Before most people noticed, the idea of employing aircraft in the national airspace without pilots aboard them -- unmanned aerial vehicles, or UAVs (and sometimes UAS, where the S is for "systems") -- has gotten to a point where their introduction is considered inevitable. Who'd have thought? After all, these machines were originally developed to conduct military missions in areas deemed too hazardous for humans, not for ordinary flying tasks. Advocates like to say UAVs are best at "3-D missions" -- those that are dull, dirty or dangerous.

Now there's palpable pressure from UAV advocates to insert the aircraft into an array of civilian missions as well. And there are two leading arguments for their use: long endurance and low cost.

It's true that replacing the weight of a pilot with an equal amount of fuel confers on the UAV a higher fuel fraction in its design gross weight, which is where the endurance comes from. A solar-powered Zephyr UAV stayed aloft for 54 hours, setting a record in 2006, and the turbofan-powered Northrop Global Hawk operated by the U.S. Air Force has a claimed 42-hour endurance. No current production human-piloted aircraft can match numbers like those, although Burt Rutan's one-off designs for record-setting aircraft prove that endurance can be had with a pilot aboard. (Rutan's Voyager flew for nine days, or 216 hours, on its nonstop, round-the-world flight. It also demonstrated that a flight of that length is hard on pilots.)

The cost-saving argument, though, may not hold up for high-end machines such as the military's Predator and Global Hawk families, which can be priced in the tens of millions (average Global Hawk in 2003: $57 million) and require extensive logistics and ground crew. Versions tailored to the less-exotic needs of civil operators are cheaper but still priced like small business jets. Economy-class UAVs are confined to much smaller vehicles in a class with radio-control scale-model hobby aircraft; they are cheaper to acquire and operate. One problem at the low end is that many would-be operators think of them as the same as recreational scale models and don't yet realize that the flight of these vehicles falls under the jurisdiction of the FAA.

Perhaps a typical example is one from The New York Times of Jan. 13, 2005. Chang Industry, Inc., of La Verne, Calif., was reported to be marketing a five-pound aircraft with a four-foot wing and a unit cost of only $5,000. The company anticipated a demonstration for the Los Angeles County Sheriff's Department to show off a video surveillance camera. With a fabric wing and kite-like structure, a service ceiling of 1,000 feet and an endurance of 20 minutes, the little airplane seemed harmless enough if things went south. But Chang knew its market: There was also a $15,000 model in the works with an eight-foot wing. An FAA spokesman quoted in the story pointed out that the agency would issue limited certificates of authorization defining where such aircraft could operate and under what conditions. And that's still the way it's done today.

There's no argument in wartime over the preference for an unmanned aircraft in combat areas where lives are at stake. The saving of an American life trumps all, and Congress has responded in kind. The Congressional Research Service (CRS) reports that between 2001 and 2004, UAV expenditures rose from $667 million to over $1.1 billion. In Washington, UAVs (now labeled "transformational," the magic word around the Department of Defense) are the darlings of procurement programs and may well have a total market somewhere north of $3 billion by now.

But the question lingers as to a UAV's comparative effectiveness and cost performing ordinary surveillance missions in the national airspace system (NAS) when the competition is a Cessna 182 with aux tanks and containing a couple of observers with night binoculars and piddle packs. Some estimates peg operating cost at six times a manned aircraft like the Cessna, while UAV advocates prefer to compare the price of their wares with a Lockheed P-3, a four-engine Navy antisubmarine patrol aircraft, or a Sikorsky Black Hawk. That argument won't be settled here, though. The issue of concern to business aviation is how the introduction of UAVs may affect our operations, particularly at the lower and middle altitudes where the civil versions are likely to fly.

Law-enforcement agencies conduct a lot of airborne surveillance missions, but within the last 30 to 40 years, eyeballs have concentrated along the borders, particularly the 2,062-mile-long one dividing the United States and Mexico. And there's lot to watch for, from smugglers running illegals across at night to drug couriers moving their goods north to urban markets. The former Customs Service ranged far and wide in its drug-interdiction missions, even employing sensor-equipped Cessna Citations to pursue blacked-out aircraft headed for landing strips and drop zones. The Border Patrol spent its days and nights in low-flying airplanes and helicopters "cutting sign" -- their term for tracking -- in the desert. Now the two agencies have been combined to form U.S. Customs and Border Protection (CBP) under the Department of Homeland Security. A 2005 report counted some 10,000 agents on duty on the southern border alone; the figure is sure to have risen.

With the exception of aerostats, simple tethered balloons mounting sensors that scan wide areas, aerial surveillance has been conducted mainly in piloted aircraft by these agencies and others. And throughout these efforts, the presence of pilots on the aircraft has ensured the safety of their operation in the NAS. In addition to the see-and-avoid doctrine governing visual flight, pilots flying surveillance could interact with ATC. Perhaps a science payload might fly IFR, but surveillance, by its very nature, connotes visual conditions. And for the most part, such flights were ordinary in every sense.

Aerostats present the same issues as tall broadcast towers and appear on charts and in NOTAMs. Impromptu deployments are a cause for concern, however. In November 2007, a Honolulu police helicopter collided with a balloon being flown above a department store and was forced to make an emergency landing with its tail rotor inoperable.

Advocates of UAVs argue that aircraft like the Predator B, operating today as the MQ-9 Unmanned Aircraft System for the CBP's Air and Marine division from a center in Riverside, Calif., fly at altitudes of up to 50,000 feet and are well above commercial and business traffic. However, the sensors deliver better images from altitudes lower than that, and operators are likely to go for the best pictures. The aircraft, manufactured by General Atomics Aeronautical Systems in San Diego, is operated remotely from the Air and Marine Operations Center by either line-of-sight direct link or Ku-band links via satellite when the horizon intervenes. Most of the General Atomics Predator family is also said to be capable of autonomous flight: programmed flight plans flown without a remote link.

When a Predator B surveillance aircraft crashed near Nogales, Ariz., in April 2006, a subsequent investigation revealed that the fuel supply to the aircraft's engine had been inadvertently shut down during a mix-up as two remote operators changed controls. The aircraft continued to descend, giving no indication of the problem, until it struck the ground. Although there were no casualties or property damage, the incident put the spotlight on a serious operational flaw. A news report at the time cited the value of the aircraft at $6.5 million (another said $14 million) and stated that it was generally operated at 15,000 feet. Another recent accident in Kinshasa, capital of the Congo, involving a European Union-operated Belgian-built UAV, reportedly killed one person and injured two others.

According to a CRS estimate, the UAV accident rate is 100 times that of manned aircraft, a factor that is seldom considered when system costs are reckoned.

Surveillance flights by UAVs are one thing when they're operating along the southern border far from congested airspace. But it's another matter to envision them performing surveillance over the major port cities on both coasts, although there is considerable enthusiasm among their advocates for employing them in just that role.

From a 2003 CRS document: "Additional roles for UAVs in the near future may include homeland security and medical resupply. The Coast Guard and Border Patrol, parts of the newly formed Department of Homeland Security, already have plans to deploy UAVs to watch coastal waters, patrol the nation's borders, and protect major oil and gas pipelines. Congressional support exists for using UAVs like the Predator for border security." Military operation would be inappropriate for such duties, the report said, so domestic agencies would carry out the missions. Still . . . medical resupply? Watch coastal waters? From another CRS paper: "The longer flight times of UAVs means that sustained coverage over a previously exposed area may improve border security."

But if the purpose is to stare at one spot for a very long time, why not install an aerostat or a camera tower? And the language justifying use of these vehicles invariably rolls out the "T" word. Has there been one proven instance of a terrorist entering the United States by overland crossing of the border in a remote area? Would a terrorist choose that route to, say, a crowded railroad terminal? So far, only government agencies have been seated around the tables where such questions are debated, and the FAA has been the sole guardian of NAS integrity.

Setting aside the unsettling feeling that the UAV movement is at least partly fueled by overwrought enthusiasm, there are the practicalities of their operation, and that's what moved the AOPA to ask for formation of a Special Committee under the Radio Technical Commission for Aeronautics (RTCA) to formulate the operational specifications of UAVs to operate in civilian airspace. The creation of SC-203 under the RTCA effectively blocks helpful suggestions from UAV advocates that the best solution to NAS flights was to revise the FARs or create semi-permanent temporary flight restrictions. The AOPA pointed out that a restricted area in a swath painted along the southern border would affect more than 100 airports.

In medicine, physicians adhere to the tenet "Do no harm." Heidi Williams, AOPA director of Air Traffic Services, states flatly, "We have gone on record to the FAA that any [UAV] integration into the NAS should be done without any harm to the existing civil airspace user." In expanding upon that, she says, the organization believes UAVs in the NAS should be regulated and equipped like other aircraft. At the heart of the discussion within circles such as SC-203 is the concept of "sense and avoid" and what that will mean. Obviously rooted in the concept of "see and avoid," the execution of an automated equivalent adds considerable complexity to the current architecture of the UAV, which is focused on surveillance below, not the airspace surrounding it. And the AOPA is rightly focused on keeping attention on the low- and mid-altitude vehicles with enough mass to create a hazard. Williams also emphasizes that the AOPA is not opposed to the introduction of UAVs into the NAS.

Envisioning actual operations leads to looming questions. How will UAVs in climb and descent phases of their missions manage encounters with other aircraft? If you instinctively think, "Light that sucker up so I can avoid him," you're not alone. Both visual and electronic beacons are needed even in daylight VMC because UAVs can be difficult to spot. But the current thinking would provide the UAV itself with an electronic awareness of its surroundings and automate its responses. That means programming relatively complicated scenarios such as the Right of Way Rule (FAR Part 91.113) governing overtaking, intersecting courses and head-on encounters. If you put yourself into any of those situations while driving your Learjet on an arrival profile, you would wish for an automated maneuver that would take the UAV out of the play completely.

The AOPA's Williams characterizes the present level of discussions as "concept and philosophy." Spokesman Chris Dancy adds, "The immediate concern the AOPA has is potential civil end users. They're getting ahead of the game but are not looking at the regulatory environment." He's talking about law-enforcement agencies, of course, in small towns everywhere who are thinking about how cool it would be to have their own little mini-Predator.