If there's one key selling point that aerospace giants Airbus and Boeing have both been using in marketing their new aircraft, it is fuel efficiency.
But neither of the two major plane makers are directly responsible for much of the promised improvement in fuel usage.
It is in fact largely down to French company Safran and GE, whose joint venture CFM International is making the LEAP motors that will power most of the next-generation of the planes which will be the future workhorses of airlines.
The promise of 15 to 20 percent fuel savings moved closer as LEAP engines made their first flight tests that are a key step for the Airbus320neo to enter service next year and Boeing's B737 MAX in 2017 as planned.
The LEAP-1B for the B737 MAX made its first test flight in California on May 7 on a modified Boeing 747.
"With this major engine milestone and the test results to date, we continue to be confident that the LEAP-1B-powered 737 MAX will provide our customers with the most fuel efficient, reliable and maintainable airplane in the single-aisle market," the head of the 737 MAX programme at Boeing, Keith Leverkuhn, said in a statement.
Meanwhile, the first A320neo equipped with LEAP-1A engines made its maiden flight in Toulouse, France, on Tuesday.
"Today marks yet another important step forward in the development of the A320neo and I would like to warmly thank all the teams who made this first flight happen," said Klaus Roewe, head of the A320 programme at Airbus.
Both Airbus and Boeing have a lot riding on the new aircraft which have driven the record orders that the two companies have racked up in recent years.
- Razor-thin margins -
Instead of developing new aircraft for the medium-haul segment, as Boeing did for long-range with its Dreamliner, both companies opted to modernise their existing planes and much of the gains are down to improvements in the engines.
Those promised new models have proved to be best sellers for Airbus and Boeing, especially with low-cost airlines which operate on razor-thin margins.
At 3,794 orders, the A320neo accounts for more than half of Airbus's backlog of 6,399 aircraft. At Boeing, the B737 MAX accounts for 2,724 out of 5,667 outstanding orders.
That has put enormous pressure on CFM International, and its parent companies Safran and GE, to deliver on the 15 percent fuel savings that the LEAP engines promise compared with CFM's current engine, the CFM56.
"I am completely confident of attaining the promised performance levels when the Leap enters service" as planned next year, said Safran chief executive Jean-Paul Herteman recently.
"The LEAP is very much on track to do what we planned it to do."
The LEAP engine's fuel efficiency comes from better performance as well being lighter thanks to new 3D weaving techniques for composite materials used to make the fan blades and fan casings.
Not only will the new materials add up to weight savings of some 500 kilos (1,100 pounds) per plane, they promise to be stronger and more durable and thus cost less for airlines to maintain.
The next-generation planes are an important source of business for CFM International.
It is the only company developing an engine for the B737 MAX, and is also developing the Leap-1C that will power the C919, the plane that Commercial Aircraft Corp. of China (COMAC) hopes will help it break the Airbus and Boeing duopoly in the market for single-aisle, medium-range passenger jets.
CFM has competition however for the A320neo from US-based Pratt & Whitney, which says its PurePower PW1100G-JM engine is optimised to improve fuel burn, reduce weight and increase component durability.
CFM said the LEAP-1A has so far scooped up 56 percent of orders for the A320neo.
The company forecasts it will produce 30,000 LEAP engines by 2020 in its factories in France and the United States, besting the 27,000 of the CFM56 in service that had propelled Safran and GE into becoming the top manufacturer of engines for passenger jets with over 100 seats.