Turbulence when flying is a fairly commons experience for everyone. But what exactly is turbulence and how does it happen?

Turbulence: What exactly is it?

An Air New Zealand plane has been forced to do a mid-air U-turn after it failed to get permission to fly over Iran — marking the second Air New Zealand flight this month that has been stifled due to a lack of clearance.

The new Airbus A321neo was on a delivery flight from Hamburg, where Airbus has a major production facility, to Auckland and was believed to be flying via Oman, Kuala Lumpur and Cairns.

Due to a “last-minute delay in the delivery process”, the plane took off for Oman before it had been given clearance to fly over Iranian airspace.

Without the necessary clearance, the plane was unable to fly over Iran and had to turn back around and return to Hamburg.

“We had received assurances by our delivery flight planning agent that the Iranian clearance paperwork would come through during the flight,” Air New Zealand said in a statement.

“Unfortunately this was not received in time and the decision was made to return to Hamburg.”

The airline called the incident “unfortunate” but said turning back was “not uncommon”.

It said: “Each clearance process is managed differently and has different application deadlines and parameters (for instance one jurisdiction may require proof of an existing clearance to land elsewhere, another may outline a set period of time when the aircraft is able to land within that airspace, and some require application within a short window of the flight departing).

“While the turn back is unfortunate, the nature of obtaining a chain of interlinked clearances means scenarios of this nature are not uncommon.

“The aircraft has since departed Hamburg and is expected to arrive in Auckland late on Thursday evening.”

It is the second time in weeks an Air New Zealand plane has been refused clearance to enter a foreign country.

Flight NZ289, carrying about 270 passengers, was turned around several hours into its journey from Auckland to Shanghai on February 9 as it wasn’t given permission to land by Chinese authorities.

“A technicality meant the particular aircraft operating this service did not have Chinese regulatory authority to land in China,” the airline said at the time.

Aviation experts pointed the finger at Air New Zealand and said there was likely to have been a “serious administrative cock-up”.

Earlier this week New Zealand Prime Minister Jacinda Ardern defended the country’s relationship with China amid suggestions of a diplomatic rift.

A major China-New Zealand tourism scheme was also delayed this month — Chinese media claiming tourists were “punishing” New Zealand for its decision last year to ban Chinese telco giant Huawei from operating in the country.

“This is a robust and mature relationship,” Ms Ardern told reporters on Monday.