Having completed a recent review of requirement, configuration and certification plans for its 737 Max, Boeing plans to fly the first re-engined narrowbody in 2016. It now holds commitments from eight customers for more than 600 aircraft with "hundreds more" on the way.
John Hamilton, Boeing 737 chief programme engineer, in a briefing to media last week said the company will fix the configuration of the updated aircraft in 2013. It has already disclosed the aircraft will feature a 173cm (68in) fan on its CFM International Leap-1B engines.
Boeing has definitively frozen the length of the fuselage and the door configuration, maintaining the dimensions found on today's 737-700, -800 and -900ER, though Hamilton says the nose of the Max will ride higher than the Next Generation models.
"We can put a 68in fan on the airplane without changing the nose gear, but we allowed our designers to remove that constraint to see if they can further optimise the engine on the airplane," said Hamilton. "We believe there's a little better optimisation that can occur when we allow the nose gear to float up a little bit."
Boeing Commercial Airplanes CEO Jim Albaugh said the nose gear extension would add 15-20cm (6-8in), though Hamilton declined to commit to that figure, saying there is "squishiness" on the final length.
"We haven't finalised exactly how much we need to grow the nose gear," said Hamilton, who said only "minor changes" to the nose landing gear well would be required to relocate components to accommodate the longer strut.
Hamilton added that the engine will ride further forward relative to today's CFM56-7BE engine.
Hamilton confirmed that Boeing will employ fly-by-wire spoilers on the 737 Max, the first such implementation of a digital flight control system on the narrowbody's design "to save weight on the airplane, improve the production flow, and allow us to improve stopping performance of the airplane."
Conceptually, the 737 Max's hydraulic system redundancy will be based on a scaled-down version of the larger 757, which can be simplified by only applying fly-by-wire to the spoilers, he said.
Other features of the Max committed to by Boeing included a reshaped tail cone to clean up aerodynamic performance, widespread structural strengthening, a new nacelle and pylon for the Leap-1B, and fuel, pneumatic systems and updated engine software.
The aircraft may also incorporate technologies that will be added to the Next Generation 737 ahead of the Max's deployment. Boeing plans 2012 tests for its EcoDemonstrator programme with American Airlines - the only disclosed customer for the Max - which will evaluate a mini-split flap, variable area fan nozzle, adaptive trailing edge technology, flight trajectory optimisation and regenerative fuel cells.
Beyond the disclosure of select details, Hamilton emphasised that many key attributes of the Max are still undefined, including the configuration of the engine's "custom core", wingtip devices, thrusts and weights based on customer requirements, which he said will be established "later next year."
The Leap-1B's "custom core", which will anchor an 11-12% improvement in fuel burn and 7% operating cost improvement on the aircraft, has evolved from its initial concept to bolster the powerplant's performance.
Hamilton said of the design: "With development programmes, you start with one engine and you start to refine it. The initial engine that CFM proposed to us probably had a lot of commonality with the C919 and [A320neo], but that was kind of our starting point. But we're going to continue to work with CFM every day on this and continue to customise the engine so that it is unique for the 737."
"We haven't finalised all the details of the engine, but we've dialed it in enough now, we know that we can get much better performance than what" was known when the evaluations first began with the baseline engine configuration, said Hamilton.
With its 2017 service entry planned, Boeing aims to certify the new 737 variant under an amended type certificate rather than a full-scale recertification.
Hamilton gave no specific timeline for the firming of the more than 600 orders and will be contingent upon establishing the performance of the aircraft, which will enable Boeing to provide contractual guarantees to its customers.