KEY POINTS:
A multimillion-dollar airport guidance system allowing planes to land in fog got its first workout yesterday when a pea-souper descended on Auckland International Airport.
Heavy fog saw dozens of flights in and out of Auckland cancelled - but planes with the new system were able to land.
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midday two international flights had been diverted: an Air Pacific flight from Nadi to Auckland was re-routed to Christchurch and an Air New Zealand flight from Sydney diverted to Wellington. An Air NZ flight to Adelaide was cancelled.
But the new Cat IIIB Instrument Landing System - which came fully on-line on Monday - meant all the international flights fitted with the receiving equipment, or suitably trained pilots, landed successfully.
In the past, a fogged-in Auckland Airport meant long-haul flights ran the risk of being diverted.
Airport spokeswoman Lucy Powell said the Cat IIIB system had worked throughout the day, with the only flights cancelled being those without the technology aboard, or untrained aircrew.
"All our landing equipment was working, but it still depends on the compatibility of the pilot."
The $4 million system fires electronic beams toward approaching aircraft, enabling the pilots to line up with the runway centreline and to descend on an ideal glide path.
Under the old (Category I) ILS, a pilot landing at Auckland needed to be able to see the ground at or above 60 metres (200ft) and have a minimum forward visibility of at least 800m to land safely.
If the minimums were not met, the crew was forced to declare a "missed approach" and climb to 915m (3000ft).
The minimum heights and distances are set by the Civil Aviation Authority and vary from airport to airport, but the higher the ILS category system, the lower an aircraft is allowed to descend without seeing the ground.
The new Cat III B allows much lower minimum altitudes, to a point where planes fitted with the technology can land in virtually any fog conditions. Auckland Airport experiences fog between 12 and 16 days a year, with diversions likely to be imposed on anywhere between three and 11 of those days.
While the fog is in, the airport's usual 40 take-offs and landings an hour are reduced to as few as six. Authorities believe that number will be boosted to around 12 by the end of this month, and up to 20 by September.
The new system sees Auckland landing minimums reduced to 15m (50ft) of altitude with a forward visibility down to a minimum 50m.
The new system works chiefly for international and domestic main trunk-flying jet aircraft, though some turbo-prop planes might have the system.
However, it is generally only the pilots of larger aircraft trained to use the system.
Airways Corporation, which ran the Cat IIIB upgrade, was required to make substantial changes to the airport's instrument landing system and lighting systems.
The upgrade included improvements to runway edge lights and approach lights. A large number of bright lights had to be installed at the point where aircraft touch down, while taxiway lights have been placed closer together to enable aircraft to taxi in poor visibility.
Other improvements are expected over the next year, culminating with the introduction of a ground radar, which will allow air traffic controllers to better direct aircraft on the tarmac.
