Global Hawk Testing Awaits Investigation
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Amy Butler and Guy Norris
USAF says Global Hawk's unmanned attributes facilitated impromptu lakebed landing Printed headline: Sortie Spoiler
Testing of the Global Hawk unmanned aerial system (UAS) at Edwards AFB, Calif., was halted pending investigation of a May 28 emergency landing that was prompted by a spoiler malfunction during a test sortie.
The landing occurred at 11:53 p.m., and is being hailed by Maj. Gen. David Eichhorn, Flight Test Center commander there, as a serendipitous success due to the availability of the massive Rogers Dry Lake Bed, which is used by the space shuttle for landings.
Global Hawk air vehicle 9 (AV-9) was about 9 hr. into a sortie designed to characterize performance of the Raytheon Enhanced Integrated Sensor Suite when the first indication of a malfunction emerged. "The descent from altitude was not proceeding according to profile and the spoilers were not responding as predicted," according to Edwards officials.
Eventually, the aircraft landed after an unexpectedly long, unpowered glide. Once on the lakebed, the Global Hawk's right main landing gear struck one of NASA's Precision Approach Path Indicator (PAPI) lights for shuttle landings that are located on the path to Runway 22L. The light fixtures are trailer-mounted, but are considered fixed units, as they are not usually moved once aligned. In the incident, the UAS's main right gear struck both the light and the trailer and was partially sheared off in the process; the UAS's sensors are intact, though. NASA adds that for the upcoming STS-127 International Space Station mission a spare light will be put in place.
The Global Hawk landing is notable for two reasons. "Landing on the lakebed would not be an option with a manned aircraft since the lakebed runways are unlit, but the unmanned Global Hawk wouldn't know the difference," Eichhorn says. The PAPI system provides slope indications for the shuttle, but is not suitable for landing manned aircraft. Secondly, the endurance of the aircraft allowed enough time for officials to come up with a contingency plan.
Engineers and contractors toiled for about 7 hr. while the aircraft remained in flight overhead--Global Hawk is designed to exceed 24 consecutive hours of flight--to "find a way to safely recover what engineers formerly believed to be an unlandable configuration," they say.
At issue was how to decrease speed enough upon approach to increase the chances of recovering the aircraft and its expensive sensors intact. "Letting the aircraft land 'normally' would result in failure of the landing gear . . . because lack of spoilers would cause the aircraft to touch down too fast and then bounce back into the air," Eichhorn says in a June 2 commentary published on Edwards AFB's web site. "It's the next landing that's extremely nasty. The aircraft would float up and up and then nose over to come back down. The subsequent landing would be hard enough to fail the gear."
At about 11:30 p.m., officials decided to shut off the engine to allow the aircraft to glide on its approach and reduce speed upon touchdown. Eichhorn comments that this was a "daring plan," noting that--as with shuttle landings, including one on May 31 on the lakebed--landing without engines eliminates the opportunity for a second attempt. "The aircraft floated . . . far further than anyone expected. . . . It landed about 6, maybe 7 mi. down the runway," Eichhorn says.
He says he is "impressed" with how well the UAS withstood the collision after touchdown. The landing gear "didn't collapse for several more thousand feet."
Eichhorn says officials are "keeping their fingers crossed" that damage will cost less than $1 million, which is the threshold for a Class A mishap review.
The plan for an unpowered landing attempt also takes a lesson from the playbook of the high-flying U-2 spy aircraft, which will eventually be replaced by the Global Hawk. Like the U-2, the Global Hawk was designed to collect intelligence above 60,000 ft., a requirement that drove engineers to design the system for a steep climb rate to maximize the time for each sortie at altitude. And with both systems, this design presents challenges for a landing. The spoiler is engineered to increase drag during descent for landing, allowing the aircraft to stably reach the ground at a safe speed.
U-2 landings require the pilot to stall the engine upon approach, effectively decreasing speed fast enough to drop the aircraft onto the runway. On its centerline landing gear, the U-2 touches down and then glides to rest on one of its wings, which are reinforced to handle this wear and tear.
An investigation is ongoing to determine why the spoiler malfunctioned.
In the meantime, Pentagon acquisition chief Ashton Carter conducted a review on May 29 of Global Hawk, during which he was briefed on the incident. The Pentagon hasn't yet released the outcome of this review. A new schedule to accommodate a delay to the initial operational test and evaluation schedule for the Block 20/30 Global Hawk hasn't yet been released; this delay existed before the May 28 incident. Pentagon officials have suggested the delay could slip those flight trials, which were set for August, by about six months.
Photo credit: Northrop Grumman
