Air New Zealand has selected its next new batch of widebody planes. Its gone with what it knows - Boeing 787 Dreamliners. But the airline has gone for a bigger version of those already in its fleet, 787-10s. These will increase flexibility for the airline and give it scope to fly to more destinations. Grant Bradley examines the order in detail, why the airline opted for the planes, new features and what they means for passengers.

What has the airline ordered?

Eight Boeing 787-10s. This is a stretched version of two earlier models which first entered commercial service in 2011. Boeing says a 787-9 (of which Air New Zealand already has 13) can carry 290 passengers while the -10 can carry 330. The plane is about 5m longer and has about 15 per cent more space for passengers and cargo. The -10 first started flying in 2017 and so far 24 have been delivered to airlines including commercial partner Singapore Airlines. It has heavier, cantilevered landing gear to allow it to carry more weight while minimising the risk of tail strike.

What will the new Dreamliners be like for passengers

While they'll look similar from the outside to existing Dreamliners, Air NZ is promising to ''reinvent'' the interiors of the new planes (and retrofit existing widebody aircraft). Work on testing new cabins began last year with frequent flyers invited into a secret test facility in Auckland - Hangar 22 - to try out new seats and cabin concepts. The airline has been investigating zero gravity style seating. Its interiors are well due a revamp as it faces fierce competition, particularly at the front of the plane from other full service airlines. It's crucial to get the business cabin right as this where the serious money can be made. Luxon says when the experts are happy with the interior transformation that will be the big breakthrough moment to set the airline on its path for the next 10 to 15 years after that. Staff will have a different look with new uniforms by 2022.

How much will they cost?

These planes have a list price of $4.1 billion but Air NZ won't be paying anything like that. Airlines can extract discounts of 50 per cent or more depending on the size of the order, how keen the plane maker is to secure it and how hot the competition is. This is why airlines play the big plane makers Boeing and Airbus off against each other. Air NZ chief executive Christopher Luxon says small airlines do this to get ''maximum contestability'' to get good pricing. What airlines pay are tightly guarded secrets but Luxon did say Kiwis are good at getting a bargain.

What are they replacing?

Eight Boeing 777-200ERs (extended range) These started entering the Air NZ fleet in 2005 and have been used on long haul routes including Vancouver, Houston and Buenos Aires. But with an average age of 13.8 years they are nearly twice the average age of the rest of the airline's fleet. An interior upgrade in 2014 brought them to the standard of more recent acquisitions and brought seat count up to 320.

What was the tipping point for the selection?

It made alot of sense for Air NZ to stick with what it knows. There will be minimal extra training needed, its staff know these planes well and by expanding the total Dreamliner fleet, existing or future longer-range 787-9s can be reconfigured to fly further than its current longest flight from Auckland to Chicago. Luxon says Auckland-New York is one the airline's ''ambitions in life.'' Passengers may not have to wait until 2022 before it takes off. He's not putting a date on a launch but work on the route is understood to be well underway and if existing 787-9 planes are modified it could happen within a year or so.

Are there other planes in the pipeline?

Yes, in addition to the eight firm orders announced today, the agreement includes options to increase the number of aircraft from eight to up to 20. The airline has also negotiated substitution rights that allow a switch from the larger 787-10 aircraft to smaller 787-9s, or a combination of the two models for future fleet and network flexibility. The delivery schedule can also be delayed or accelerated according to market demand. This suggests Air NZ was able to extract a good deal out of Boeing. Luxon says the extra 12 aircraft would be for growth - if and when it happens. The airline has eight 777-300s and they will need replacing from the middle of next decade and this is where the exciting 777X may come in - while its large size and untested status made ruled it out for the current tranche, Air NZ remains interested in the plane, which is due to start flying commercially next year for launch customer Emirates. Airbus' elegant A350-1000 will be in the running again too.

Air NZ's profit's down, why's it spending on new planes?

Within hours of the big announcement there was further bad news about profit forecast after it was downgraded earlier this year and the airline is having to cut head office costs. But it's still well in the black, has one of the best investment grade credit ratings around and airlines need new planes to at least keep up with the opposition. It owns around 70 per cent of its aircraft (the rest are leased) and will decide how many of the new planes it will buy outright closer to delivery date. These new Dreamliners will be up to 25 per cent more efficient than the 777-200s and with fuel a third or more of total costs on long haul routes (of which Air NZ flies a high proportion), the attraction of new planes is clear.

Air New Zealand chief executive Christopher Luxon says his airline got a good deal. Photo / Greg Bowker

What about the engines?

Buying new planes gives airlines the opportunity to go with a new engine maker and Air NZ did just that - going with General Electric GEnx-1b power plants. Rolls-Royce Trent 1000 Package C engines on Air NZ planes are susceptible to premature corrosion and turbine blades have needed replacement meaning up to five Air NZ Dreamliners at a time have been grounded. Air NZ says in this financial year it could cost it up to $40 million. The engines have needed blade repairs in Singapore well ahead of expectations. Air NZ is almost through the repairs and is publicly very kind to Rolls, with Luxon saying he had been impressed with its engineering capability. Airlines are pressuring the plane makers which are in turn are putting the heat on the three big engine builders - GE, Rolls and Pratt & Whitney - to come up with ever more efficient machines. And this is where the rush to market sometimes means they don't spend long enough in test beds. GE engines now power Dreamliners which fly 18 of 20 longest routes, including Qantas' Perth-London service which had more than 99 per cent reliability in its first year. The carbon-fiber composite fan blades on the GEnx engine feature a new, more efficient design, a reduced blade count (from 22 to 18 fan blades) and a composite fan case for further weight reduction.

Haven't there also been problems with Dreamliners too?

Yes. Right from the start. The plane was first dreamed up around the turn of the millennium when it changed course from the delta wing high speed sonic cruiser, rightly assessing cash-strapped airlines' demand for highly fuel efficient (and not faster) mid-size planes. Extensive use of lightweight but extremely strong carbon fibre, more efficient aerodynamic design and more fuel-efficient engines made it attractive to airlines recovering from the impact of the 2001 terror attacks. But Boeing's new manufacturing processes that meant parts of the plane were made around the world and then assembled at its plant in Seattle soon led to problems, the supply chain broke down, some components didn't fit.

Delivery was delayed for years and the plane was dubbed the ''Seven Late Seven.'' Soon after it was introduced to flying (by Japanese airline ANA) in 2011 battery fires resulted in the global fleet being grounded. Better battery cell quality, venting of battery compartments and fire-proof technology allowed the plane to get back in the air and it has had an excellent reliability and safety record since. There have been concerns raised about workmanship at the non-union new Boeing plant in South Carolina, the plane maker has said there is no threat to safety.

Boeing has another massive problem, how does this affect Dreamliners?

New generation Boeing 737 Max planes have been grounded after two crashes that have killed more than 340 people. New flight control system software have been linked to the tragedies and the plane maker has been under intense global scrutiny for its introduction and how its handled the crisis. But the Max is a completely different plane - it's a narrow body aircraft and has different systems to those that have been on Dreamliners for close to a decade. Luxon says his airline had no problem going with Boeing, where it had bought planes for 50 years. ''It (the Max issues) didn't figure at all. The 787 family of aircraft have none of the challenges around the Max system.''