Boeing expects the pace of testing KC-46A mission systems, exterior lighting and ultimately aerial refueling to accelerate with the completion of the baseline airworthiness testing of the second functional tanker scheduled for March 8.

The aircraft completed its initial flight from Paine Field, Wash. on March 2, but landed back at Seattle’s Boeing Field after only an hour. Although Boeing completed an intermediate post-flight inspection over the intervening period it still aims to complete the remainder of the first flight profile on March 8 before inducting the aircraft into the flight test program.

Boeing says the first flight was shorter than normal because take-off was relatively late in the day and because the aircraft had to be prepared for inspection by Secretary of Defense Ash Carter during his visit to Seattle the following day.

The aircraft, dubbed EMD 4, is the third of four tanker prototypes to be tested under the U.S. Air Force’s KC-46A engineering and manufacturing development (EMD) contract. The first aircraft, EMD 1, which began the test program with its maiden flight in December 2014, has so far amassed more than 260 flight hours. The second aircraft, EMD 2, has flown for almost 190 hours since its initial sortie in September 2015. EMD 1 is a 767-2C “provisioned freighter” and will be converted into a full tanker later in the program, while EMD 2 is the first fully configured for tanking.

Since completing its first aerial refueling on Jan. 24, EMD 2 has refueled an F-16 and an F/A-18, as well as taken on fuel from a KC-10. On March 1, the tanker also refueled a U.S. Marine Corps AV-8B for the first time during a test flight in a section of the R-2508 restricted airspace complex close to the China Lake Naval Air Weapons Station in California.

Additional air refueling trials with the aircraft’s boom are expected to take place shortly with a C-17 and an A-10 to fill out the Air Force requirement to demonstrate refueling capability with an initial mix of aircraft. The A-10 represents the light/slow receiver and the C-17 a heavy receiver. Flight tests employing the centerline drogue system and wing aerial refueling pods were conducted with the F/A-18 as the light/fast receiver and with the AV-8B as a light/slow receiver.

With the addition of EMD 4, Boeing expects to accelerate demonstrations of refueling to include a total of 18 different aircraft types as it closes on the Air Force’s Milestone C decision covering low rate initial production of the first batch of aircraft. The Air Force is expected to make the Milestone C decision in April, enabling the award of LRIP Lot 1 in May and Lot 2 in June. The LRIP 1 award will cover seven aircraft, while LRIP 2 will cover 12. Between the two contracts the Air Force aims to have 18 combat-ready aircraft to meet the Required Assets Available milestone target by August 2017.