In 1992, Scaled Composites built a radio-controlled UAV intended for 48-hour flights at 65,000 feet. Called Quiver (it was later changed to Raptor, for “Responsive Aircraft Program for Theater Operations”), it had a wingspan of 66 feet and an 80 hp Rotax engine. Scaled also home-brewed the autopilot, and there was some uncertainty about how it might behave before its rates and gains had been properly adjusted. In order to avoid losing the prototype on its first flight, Burt Rutan came up with the idea of providing it with a human safety pilot who could take over in case something went wrong.
Now, the Quiver was designed to carry a 150-pound payload, including a couple of underwing anti-missile missiles, but its skinny fuselage did not have a cockpit, or even room for one. Rutan solved the problem with his customary ingenuity and sublime indifference to human comfort. A backrest and safety belts — but no windshield — were added on top of the fuselage, along with makeshift links to the primary flight controls. Test pilots Mike Melvill and Doug Shane, the latter now Scaled’s CEO, climbed into the makeshift saddle for the first flights.
Shane later described flying in the open air, astride the airplane and behind the beating propeller, as “a new and unwelcome experience.” Landings were particularly harrowing. Melvill recalled “how hard it was to let [the remote pilot] land and not grab the controls.”
On a seemingly unrelated topic, I remember watching in awe, as a small boy inside New York skyscrapers, as a liveried elevator man made a series of subtle adjustments with an ornate brass lever to bring the floor of the elevator to rest in perfect alignment with the floor outside. It seemed like a beautiful example of human skill and adaptability; how cruel to discover that elevators could be made to mind themselves!
Did the first riders in automatic elevators, invited to believe that the touch of a button would carry them up that terrible dark shaft and deposit them safely at their destination, feel the same qualm as Shane and Melvill surely did as their fingers first followed the tremors of a stick controlled from afar?
Or as our children will, when they first board an airplane without a pilot or a cockpit?
The idea of passenger-carrying airplanes without pilots usually comes up in relation to the increasing automation of airliners and — it is rumored — the withering away of basic flying skills in their pilots. But the designers of our future, when they are figuring out how (but not why) to put people on Mars, are meditating a different kind of autonomous flight. The concept is something we have seen in movies and illustrations depicting cities 100 years hence: aerial taxis whizzing among the towers, delivering their occupants to destinations they would have reached, in the olden days, by taxi or light rail.
These are PAVs — personal aerial vehicles — and in their taxi-like commercial form, NASA Langley’s Mark Moore calls them Zip Aircraft, after the Zipcar model of distributed car rental. In the Zip model, locked cars are left by their drivers at their destinations; a person needing a car finds one nearby and makes a reservation through an online service, then opens and drives the car with a smart membership card.
The assumption underlying Zipcars is that most people know how to drive. The obvious difficulty, when considering an aerial version, is that most people do not know how to fly. The solution would be an airplane that flies itself — you tell it where you want to go, and it takes you there.
Although Moore is careful to describe his studies to date as merely exploratory, he is optimistic about the future of PAVs and particularly about the potential of electric power. Electric motors open up possibilities for structural and aerodynamic advances, increased reliability, reduced noise and pollution, and reduced acquisition and operating costs, but they suffer today from the inadequacy of even the best current batteries. For a given powerplant weight, an airplane cannot go nearly as far on battery power as it can on a like mass of liquid fuel. Moore argues, however, that experimental batteries now in development, which should be commercially available by the end of the decade, could provide a four-seat airplane with a range of 200 miles, and that would be sufficient for most PAV trips. Hybridizing the powerplant with a small, range-extending internal-combustion engine would take care of longer trips, at least until still-better batteries arrive.
NASA’s Kenneth Goodrich, who is studying the problems of autonomous flight, imagines pilot and airplane sharing responsibilities. He speaks of “inner loop” and “outer loop” skills. The inner loops consist of basic ship-handling: staying right side up, managing power, maneuvering, maintaining speed and altitude, navigating among defined waypoints, even controlling the approach and landing. These are tasks, some more complex than others, that “are dealing with relatively straightforward/deterministic signals and physics.” In other words, either things are where they should be, or some clearly defined action is required to get them there; there are no ambiguities.
Goodrich compares a semi-autonomous airplane — one with just inner-loop capabilities — to a well-trained horse. “The airplane has instinctive or reactive intelligence (which is much simpler than general human intelligence) relative to expected environmental factors and is generally biased toward self-preservation in the absence of decisive pilot direction.” If you do the wrong thing, or do nothing, the airplane finds its way to some safe condition.
Outer loops involve more abstract types of perception and decision-making, ones for which we now consider the human mind indispensable. The variety of situations that can arise in flight, and the complexities of dealing with them, seem far beyond the grasp of any imaginable computer program. It is difficult to imagine a machine possessing the combination of situational awareness, initiative, judgment and resourcefulness that a good pilot possesses, and so pilots — not to mention everybody else — tend to be skeptical of the idea that full responsibility for the execution of a flight could be entrusted to automata. It is sufficient to mention Sullenberger and the Hudson, and the case is closed.
But even full autonomy may prove more attainable than we suppose. I suspect that in 1970 the people who operated what then passed for digital computers would have said that no non-professional could ever be expected to manage one; yet today we all use them routinely. It’s partly a matter of people learning new skills, and partly one of tasks being redefined to allow computers to handle them.
I can imagine — Moore and Goodrich suggest nothing of this sort — airplanes without pilots operating in a highly regimented environment under some sort of central or distributed external control. They would fly at altitudes and along routes chosen to mesh with other flights. A PAV might join a flock of others moving along a sort of three-dimensional city street, and formate more closely with them than normal pilots would dare. Conflicts would be avoided not by improvising a response to each new event, as humans do, but by ensuring that no unexpected event occurs. Philosophically, however, this model is opposite to that of the coming NextGen air traffic system, in which the role of central control is diminished rather than increased and decision-making is distributed among the airborne participants.
Whatever mix of autonomous control and piloting skills flying might eventually require, the implementation of the Zip Aircraft concept does not imply the extinction of aviation as we know it today. One area of current study is how to integrate large numbers of PAVs into present traffic. PAVs are expected to operate at low altitudes, from special airports or special parts of existing airports, and on routes that would avoid conflict with other types of traffic.
Of course, we know that pilotless airplanes are already here. It’s certain that they will increase in number and take on more and more diverse tasks, including the carriage of cargo, and will learn to mingle unobtrusively with piloted airplanes. But will they ever carry people? Before we prepare to hang up our goggles and scarves in the temple of Daedalus, we should take some comfort from Ken Goodrich. “Elevator-like autonomy,” he says, “could be an option in the distant future (20 to 30-plus years), but it’s far beyond the state of the art today.”
