BOSTON - S&P Global Ratings assigned its 'AA-' long-term rating to Metropolitan Washington Airports Authority (MWAA), D.C.'s nearly $598.3 million, series 2017A airport system revenue refunding bonds. At the same time, S&P Global Ratings affirmed its ratings on parity debt outstanding. The outlook is stable. "The ratings reflect our opinion of MWAA's solid market share and service-area strengths combined with very good system-wide airline diversity and generally stable system-wide enplanements trends that have grown since 2014," said S&P Global Ratings credit analyst Kurt Forsgren. Solid progress in recent years developing non-aeronautical revenue has reduced costs to airlines, although the authority's debt per enplanement is high on an absolute and relative basis compared with that of most U.S. airports, resulting in higher airline cost per enplanement. We expect bond proceeds to refund up to $380 million of the series 2007A-B bonds, generating approximately $50 million in debt service savings; fund up to $200 million in new money for capital spending; fund reserves; and pay costs of issuance. Traffic at the airport system is growing, reflecting the strong and diverse service area economy. It has a population of 6.1 million (making it the sixth-largest metro region fifth-fastest growing in the U.S.) which enplaned 22.6 million passengers in 2016, exceeding 2005's all-time peak of 22.3 million. DCA (where American Airlines Inc. has a 50% market share, ranks as the eighth-largest domestic market in the airline's network) experienced annual growth rates of 9.9% and 2.4% over the past two fiscal years, totaling 11.7 million enplanements in 2016. This set a passenger traffic record due in part to growth by Southwest Airlines Co. which now has the second largest market share at National at 14.6% . DCA generates 52% of the system's total passenger traffic. IAD's growth has been more limited, at 0.3% and 1.4% over the past two years and totaling 10.8 million enplanements in 2016. The stable outlook reflects our expectations that MWAA will manage its large capital plan without significant cost overruns or material scope changes while meeting projected financial margins and containing cost levels during the two-year outlook period. Passenger growth, on time and budget delivery of the CCP, continued improvement on non-aeronautical revenue development, and DSC (S&P Global Ratings-calculated) staying at recent levels that we view as sustainable could result in an upgrade over the two-year outlook horizon. Significant declines in activity levels or financial performance materially below forecast could result in a lower rating. Certain terms used in this report, particularly certain adjectives used to express our view on rating relevant factors, have specific meanings ascribed to them in our criteria, and should therefore be read in conjunction with such criteria. Please see Ratings Criteria at www.standardandpoors.com for further information. Complete ratings information is available to subscribers of RatingsDirect at www.globalcreditportal.com and at www.spcapitaliq.com. All ratings affected by this rating action can be found on the S&P Global Ratings' public website at www.standardandpoors.com. Use the Ratings search box located in the left column.