July 12, 2013
Credit: Amy Butler
The U.S. Navy’s third attempt to land the X-47B onboard the USS George H.W. Bush off the Virginia coast on July 10 — following the first-ever landing of the stealthy, tailless unmanned aircraft on the carrier earlier that day — resulted in a divert to its backup airfield at Wallops Island, Va.
Roughly 4 mi. out from the carrier deck and about 2 min. prior to landing, the second X-47B demonstrator — designated Salty Dog 502 — detected an anomaly in one of three onboard navigation computers; the aircraft runs constant health diagnostics for these computers during missions. This prompted the aircraft to autonomously execute a wave off; it then flew back into the pattern to await feedback from the ship-based mission operator.
The mission operator — housed in a container in the carrier’s hangar — then ordered the aircraft to follow its preprogrammed routine for a landing at Wallops. “We had decided that we’d run enough for the day,” says Navy Capt. Jaime Engdhal, program manager for the $1.4 billion Unmanned Combat Air System demonstration project.
Each of the two Northrop Grumman X-47B demonstrators has three redundant navigation computers and another three are on the ship. There is also triple redundancy in the data links between the two. The system uses precision relative GPS to get the aircraft safely on the ship. In other words, the aircraft knows where it is relative to the ship, which is moving forward and pitching and rolling on the waves.
“Nothing went wrong with the aircraft. In fact, everything went right,” Engdahl says.
Rear Adm. Mat Winter, Navy program executive officer for unmanned aviation and strike platforms, said, “We learned a lot in that off-nominal … scenario.” Without the actual event, officials would have relied only on modeling and simulation for such a sequence of events.
The test team is working through the air vehicle’s data now and Carl Johnson, vice president overseeing Northrop Grumman’s UCAS program, says they will probably find it is “a minor issue” that will be resolved once the computers are reset.
Monday, July 15 is the X-47B’s next opportunity for ship landings. Engdahl plans to get one of the demonstrators back onboard to repeat more landings if possible.