CFM has frozen the design of its next-generation Leap turbofan destined for Boeing’s 737 MAX family of narrowbodies.

The joint venture formed by General Electric (GE) and Snecma also has started the assembly of the first of 28 Leap engines, which will be used for ground and airborne tests between now and early 2015, when the first variant of the new engine is due to be certified.

Gareth Richards, Leap’s program manager, during a May 2 press conference in London said the design freeze on the Leap-1B variant was completed on schedule on April 29. Design freeze of the -1A and -1C variants of the engine, destined for the Airbus A320NEO and the Comac C919, respectively, was completed in mid-2012.

Richards said the design freeze was a significant phase in the -1B’s development and will allow engineers to begin producing parts by the end of the year.

The schedule for the -1B is significantly different from the other variants. The -1A and -1C, which differ only in aircraft integration and fitting, are due to be tested in September, with certification due in 2015. For the -1B, the first engine will be ready for tests in June 2014, with certification due in 2016. According to CFM, the scheduling differences were requested by Boeing.

CFM is working to increase production capacity for the engines and supplier Albany Engineered Composites has opened a new plant in Rochester, N.H., to build the composite main fan blades to be used in the engine. A second facility in France will follow later this year.

CFM will build around 1,500 CFM56 engines in 2013, but expects Leap production to increase to 1,700 engines a year by 2020. This would require production of 36,000 fan blades each year.

Production of the CFM56 should begin to fall before then and will probably cross over with the Leap engine for about three years through to 2019. The only CFM56 production after that would be for spare engines.

The -1C will be the first Leap to fly on GE’s Boeing 747 flying testbed at the end of April 2014. The C919 first flight is expected shortly after that, followed by U.S. engine certification at the end of March 2015 and entry-into-service in the second quarter of 2016.

The NEO Leap engine currently is scheduled to become airborne on the flying testbed at the end of September 2014, with FAR 33 engine certification expected the following summer. First flight on the A320NEO is due around the third quarter of 2015 with entry-into-service the following year.

Certification of the -1B is expected in the first quarter of 2016, with first flight of the 737 MAX due later that year.