The complicated business of integrating unmanned aircraft into U.S. skies full of small planes and airliners will begin at six test sites and require several years of testing, the Federal Aviation Administration said Thursday.

Twenty-four states — including Virginia and Maryland — have applied to host test sites, but the FAA has yet to determine where they will be located. In presenting what it called a “road map” for the introduction of unmanned planes into the system, the FAA acknowledged that it will take longer than Congress had hoped when it set a September 2015 deadline for granting drones general access to the skies.

“Government and industry face significant challenges as unmanned aircraft move into the aviation mainstream,” Transportation Secretary Anthony Foxx said in a statement. “This road map is an important step forward that will help stakeholders understand the operational goals and safety issues we need to consider when planning for the future of our airspace.”

Until the testing is complete, the FAA said, it will grant flight privileges to unmanned aircraft operators on a case-by-case basis.

Unmanned aircraft come in many shapes and sizes. Military drones used for surveillance or missile strikes in Afghanistan, Pakistan and the Middle East have received the greatest measure of public attention, and similar aircraft could be used domestically to monitor traffic conditions, weather patterns and assorted other things.

“From advancing scientific research and responding to natural disasters to locating missing persons and helping to fight wildfires, [unmanned aircraft] can save time, save money, and, most importantly, save lives,” Michael Toscano, president of the Association for Unmanned Vehicle Systems International, said in applauding the FAA road map.

The same technology that allows pilots to fly those relatively small aircraft from afar can be used to fly larger planes. Although public reaction would probably prevent airlines from replacing pilots on passenger planes, it’s possible that similarly sized planes carrying packages and other freight could be flown by ground-based pilots.

Public fear of a spy in the sky, peering down like the drones that hunt Taliban and al-Qaeda members in Pakistan, has called attention to privacy issues as unmanned aircraft take flight in the United States.

The FAA said Thursday that it will require that the operators of the six test sites establish and follow practices to prevent encroachment on the privacy of those below.

“In requiring test sites to have a written plan for data use and retention, the FAA also appropriately focuses on the real issue when it comes to privacy — the use, storage and sharing of data, or whether data collected must be deleted,” Toscano said. “Privacy laws must be platform-neutral, treating manned and unmanned platforms the same. “

The road map also deals with policies, regulations and procedures required to integrate the coming flock of unmanned aircraft into a revolutionary new aviation-control system known as NextGen.

The goal of NextGen — often described as based on the Global Positioning System but actually far more complex than that — is to replace a 60-year-old radar-based system that directs landings and takeoffs and keeps planes from colliding in midair.

The new system should result in significant fuel savings, less pollution, more-direct routing of flights and a general expansion of air traffic to accommodate projected demand. Tossing an as-yet-unknown number of unmanned aircraft into that mix complicates the equation.

The road map pointed to areas that require resolution or regulation, such as training for pilots who handle unmanned aircraft as new technology emerges for their operation. Certain systems used in piloted planes — such as an automatic collision-avoidance system known as TCAS — must be refined for use in unmanned aircraft, the road map said.

“In that airspace of the future, we will have new users,” FAA Administrator Michael Huerta said in presenting the road map. “We will have more commercial space launches, and we’ll have more unmanned aircraft systems. As you know, it requires significant consensus of how we can safely integrate game-changing technologies such as these, and I’m pleased to say we’ve made very solid progress. We are dedicated to moving this exciting new technology along as quickly and safely as possible.”