ABOUT 50 aircraft were in the air above New Zealand when an unexplained glitch knocked out air traffic control radar, forcing them land without its help and grounding those ready to take off.

An apologetic Airways New Zealand, responsible for air traffic control, is blaming an “internal network outage” for disrupting its communication and surveillance systems.

The outage hit at 2.41pm (1241 AEST) on Tuesday and the radar system wasn’t fully back running until 4.30pm.

State-owned Airways says there were about 50 aircraft in the air at the time, but none were in danger.

The Airline Pilots’ Association says it doesn’t yet know what caused the fault but it was obviously concerning.

“Clearly that is a concern to us,” spokeswoman Lisa Williams said.

However, she said air traffic controllers should be commended for still safely landing planes based on getting their airspeed, altitude and location from pilots via radio.

“Air traffic controllers train for events like this, and today put them to the test but they did brilliantly.” Pilots in the air would have followed the rules about what to do in such an “extremely rare” situation, she said.

“They kick into pilot mode to ensure they are getting their plane on the ground safely and their passengers happy and safe.” Auckland Airport’s backlog of five international flights were quickly cleared. However, eight domestic flights were delayed and two cancelled, spokesman Simon Lambourne told Radio New Zealand. Wellington Airport was reporting more than a dozen delayed domestic flights yet to take off on Tuesday evening and Christchurch Airport nine delayed flights.

Air New Zealand says the problem affected about 160 domestic and international flights.

“The backlog will take some time to clear,” a spokeswoman said. Jetstar only had six domestic flights affected, according to reports.

Airways NZ’s Radar Centre is based in Christchurch with radar equipment located around the country.

In a statement, spokeswoman Emily Davies says it has back-up processes to safely deal with network failures.

“Airways quickly worked to resolve the issue and has thoroughly tested the integrity of the system.” It was completely confident in the safety of all its systems when it resumed departures, she said.

The Civil Aviation Authority is investigating the outage, which may take a couple of months, a spokesman said.