The U.S. Navy and Northrop Grumman failed to execute a fourth landing attempt of the X-47B onboard the USS George H.W. Bush carrier, and the Unmanned Combat Air System (UCAS) demonstration flight test program has completed its planned “at-sea” trials.

The first X-47B demonstrator — Salty Dog 501 — launched from NAS Patuxent River, Md., July 15 but experienced a “minor test instrumentation issue” that prompted officials to scuttle the landing attempt and recall it to the land base, says Jamie Cosgrove, a spokeswoman for the Navy UCAS program. The aircraft landed safely at Patuxent River.

The second X-47B — Salty Dog 502 — conducted the first two landings of a stealthy, tailless unmanned aircraft onboard an aircraft carrier on July 10, a major aviation history milestone. The third attempt by that aircraft later in the day, however, was scrapped due to a self-detected navigation computer anomaly, and the air vehicle commanded a wave-off about 4 mi. out from the ship. Though the aircraft carries three of these computers for redundancy, testers opted to command the aircraft to divert to the runway on Wallops Island, Va., its pre-programmed backup landing site.

No further carrier landing tests are planned, and the George H.W. Bush returned to port July 16.

Though the last two attempts failed, the first two represent “a decade of Navy unmanned integration efforts that show the Navy’s readiness to move forward with unmanned carrier aviation,” according to Rear Adm. Mat Winter, Navy program executive officer for unmanned aviation and strike weapons.

The Navy and Northrop Grumman officials are still combing through data collected from the July 10 tests. Winter says he will continue to look for opportunities for the air vehicles to be used for other Navy test efforts, but none are planned.

The aircraft are expected to find homes at the naval museums at Patuxent River and Pensacola, Fla.