Jetstar and Qantas have cancelled a number of trans-Tasman and domestic flights as a safety measure, with all checkout counters at Auckland International Airport closed this morning.
Air New Zealand general manager airline operations and safety Captain David Morgan said it would have been easier to simply cancel flights and it had taken a lot of effort to develop alternative flight plans to continue to get passengers to their destinations.
In order to avoid the ash, domestic services had been operating up to a maximum 5500 metres, while trans-Tasman flights departing Christchurch and Wellington were given new flight paths heading much further north than normal before crossing the Tasman.
"The extra distance involved required the use of 10 percent more fuel, but has meant customers were able to safely get to where they needed to go."
Capt Morgan said the MetService had advised that the ash cloud was now much higher and the Civil Aviation Authority was comfortable for domestic and trans-Tasman services to continue to operate.
"We will not fly through ash and are constantly taking guidance from CAA and the MetService to ensure we can continue to carry passengers only where safe routes and altitudes are available."
However, the airline said that if the ash cloud sitting over New Zealand moved further north the airline may have to start cancelling flights.
This afternoon Qantas said it would resume flights to and from Melbourne from 1pm (Australian time) today.
All Qantas services to and from Melbourne were cancelled from 6pm yesterday because of the ash cloud, but based on its latest risk assessment, the airline had determined that it was now safe to recommence Melbourne operations.
Flights to and from Tasmania and New Zealand remained cancelled today.
Reports from Australia said a second volcanic ash cloud could drift across south eastern Australia within the next two days, with the potential to cause further disruptions to airline schedules.
Supervising meteorologist at the Volcanic Ash Advisory Centre in Darwin, Gordon Jackson, said the cloud currently over Victoria and Tasmania would remain for the next 12 to 36 hours.
"It's in an area where there is very little wind therefore it's not going to be moving northward, so (it will be) moving to the east and to the west and spreading out a bit," he told AAP.
Mr Jackson said the centre was monitoring a second ash plume hovering over the ocean south of Australia.
"It depends on how the wind is moving. It may actually be brought up over Tasmania and Victoria in 48 hours," he said.
Engineer and aircraft maintenance expert Peter Marosszeky said the volcanic ash was like sandpaper that could choke up a plane's engine.
"Volcanic ash in the air has disastrous consequences on aircraft that fly through the cloud," he said.
Virgin Australia to resume flights
Meanwhile Virgin Australia announced it had resumed flights out of Melbourne, Tasmania and New Zealand from 7am today.
"Overnight we have been monitoring closely the situation and we now believe that conditions are safe to operate," group executive operations Sean Donohue said in a statement.
Jetstar, Qantas cancellations
Jetstar and Qantas have cancelled a number of their trans-Tasman and domestic flights as a safety measure, with all checkout counters at Auckland International Airport closed this morning.
Jetstar has cancelled 24 domestic and international flights today, including one flight from Auckland to Singapore and one flight from Singapore to Auckland.
A decision on afternoon flights will be made at 3.30pm NZT.
Cloud could cover North Island
Civil Aviation meteorologist Peter Lechner told NZPA the cloud could be over most of the North Island by this morning, which would affect airline operations.
To people on the ground the ash particles, which were finer than house dust, would appear as high, thin cloud.
"You can imagine how much volume of air is being sucked through a jet engine, you can accrue a lot of ash, and the turbine blades are very hot so it just sticks to the blades, melts and builds up and eventually stalls the engine."
The ash was at an altitude of between 6000m and 10,700m.
Jets normally cruised between 8000m and 9800m, and turboprops operated at between 5500m and 6100m.
Aircraft could operate under that band safely, although jets would use more fuel at lower altitudes, making it more expensive for the airlines, Mr Lechner said.
Civil Aviation Authority spokesman Bill Sommer said the volcano was still ejecting particles, so further plumes could shroud the country until the eruption was over.
Unlike overseas aviation authorities, the CAA does not have the power to ground planes, and can only advise airlines.
When the volcano first erupted on June 4, ash was thrust 15,000m into the air, which meant the particles travelled through the stratosphere without being broken up by weather patterns.
It has since travelled 9000km over Argentina, Uruguay, South Africa and Australasia and is expected to circumnavigate the globe by the end of the week.
New Zealand is particularly vulnerable to the plumes because it lies on the same latitude as Chile. The westerly band of trade winds known as the "Roaring Forties" moves quickly, because there is little land south of 40 degrees. These winds have pushed the ash cloud directly towards New Zealand before it has had time to diffuse.
The volcanic ash can lead to a loss of thrust or engine failure - known as "flame-out" - on planes.
The ash can also wear down engine parts and clog fuel and cooling systems.
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