This morning fog has once again affected flights at Auckland Airport for the second time in three weeks. On June 14 close to 100 flights were delayed or cancelled due to fog.

Today the airport is advising passengers to check its website for information on delays. e Why does fog cause such widespread problems for planes and why are some flights likely to face more disruption than others?

What happens when fog sets in?

The fog season for airports runs from April to September and special procedure's come in when visibility reduces to less than 2000 metres, or the cloud base is below 300ft, Fog is most challenging for aircraft taxiing, taking off, or landing.

How does special equipment work?

An Instrument Landing System (ILS)sends out a signal which guides aircraft down onto the runway. It is always in operation at the airport but becomes even more important when pilots haven't got a clear view for landing. Being able to operate in fog requires three essential components - ground based systems to guide pilots to the runway and equipment on board that is compatible and crew who are certified to use this equipment. Senior technical officer for the New Zealand Air Line Pilots Association, David Reynolds, says although the plane can land on autopilot, those on flight deck must be able to see a certain number of runway lights or else they will abort a landing and go around.

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What about taxi-ing
On the ground, aircraft are guided by an advanced lighting system. It's essentially a set of traffic lights embedded in the ground to divide up the taxi ways. Aircraft sit at these hold points and once cleared by the controllers, pilots move forward when the red lights go out.

Why are more regional flights cancelled?
The difference between regional aircraft and international aircraft is the level of technology onboard. While most jet aircraft are fitted with systems compatible with the ILS, along with certified crews, and this means most aircraft can operate in these conditions using the ILS at Auckland Airport. Regional turbo-props generally aren't ILS enabled. Reynolds says airlines generally make decisions on economic grounds - diverting or cancelling an international flight is far more disruptive and expensive than a smaller regional plane.

The Christchurch control tower during a foggy morning. Photo / Grant Bradley
The Christchurch control tower during a foggy morning. Photo / Grant Bradley

How often does fog hit?

Airways says on average about 10 days a year in Auckland. Fog slows traffic down and so inevitably there will be delays. In saying that, we are still able to handle about 22 to 25 arrivals and departures at Auckland. It hits about five times a year in Wellington.

How dangerous can it be?

Extremely. Forty-two years ago on the Spanish island of Tenerife, in the Canary Islands. two Boeing 747s - one belonging to KLM, the other to Pan Am - collided on a foggy runway. In what remains the worst air disaster in history 583 people were killed.