Flight testing of the Mitsubishi Aircraft MRJ has addressed the areas of greatest risk, the company said.

These include the extremes of weights and center-of-gravity positions, buffet boundaries and performance in stalls. No flutter has been encountered when reaching design speeds, Mitsubishi Aircraft said.

The company plans to add two aircraft to the flight-testing program to verify design changes prompted by a reassessment of certification requirements in late 2016. This will bring the flight-test fleet to the unusual total of seven aircraft, but the two additional units will be good candidates for later sales to customers, said Yugo Fukuhara, general manager of sales and marketing.

Four MRJs are flying, all at Moses Lake, Washington. They have been built to the design of the MRJ90 version, which is intended to seat 88 passengers in an all-economy configuration. Since mid-2017, the flight-test fleet has been used to verify performance with various weight loads and distributions, the points at which buffet is experienced and how the aircraft behaves in a stall. Other tests have evaluated the operation of the direct mode of the fly-by-wire system and specific fuel consumption of the Pratt & Whitney PW1200 engines, which has been as expected. No show-stoppers have been discovered, the company said.

Mitsubishi Aircraft also is developing the MR70, with seats for 76 all-economy passengers.

A fifth MRJ90 is complete but has so far been used only for ground tests at the program’s base at Nagoya, Japan.

Two redesign efforts were required after the company realized in 2016 that vulnerability to underfloor water ingress and to bomb damage could imperil the MRJ’s airworthiness certification. One change was a reshuffling of avionics in and between the forward and aft avionics bays, the design of which was completed in 2017. The other was changing wiring harnesses. For that, the architecture-level design is complete but detail design is not, the company says.

When launched in 2008, the MRJ was due for delivery in late 2013. It has since been repeatedly delayed, usually because of some difficulty in complying with certification requirements.