United Airlines is expanding its partnership with regional provider Mesa Airlines through the introduction of 12 new Embraer E175 regional jets beginning in May of 2017.

The additional E175s will grow United’s overall E175 fleet to 60 with Mesa Airlines and 100 overall once all ordered aircraft are delivered. Republic Airline also operates 16 E175s for United with 24 more on order, while Mesa operates 48 E175s and 20 of rival Bombardier’s CRJ-700 regional jets. In all, United has ordered 135 E175s to date, and has yet to place the remaining 35

United’s E175s seat 76 passengers in a three class configuration with 12 First Class seats, 16 Economy Plus ones, and 48 Economy Class seats. At present, the E175s operate primarily from Houston, Newark, and Chicago O’Hare with limited flying from Denver and Washington Dulles.

United more broadly is in the middle of a transformation of its regional fleet which will see the volume of regional aircraft (both jet and turboprop) in the United Express fleet with fewer than 50 seats drop to less than 100 by 2019.

Today, United operates more than 286 including 195 Embraer E145 RJs, 63 Bombardier CRJ-200s, 5 37-seat Embraer E135s, and 23 turboprops (2 ATR 42s and 21 Bombardier turboprops).

It is unclear how the recent deferral of United’s 61 outstanding orders for Boeing 737-700 jets will affect the plans to reduce the fleet of small RJs. When those orders (for 40 and 25 in two tranches) were announced, they were positioned as allowing United to up-gauge certain large RJ flights to mainline, thereby creating a chain reaction of up-gauging small RJs to large RJs and retiring the small RJs (and props) all together.

But United more broadly has lagged behind peer airlines American and Delta in reducing its reliance on small regional jets. United has a higher proportion of RJs on its domestic flying, including on massive routes like San Francisco – Dallas Fort Worth, and that is widely seen as having contributed to United’s degraded customer perception over the past half-decade (on a relative basis). United has taken several steps to improve its customer experience over the last 2 years under the leadership of Oscar Munoz, but the RJ issue remains a major sticking point for those moves.