimums at Sydney Airport. Only through the effective collaboration of Sydney Airport, the airlines, Airservices and CASA could the new CAT II ILS have reached fruition.

In an operational sense, crews now have at their disposal enhanced approach lighting and an instrument approach that brings them within 100ft of the runway. These benefits will be of value not only when the weather lurks at its very lowest levels, but will enhance the transition from instrument flight to landing visually whenever low visibility procedures are in effect at Sydney Airport. Crews will stillneed to carry the fuel to divert when the weather sits below the alternate minima, but the chance of arriving at their planned destination has been increased by virtue of the lower levels of required cloud base and visibility.

Technology will continue to move forward. Devices such as head-up guidance systems (HGS) will work in conjunction with instrument approaches while the approaches themselves will continue to evolve beyond the use of ground-based aids. In the meantime, the ILS remains the backbone of airport approach systems around the world and now Sydney Airport has joined the ranks of those approved to guide its arrivals via a CAT II ILS. For airports, regulators and aircrews alike, the challenge will always lie in safely flying down to the minima.