In Paris Saturday, CFM executives said development of the LEAP engine for new single-aisle jets is going very well but that Airbus and Boeing each raising production as high as 60 jets per month would stretch the limits of what’s possible.

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PARIS — Boeing’s plan to sharply increase 737 producion in Renton while simultaneously introducing the new MAX model depends crucially on engine maker CFM International smoothly conducting its own daunting production ramp-up.

In Paris Saturday, CFM executives said the engine’s development is going very well. However, they also indicated that if Boeing and Airbus were to raise production rates beyond current plans by decade’s end –  to as many as 60 single-aisle jets per month each, as both jet manufacturers have discussed – that would stretch the limits of what the engine-maker can supply.

CFM, a 50-50 joint venture between GE and French aerospace giant Safran, builds all Boeing’s 737 engines and about half of the engines for Airbus’s A320 family.

Its CFM56 engine powers the current generation of 737NGs and A320s. The new LEAP engine now in development will power the forthcoming 737 MAX, the Chinese COMAC C919, and half or more of the A320neo jet families.

CFM president Jean Paul Ebanga said that by 2020 the company will reduce the CFM56 production rate to a very low level while at the same time accelerating from building zero LEAP engines to building 1,800 a year.

GE and Safran have mounted a huge industrial push to create the LEAP capacity in plants around the world.

“The industry is turning from record orders to production,” Ebanga said. CFM, he said, is “preparing and gearing up to be able to address the coming ramp-up of the LEAP.”

Still, 1,800 engines a year is barely enough to satisfy the current planned rate increases by both Boeing and Airbus. Boeing is going to 52 jets per month in 2018. Airbus is targeting 50 per month in 2017.

In a briefing at Le Bourget, the site of the Paris Air Show that starts Monday, CFM executive vice president Allen Paxon, who heads the GE side of the joint venture, said “there are restrictions on how fast we can ramp up.”

“We cannot go zero to 1,800 in a year,” said Paxon. “There are limits. We are at those limits now.”

Paxon, and his Safran counterpart, CFM executive vice president François Bastin, added that it could be possible to ramp up higher with the existing CFM56 single-aisle engine.

“If the market wanted more CFM56 engines we could do it,” Bastin said. “Everything is possible with the right amount of time.”

The implication was that 60 jets per month may still be possible before decade’s end with a combination of the stepped up LEAP engine production – based on the manufacturers’ plans to switch over largely to the MAX and neo by 2020 –  plus maintaining higher production of the CFM56 engines.

But clearly it will be a challenge to push the engine supply higher than the current plan.

GE and Safran have together invested heavily to build new facilities and improve existing ones for production of the engine, which has already won more than 8,900 orders thanks to the huge sales successes of Boeing’s MAX and Airbus’s neo.

The CFM partners have built a new final assembly plant for the LEAP in Lafayette, Ind., and new engine parts plants in Auburn, Ala.; Rochester, N.H.; Ellisville, Miss.; and Greenville, S.C.

The final assembly plant in Villaroche, France, has been transformed with the creation of a new pulsed moving line for the LEAP work. Parts are also made in China, India, Germany, Mexico, Poland and Belgium.

The new plants are needed because of the new technology in the engine, which is larger but contains parts made from lightweight materials so that it will burn 15 percent less fuel than today’s CFM56 engines.

The blades on the 78-inch diameter fan are woven composites; the shroud around the hot core is made from composite matrix ceramics; the low pressure turbine blades at the rear of the engine are made from titanium aluminide; the tip of the fuel nozzle that sprays jet fuel into the combustion chamber, once made from 28 pieces of metal braised together, is now 3-D printed in a single piece made from a nickel cobalt alloy.

Paxon said a key requirement for all of these new materials was to be as robust as the corresponding parts on the CFM56, which is such a reliable engine that on average it spends 8 years on the wing of a jet before needing to be taken off for maintenance.

With so much innovation, the program has to be carefully controlled. Ebanga said Saturday that everything so far is going according to plan in terms of both performance and test schedule.

Flight tests on the Airbus A320neo engine began in May and are well advanced. Bird strike and other in-flight ingestion tests are also complete. That engine may be certified to fly by the aviation authorities later this summer.

The version for the Boeing 737 MAX, which is lagging the Airbus plane, will fly next year. Boeing is depending on CFM continuing to get it right.