Mitsubishi Aircraft received a huge dose of credibility at the Farnborough International airshow yesterday by announcing a 100-aircraft commitment for MRJ90s from the largest regional airline holding company in the world–SkyWest Airlines. The agreement in principle, signed just this week, potentially raises the MRJ regional jet family backlog to 170 airplanes and gives Mitsubishi its second major U.S. customer.

“This is the market we’ve been targeting all along,” Mitsubishi Aircraft vice president of sales Hank Iwasa told AIN. Restrictions in U.S. major airline union contracts that limit the size of airplanes that regional airlines may operate have long hindered the company’s efforts to break into the U.S. market, but Iwasa expressed confidence that one or more of SkyWest’s major airline partners would negotiate less restrictive scope clauses by the time the first MRJ gets delivered.

The MRJ90 holds 92 seats in a single-class configuration, but SkyWest could opt for a dual-class cabin that would likely hold closer to 80 seats, said Iwasa. U.S. scope clauses typically limit the size of regional jets to 76 seats.

The contract, which gives SkyWest the option to convert part of its order to positions on 76-seat MRJ70s, calls for delivery of all 100 airplanes over a three-year span, between 2017 and 2020. By that time, said Mitsubishi Aircraft CEO Hideo Egawa, the company expects the MRJ’s production rate to reach “four or five” per month.

The MRJ program has faced its share of obstacles since Mitsubishi Heavy Industries began assembling the first aircraft in April 2011, due most notably to the manufacturer’s failure to properly document engineering and production processes. In fact, this spring Mitsubishi announced it had to delay first flight of the MRJ90 for more than a year, until some time during next year’s fourth quarter. As a result, the airplane won’t reach the market until the summer “or later half” of the 2015 Japanese fiscal year, which runs from April 1, 2015, to Mar. 31,2016, according to new program schedules.

The program’s rather abrupt interruption came at the behest of the Japan Civil Aviation Bureau (JCAB), which, in concert with the U.S. Federal Aviation Administration (FAA) and the European Aviation Safety Agency (EASA), carries responsibility for issuing type and production certification for theairplane.

Since the JCAB intervened, MHI has had to remanufacture virtually all the parts for the first prototype, explained Mitsubishi Aircraft marketing director Yugo Fukuhara. The company has now begun refabricating the large structural components, most notably the forward part of thefuselage.

Seemingly unfazed by the setback, Pratt & Whitney hasn’t slowed testing on the engine chosen to power the MRJ–the PW1217G geared turbofan, which finished its first flight test program on June 21 following 23 flights and 127 hours on one of the engine company’s two Boeing 747SP flying test beds. The flight-test program, which began on April 30, successfully validated the PW1217G’s in-flight performance, operability and control systems, said Pratt & Whitney. So far the company has run the engine for more than 500 hours–and nearly 6,000 cycles.