There is no doubt that the first-ever arrested landing of an unmanned aircraft on a carrier deck, a feat achieved last week by the U.S. Navy, will be among the most notable events in aerospace history.

But the dazzling display 70 mi. off the U.S. East Coast was followed by challenges, underscoring the complexity of achieving the Navy's longtime goal of marrying the power projection of its aircraft carrier fleet with the endurance and range of unmanned aircraft. Anomalies with the Unmanned Combat Air System's (UCAS) ship controls and navigation computer—experienced after the seminal landing—point to areas that will need work as the Navy eyes a carrier UAV purchase as soon as next year.

The Navy's second Northrop Grumman X-47B demonstrator, Salty Dog 502, made aviation history July 10 with the landing, which Navy Secretary Ray Mabus said helps to “make sure that we keep the technological edge” as countries such as China work to build stealthy aircraft and unmanned vehicles.

Though it is an achievement, the event underscores that the $1.4 billion program is experimental, as operators experienced an anomalous shift of the aircraft's controls after the landing. A third arrested-landing attempt was aborted, prompting the vehicle to divert to the backup landing site at Wallops Island Air Field, Va.

The first landing snagged the No. 3 wire as planned just after midday. The second followed a catapult takeoff from the deck and touched down with the No. 2, nine vertical inches (translating to a few horizontal feet) from the projected impact point, says Capt. Jaime Engdahl, Navy UCAS program manager.

Prior to the first landing, the on-deck landing systems officer (LSO) ordered a go-around, an intentional use of a standard procedure, to ensure the algorithms responded as needed. The LSO uses a digital interface to relay commands to the UCAS, and its software translates them.

Once the media and senior-level onlookers departed, a third landing attempt failed. The X-47B conducted an autonomous wave-off about 4 mi. from the carrier after it detected an anomaly during a routine check of the three onboard navigation computers. The aircraft then rejoined the pattern until a ship-based operator commanded a diversion to Wallops.

A separate anomaly cropped up earlier as UCAS operators attempted to hand off control of the air vehicle from an on-deck operator—which uses an arm-mounted device to taxi the aircraft—to a mission operator in the aircraft's hangar after the first landing. The blue light, a signal of a successful handoff, did not illuminate; instead, operators saw a red light, indicating a faulty one.

The deck crew quickly worked with a backup arm-mounted deck controller, achieving proper handoff, according to Engdahl. The backup system was there to ensure the aircraft was not stuck on deck for a long period of time, which would be a problem in an active air wing where aircraft are constantly rotating on deck.

The arrested landing is the capstone and most difficult test for UCAS, following May's first catapult launch. The landing is more complex, as the aircraft's autonomous software and precision-relative GPS must compensate for the motion of the carrier, says Rear Adm. Mat Winter, Navy program executive officer for unmanned aviation and strike weapons. The next at-sea trial for the UCAS is scheduled for July 15, during which one of the two demonstrators will attempt more landings.

However, the air vehicles are likely nearing the end of their flight careers. Both are slated to take permanent residence at naval museums, one in Pensacola, Fla., and the other at NAS Patuxent River, Md., Winter says. Until the program formally wraps up at year-end, he says the Navy will continue to assess whether the aircraft could be used for other projects.

The UCAS is a precursor to the Navy's Unmanned Carrier-Launched Airborne Surveillance and Strike program through which the Navy plans to buy at least two “orbits”—the ability to conduct two, separate 24/7 operations from a ship—for $150 million or less, not including the technology demonstration price.

Boeing, Northrop Grumman, Lockheed Martin and General Atomics are all vying for the contract, and the number of aircraft sold will vary depending on how the contractors design an orbit. Air vehicles proposed by each are being reviewed by the Navy through a nine-month preliminary design review phase.

In parallel, the Navy plans to issue a draft request for proposals in August to kick off the competition for the aircraft contract. A formal request for proposals is expected in the second quarter of fiscal 2014 with a downselect by October 2013.

Watch videos of the UCAS arrested landings on our Ares blog at ow.ly/mS64L