"I think it's a real question: is there a need for larger civil UAS?" says Lambert Dopping-Hepenstal. At first glance, this is a surprise: Dopping-Hepenstal is the program director of ASTRAEA, a partnership involving industry, government and regulators in the UK aimed at solving the various problems preventing the flight of large and increasingly autonomous unmanned air systems. But the point he raises is crucial to the future of what many observers expect to remain aviation's fastest-growing market.

There is no lack of enthusiasm or ideas when it comes to realizing the potential of small UAS. But the promise of larger systems has yet to be fulfilled. The significant technical and legal impediments to operating large unmanned aircraft in unsegregated airspace appear to be holding back would-be users. Yet it is also possible to argue that if a compelling enough use case existed, resources would be made available to surmount the challenges. In short, the UAS sector is still lacking what the computer world calls a "killer app."

"It seems unimaginable that in the coming decades unmanned aircraft won’t be commonplace," Dopping-Hepenstal says. "Yet equally there have been all sorts of market failures. If we'd been having this conversation 30 years ago, we'd have been predicting that there'd be hundreds of supersonic airliners flying today."

Assessing how long-established rules of the air apply to aircraft with no human on board is difficult. Regulators, whose primary role is to ensure safe use of the air, want to see if unmanned systems fit existing certification and flight-safety rules, rather than try to draft new ones specifically for UAS. Industry is unwilling to spend too much time and money developing an aircraft for civil use if regulators will not allow it to fly.

There are several pan-European or international efforts to develop standards for remotely piloted air systems (RPAS), such as JARUS (Joint Authorities for Rulemaking on Unmanned Systems), the relevant working groups of ICAO and EUROCAE, and within the European Aviation Safety Agency (EASA). The European Union's SESAR JU initiative published its European UAS "road map" at the Paris Air Show in 2013. Yet flights by large UAS in Europe are still rare.

"Technically it isn't a huge challenge – we're not going to have to invent new science," says Dopping-Hepenstal. "But the whole system is complex. For example, you've got to add to the regulations and amend legislation. You've got to create an insurance market that understands the risk. You have to analyze the training needs for pilots, maintenance crews and operators."

And because the questions are open-ended, the business case is difficult to make. There are risks to investing in the process, but not doing so may slow progress across Europe – potentially jeopardizing the gains the European aerospace sector has made, and allowing other regions to take the lead.

"The European road map calls for some initial flying by 2018, with it being commonplace by 2028," Dopping-Hepenstal says. "The U.S. has developed a similar road map. Aviation is an international market and regulations have to be recognized internationally.”