Technology Focus: Unmanned Vehicle Maintenance

Unmanned aircraft systems (UAS) are a growing factor in the aviation community, spurred by the military but looking to expand into the civilian commercial market. What are the maintenance requirements for this new genre of aircraft and what skills should be required of maintainers?

Spurred by military needs, the unmanned vehicle industry is booming and manufacturers are targeting commercial niches, from aerial photography to cargo transport. Unmanned aircraft systems (UAS), as the Federal Aviation Administration (FAA) terms them, promise lower-cost aviation operations.

But what maintenance standards will apply? Since unmanned aircraft systems move pilots and cockpits to the ground, is the risk increased? Should maintenance and design requirements be as high — or even higher — than those that apply to conventional airplanes? And what skills should be required of maintainers?

Opinions range from establishing maintenance standards equivalent to conventional aviation across the board to tailoring requirements to the size and mission of the UAS. The latter view appears to be more common, given the range of aircraft: from gadgets you can hold in your hand to 32,000-pound Global Hawks. The U.S. military has the most experience with unmanned air vehicles. According to one Army observer, there is broad agreement that UAS the size of a Hunter surveillance vehicle (1,600 pounds) and up need to be maintained similarly to a manned aircraft. The smaller (375-pound) Shadow tactical reconnaissance, surveillance and target acquisition UAS likewise boasts a sophisticated maintenance program. But commanders are reluctant to saddle themselves with much maintenance overhead for vehicles that "look, sound and act like a model airplane," the observer said. The FAA appears to sympathize with this position and may regulate the latter class of UAS very lightly.

The UAS industry is young compared with conventional aviation, and there are no widely agreed upon standards for design, not to mention maintenance. Despite the eagerness of the UAS industry to fly commercial applications routinely in civil-controlled airspace, it will not happen immediately. Even the biggest boosters don’t see unmanned aircraft in significant commercial use for another decade — and not, at first, in U.S. airspace. The FAA is still in the initial stages of analysis, granting experimental airworthiness certificates (EACs) without hefty maintenance requirements.

In No Hurry

Maintenance is the last piece of the UAS regulatory puzzle that will be put in place, according to FAA. "We need [maintenance standards] in place when all the other things come together, when we have technical standards in place so that we can certificate aircraft — when we have operational rules and regulations in place to support the rules for integration of unmanned aircraft," said Doug Davis, manager of the agency’s UAS program office. He predicts that all of this activity will be a "several-year" effort, and that maintenance procedures, policies and regulations will be "a subset of all that."

Maintenance standards will follow design standards, emphasized Susan Cabler, deputy manager of FAA’s aircraft engineering division in the certification area. "Until we have design standards out there, we won’t know what the right maintenance and continued operational safety programs need to look like. But, we’re still years away from having design standards for unmanned aircraft," she added.

Is the risk higher for remotely operated aircraft than for piloted vehicles? "In some areas the risk may be higher," Davis said, "but in some areas it may not be." FAA does not have the data yet to make conclusions either way, he said. Standards may differ depending on the airspace used, suggested John Walker, co-chair of RTCA’s Special Committee (SC) 203, charged with developing UAS recommendations to FAA.

In some ways regulation could depend on an operator’s business case. A vehicle’s expected lifespan, mission and area of operation are some of the most obvious questions. If cargo needs to be carried from point A to point B, for example, "I may want a higher standard because I need to make sure it gets there," Davis said.

For the time being, companies seeking permission to fly non-governmental applications in the National Airspace System (NAS) must apply for experimental airworthiness certificates. "We’re concerned about safe operations," Cabler said, "but to tell you the truth, if these things don’t make two flights, we don’t care if they’re not maintained so that they have an operating license. [FAA is] trying to work on over arching standards, over arching issues, at this point, not specific, in-the-weeds stuff," she said. The agency is "just concerned that they operate safely in the NAS." So far FAA has issued 14 EACS. All the experimental certificates for UAS were issued at private locations except for two, the AAI Shadow, at Benson Airport, Benson, Ariz. in February 2007 and the Aurora Flight Sciences GE50, at Golden Triangle Regional Airport, Columbus, Miss. in May of 2007.

Applicants for experimental airworthiness certificates must submit a maintenance plan as part of their package, Davis said. "But it’s geared to ensure its safe operation — not any kind of life expectancy — at this point." FAA also is working with the Department of Defense (DoD) on certification standards. But that’s "a very initial, low-level effort right now," Davis cautioned.

Another difficulty is the sheer variety of unmanned aircraft. "The question is, should something that weighs five pounds be held to the same maintenance standard as a 767?" said Davis. "The obvious answer should be, ‘probably not.’" The agency’s Notice of Policy, dated Feb. 13, 2007, disclosed that it is examining the feasibility of "creating a different category" of unmanned aircraft that are small, slow and operated within visual line of sight. This new class of UAS, which some speculate will be the first to fly in numbers in U.S. civil airspace, "may not require a certificate of airworthiness," FAA said.

FAA is looking into a small UAS policy that "will take a nontraditional approach to certification because of its very nature and because of the hazard or the risk amount that they may or may not bring into the airspace system," Davis said. A ballpark range for these prospective vehicles is five to 20 pounds. One of the drivers behind this initiative is local police forces that want to use small unmanned vehicles.

The agency currently has projects with two police departments. One department is looking at three to four months’ use of a small UAS in a tightly controlled situation, Davis said. The small vehicle would be used to gather data. FAA, however, does not typically set airworthiness standards for public organizations, Davis said. Such entities go through a certificate of authorization (COA) process, which includes an "airworthiness statement." Maintenance is embodied in this airworthiness statement, he said. "There is an expectation that they are doing a fundamental level of risk analysis and safety analysis."

Another challenge in developing standards is the scarcity of reliability data. Under the COAs and EACs, FAA is beginning to require applicants and holders to submit activity data on a routine basis, but that’s not part failure data, Davis said. The agency is waiting to have access to DoD’s UAS safety databases, as well. But data from the battlefield, as he pointed out, is often ambiguous and hard to compare with data gathered in normal operating conditions.

Army Experience

The U.S. Army’s Shadow UAS has a sophisticated maintenance program. In use since 2002, the vehicles have flown about 220,000 flight hours, and are now achieving about 12,000 flight hours a month in round-the-clock combat operations. Shadow’s maintenance figures are steadily improving.

With Shadow, the Army uses a performance-based logistics (PBL) approach that translates to "buying outcomes versus components," according to Tim Owings, deputy program manager for the service’s UAS program office under the Program Executive Officer (PEO) for aviation. That means "contracting for readiness rates, contracting for core reliability," he said.

This approach encourages the contractor to make trade-offs that weren’t being made before, the thinking goes. Instead of just buying more spare parts, the contractor can change the design to make it more reliable and use fewer parts — and the company can be rewarded through the contract for making those investments. The PBL process, for example, resulted in the design of a new magneto, which has helped reduce related accidents from 400 per 100,000 flight hours to only four per 100,000 flight hours.

AAI Corp., Hunt Valley, Md., is both the prime contractor and the "depot" for overhauling the Shadow system. As part of the latter responsibility, the company has forged a public-private partnership with the Army’s Tobyhana (Pa.) and Letterkenny (Pa.) depots. These facilities assist AAI to "strip the systems down" and "reset" them to Army standards when they come back from the war zone every 13 to 15 months, Owings said.

Field maintenance on the Shadow is performed by a combination of Army and AAI technicians. Although AAI has field service representatives forward, the Army’s primary field-level maintenance is "organic," typically performed by soldiers turning wrenches. A second type of support activity, known as sustainment-level maintenance, is performed by contractor reps in the field or depot. Soldiers handle jobs whose mean time to repair (MTTR) is less than 30 minutes, according to Chief Warrant Officer Tim Steckel, who covers Shadow maintenance and supply chain management. Soldiers do jobs such as removing and replacing major components like wings, engines and the flight control computer. AAI field service reps work on more time-consuming data link and avionics issues.

In its totality, Shadow is more complex than a conventional aircraft, and maintenance is correspondingly more complex. The ground control station, for example, contains computer rack equipment, software applications and communications gear. Army maintainers use a "general mechanic’s tool kit," stocked with basic items such as sockets, hammers, wrenches and screwdrivers, according to the UAS program office. The Air Vehicle Transport, used to move the vehicles, also is employed for refueling and oil servicing. A platoon’s maintenance complement also includes a Maintenance Section Multifunctional (MSM), which comprises two Humvees — one with mounted shelter and trailer and one with personnel, equipment and spares, according to an Army handbook. A Mobile Maintenance Facility, operated by contractor reps, supports the MSM, the handbook says.

Improving Figures

Shadow’s reliability has improved as the system has matured. The improved reliability of the new engine’s components has allowed the Army to increase time between overhauls from 118 hours to 250 hours.

The Army tracks and documents component failures. Its Critical Safety Items program "tracks flight-critical items all the way back up the chain to the supplier, to make sure that you don’t get inadvertent changes in a supplier or a component, and to make sure you don’t get creeping reliability issues," Owings said. "We know what the high-failure items are on the system, and that’s where we concentrate engineering hours." The Army keeps an accident database for Shadow and other unmanned aircraft, and looks at mean time between failure information on every part.

The service also focuses on human error. Design changes were made, for example, to make it impossible for external power umbilicals to remain attached to the vehicle on launch. Another example concerns engine maintenance. "Early in the Shadow program we found a problem with the engine lugs," recalled Owings. "It was fine if it was daylight, but when you went to change engines in pitch black, it was much more difficult to do." To fix the problem, the engine attachment process was simplified so that it could be done with fewer operations, making night maintenance operations more efficient.

Today there are typically seven Army maintainers and two field service reps supporting the Shadow at the platoon level. (One platoon’s UAS complement includes four air vehicles and two ground control stations.) The service uses contractor reps for basically two reasons, according to the program office. First, Shadow was purchased as a commercial off-the-shelf (COTS) item, so that the Army had little experience with it, initially. Second, "there is a validated manning shortage within the current force structure." But the long-term intent is for Army personnel to perform more of the maintenance work, in parallel with a planned increase in platoon size from 22 to 28 or 29 soldiers. If the force structure increases as expected, there could be an additional three Army maintainers per platoon. If the force structure increases or the current 24-hour operational tempo decreases, the number of contractor reps per unit is expected to decrease.

Shadow maintainers are trained at Fort Huachuca, Ariz. Their curriculum includes items such as engine repair, the removal and replacement of boxes inside the airplane and the ground control station, and a level of troubleshooting. Individuals also undergo 10 weeks of team training.

New Specialty

AAI has developed a "UAS systems specialist" job category aimed at field support, using internal training resources. The company has been talking to FAA about perhaps "ultimately even certifying a program such as ours to start to create the equivalent of an A&P mechanic [for UAS]," said Steve Reid, the company’s vice president of unmanned aircraft systems. Reid said some of AAI’s design engineers teach maintenance to the trainees, "which helps us to design products for maintainability."

UAS maintainers need to know a lot. "You’ve got to know your bits and bytes," Reid said, but "at the same time, you’ve got to be able to adjust the carburetor." AAI supports the Army’s Critical Safety Items program. A single-point failure of a critical safety item, for example, would result in a "stringent profile," involving oversight at the manufacturer’s facility, testing and additional inspection up through the supply chain. The Army’s partnership with AAI for Shadow maintenance has worked well so far. The company, which earns a fee based on product availability, claims to have achieved 95 percent availability.

Human Factors

Although the FAA UAS program office appears to have put maintenance on the back burner for now, the agency has been contracting through NASA to study human factors associated with the maintenance of small and mid-sized unmanned vehicles. Researchers have been surveying UAS maintainers since 2005, revealing many challenges. The work could prove to be important if small vehicles are the first to enter the NAS in significant numbers.

"Maintenance is the last piece of the UAS regulatory puzzle that will be put in place, according to FAA."

John Goglia, an aviation consultant and former member of the U.S. National Transportation Safety Board, anticipates a new specialty for next-generation, large unmanned cargo haulers, which he asserts will be first commercially significant UAS on the scene, though not initially in U.S. airspace. The "digital A&Ps" who will work on these complex aircraft will have to have not only all the basic knowledge of what makes an airplane work, but also an understanding of electronics far greater than what’s typically found today. The top priorities for these maintainers will be repair of composite materials and electronics, Goglia said.

"Nobody talks to maintenance [standards] yet," he said. And what military data there is "doesn’t fit because, to make [UAS] commercially viable, the [systems] will be much bigger." These unmanned cargo haulers are being designed to existing manned aircraft standards, he said, but with greater electronics integration and computer redundancy. To make such operations commercially viable, the manned ground support footprint will be minimal, but very high quality.

Recent articles published by FAA-funded NASA contractors, Alan Hobbs and Stanley Herwitz, point out the complexity of maintaining small- and mid-sized unmanned aircraft. One publication listed 21 topics pertinent to UAS maintenance, none of which is included in FAR 147, which lists the subjects an aviation mechanic should know. A few of these areas are computer management; software, networks and Ethernet hubs; computer operating systems; electronics; radio transmission and theory of radio control; RF interference and shielding; and ground transmission equipment and antennas.

A second article by these researchers points out that "reliance on laptop computer[s] for UAV [unmanned air vehicle] operations means that the support and maintenance of a computer system and associated software is now an airworthiness task." It continues: "As a result, human-computer interaction and computer system knowledge will be important human factors considerations for UAV maintenance personnel."

Both articles raise issues that will be important for FAA to consider, especially regarding smaller unmanned vehicles. Among the concerns were the absence of information on component failure modes and rates, lack of onboard meters for recording of airframe or engine flight hours, use of unconventional propulsion systems and the extensive presence of composite materials. Lack of detailed information on service life and failure rates makes reliability-based maintenance programs difficult to develop and runs parallel risks of under- and over-maintenance. Although the maintenance picture with high-flight-hour, military systems is much brighter, the industry is still in its early days and much remains to be done, particularly at the lower end of the scale.