LOS ANGELESCFM International has begun flight-testing the Leap-1B engine for the Boeing 737 MAX, laying the foundation for the start of flight trials of the new Boeing airliner in 2016.

The enginewhich flew for 5.5 hr. in the No. 2 position on General Electric’s 747-100 flying testbed at Victorville, California, on April 29becomes the third member of the family to reach the flight-test stage, joining the Leap-1A and -1C for the Airbus A320neo and Comac C919, respectively. The Leap-1B is in development as the exclusive powerplant for the 737 MAX and differs from its stablemates in having a smaller, 69.4-in. diameter fan and two fewer low-pressure turbine stages.

The launch of -1B flight tests comes amidst an increasingly hectic test and certification effort for CFM that has accumulated more than 5,000 cycles over the three engine models.

“We have 26 engines either on, or that have been on, test stands as well as another 10 on the way in Cincinnati or Villaroche,” CFM Executive Vice President Allen Paxson says. “We have also contracted test facilities at Air France and in the Techspace Aero facility and have two engines in Victorville. So we are literally clicking through test points at a rate our business has never seen before.”

On the same day as the first flight of the Leap-1B on the flying testbed, Paxson says CFM was simultaneously conducting a blade-out test as well as performing ground tests of the Leap-1A on the A320neo in Toulouse. CFM, a joint venture between General Electric and Snecma, began test runs of the first Leap-1A at GE’s Peebles, Ohio, test facility on Sept. 4, 2013. Assembly of the first NEO flight-test engines began last summer at GE’s Durham, North Carolina, site, while flight tests of the mechanically identical -1C variant began in Victorville, California, on Oct. 6, 2014. Ground tests of the first -1B, meanwhile, began in Villaroche on June 13, 2014.

Initial Leap-1B flight tests will focus on engine operability throughout the flight envelope and include characterization during wind-up turns and windmill relights. Airborne-testing will also provide Boeing and CFM with the most detailed assessment yet on real-world reliability and fuel-burn performance--all-important parameters in the highly competitive marketing battle with the A320neo. The Leap-1B makes up the bulk of what Boeing predicts will be a 14% fuel-burn improvement over the contemporary CFM56-7B-powered 737. The current version has itself seen a 6% improvement in fuel burn since entering service in the late 1990s.

Although it is too early in the test effort for service-representative fuel-burn performance results, Paxson says data from ground-test engines show overall performance “is exactly what” CFM expected.

“When we build development engines they are heavily instrumented and built to accomplish extreme test conditions and durability,” he explains. “They are intentionally deteriorated and have open clearances because they are built for the ‘corner point’ in the test effort. We do pre-test predictions and we are within 0.5% of every one, so we are right on track. We fully expect to be right on our commitment as we enter into service.”

Keith Leverkuhn, Boeing 737 MAX vice president and general manager, adds that at this stage, neither fuel-burn performance nor operability is an issue.

“Am I seeing anything that gives me cause for concern? The answer is no,” he says. “Obviously over the next several months with the flying testbed is when we will really start learning about the performance of the engine.”

However he adds pre-text predictions by CFM engineers “have been awfully close to what’s actually happening in the flow path. They’ve been really spot on and, also looking at how the engine is doing from an operability standpoint, the engine has been really good and we are pleased with what we are seeing.”

Paxson concedes, however, that CFM has encountered a few “bumps along the way” in the initial development of the Leap-1B, but adds these will not delay the program. Improvements made as a result of test lessons include a change to the abradable coating in the compressor after parts began to “rub off and spray on the compressor airfoils,” he says. Further refinements have also been made to the control system for the engine’s active-clearance control device, while the leading edge of the stage 1 blisk has also been “ruggedized” to improve durability against debris entering the core.

The Leap-1B flight tests also represent crucial tests of the integrated Boeing nacelle, which is designed and built at the company’s recently established propulsion facility in Charleston, South Carolina. “It’s the biggest first step we’ve taken in getting back into the business of nacelle design and build,” says Leverkuhn, adding that the MAX unit is the first “100% designed and built by Boeing.”

The slimmer nacelle design incorporates a new lip skin and laminar flow “sculpted” inlet. The inner wall of the Spirit Aerosystems-provided thrust-reverser assembly is, for the first time, made of titanium and supplied by GKN Aerospace. Seattle-based Exotic Metals, a current provider on the 737, supplies the nozzle and plug.