Oh silly question. Regulations of course:
But there can be many delays along the road from concept to certification. For instance, government officials and the designers have had to determine which regulations — aircraft or automotive — take precedence when the vehicle in question is both.
In the United States, development of the flying car was given some breathing room eight years ago when the F.A.A. created a new classification, the light-sport category, to encourage the design of small, easy-to-fly aircraft. To meet the light-sport definition, the aircraft must have a single engine and an unpressurized cabin, have one or two seats and weigh no more than 1,320 pounds; maximum air speed is limited to 138 miles per hour. . . .
¶In 2010, the $94,000 Maverick, a rudimentary buggy that takes to the air under a powered parachute, earned certification as a light-sport aircraft. Troy Townsend, design manager and chief test pilot for the company, based in Dunnellon, Fla., said he spent spent nearly all of his time over the course of three years working through the bureaucratic snags.
¶“There was a lot of red tape,” Mr. Townsend said. “The certification process went all the way to Oklahoma and Washington, D.C.”
¶To avoid having to meet federal safety standards, the Maverick is not sold as a road vehicle, but as a kit car. The buyer licenses it according to the prevailing state requirements.
¶At Terrafugia, engineers decided to classify the car side of the Transition as a multipurpose vehicle. Compliance with National Highway Traffic Safety Administration crash standards will be done by submitting the results of computer-based simulations showing how the seat belts, crumple zones and crash structure performed.
¶Terrafugia sought waivers from N.H.T.S.A. on two requirements. It obtained permission to use motorcycle tires and wheels, rather than truck-rated parts, to save weight, and it received approval to use polycarbonate plastics for the windows, rather than automotive glass, which could be shattered by a bird strike.
¶“The F.A.A. coordinated with the National Highway Transportation Safety Administration for the Terrafugia to determine the clear responsibility between the requirements for road operations and air operations,” the F.A.A. said.
¶Even though the Transition “meets the regulatory definition of a motor vehicle,” its 4-cylinder Rotax engine is not required to comply with federal emissions or fuel-economy standards. The Environmental Protection Agency decided to treat flying cars as aircraft, though it may reconsider if they “become commonplace.”
¶Test pilots must successfully fly the Transition to demonstrate its durability in dives and spins and its ability to recover from unexpected events in flight. Then regulators will determine if it can be easily handled by pilots with light-sport licenses, which can be obtained with as little as 20 hours of instruction.
¶Obtaining road certification for the Dutch PAL-V ought to be straightforward. The on-road duties of the 1,500-pound, two-seat gyrocopter are based on a three-wheel chassis that leans into turns, cornering like a motorcycle.
¶With three wheels, it is classified as a motorcycle by N.H.T.S.A “and subject to federal motor vehicle safety standards that are applicable to motorcycles,” according to the agency. The same is true in Europe, said Robert Dingemanse, PAL-V’s co-founder and chief executive.