As U.S. skies have opened up to drones in recent years, concerns about civil liberties have come to the fore, but as the fiery crash of a mammoth unmanned Navy surveillance drone in the Chesapeake Bay a year ago demonstrated, safety is cause for concern as well.

A Navy investigation, reported here for the first time, has concluded that the June 11, 2012, crash was caused by a combination of a mechanical failure and pilot error. A breakdown in the aircraft's rudder mechanism compounded by the failure of a civilian pilot to follow emergency procedures sent the Global Hawk drone, with a wingspan of 131 feet, spiraling into remote wetlands in southern Maryland.

The Global Hawk, a 22,982-pound aircraft, spiraled downward at a 55-degree angle reaching a speed in excess of 2,000 feet per minute. Carrying more than 11,000 pounds of fuel, the drone burst into flames upon impact. The crash destroyed the $176 million vehicle, but caused no property damage or injuries and caused "minimal" environmental damage, the report found.

The Navy report, obtained under the Freedom of Information Act, illuminates the safety challenges posed by increased of use of drones in U.S. airspace. With the military's use of drones growing rapidly, training exercises in the United States are also expanding. While the report credits the pilot, a civilian employed by Northrop Grumman, with steering the malfunctioning aircraft away from a populated area, Navy Cmdr. C.P. Ramsden said the pilot error as the rudder system broke down could have led to disaster.

"Failure to adhere to emergency protocols did not produce disastrous results in this particular event," Ramsden wrote. "However, future breaches of established procedure could produce a different outcome."

A trail of wreckage and a massive plume of smoke captured on video revealed that the robotic aircraft had crashed 22 miles east of the naval air station on the Patuxent River, where Northrop Grumman was training a team of civilian contactors to pilot the aircraft.

The Global Hawk is at the forefront of the drone revolution that is transforming the U.S. armed forces and, increasingly, domestic law-enforcement organizations, including the FBI. As Wired reported last June, the Navy was introducing an enhanced version of the Global Hawk at the Patuxent station.

"as part of the Navy’s newest iteration of its Broad Area Maritime Surveillance Program. BAMS, as it’s known, uses a Global Hawk outfitted with Navy-specific sensors to spy on a whole lot of ocean and beach. In this case, the Navy was set to debut two new, powerful 360-degree radars aboard its Global Hawks, with range in the hundreds of miles, as part of a $1.16 billion contract signed in 2008."

Ramsden's 381-page report found that the flight was troubled from the start. Within two minutes of takeoff, the operator's control panel started reporting a motor failure in the right inboard ruddervator, the movable hinged section of the V-shaped tail surfaces on the rear of the drone.

As the drone foundered, the pilot overrode the aircraft's default settings, steering it to the east to avoid flying over a populated area of St. Mary's County, Maryland, while climbing to an altitude of 40,000 feet.

The Navy report noted that the emergency protocol for the aircraft does not call for climbing above 40,000 feet. Had the pilot stayed at a lower altitude, the automatic settings on the craft's spoiler would have remained in effect, according to the report.

The higher altitude allowed the pilot to check the spoiler settings himself. He concluded that the aircraft was controlable and prepared to follow the rest of the protocol, which was to land as soon as possible. "Intermittent failure" of the motor mechanism resulted in "uncommanded movements" of the ruddervators as the pilot prepared to return the aircraft for an emergency landing at the Patuxent River base. The pilot then lost radio contact with the Global Hawk.

"We have lost control of the aircraft," he advised the control tower at the base, according to a transcript obtained by investigators. "The aircraft is going to crash."

"It is unknown whether locking the spoilers to a lower setting may have prevented the aircraft from departing controlled flight," the report concluded.

When the motor of the ruddervator was recovered from the crash site, investigators found that "voltage instability" in a capacitor and transformer had caused it to malfunction. The engineer who examined the parts said the breakdown was "due to random component failure not a manufacturing, design, test etc. issue" with the Global Hawk.

Ramsden recommended that the contract of the pilot, whose name was withheld for privacy reasons from the publicly released version of the report, be "thoroughly reviewed."

The report notes that the “unique makeup of the BAMS-D program,”—which largely includes contractors, who “in the case of the mishap [made up] all of the personnel directly involved”—caused some delays in the investigation into the crash’s causes, as Northrop Grumman deliberated about whether to have lawyers present for its staff during the Navy’s probe. In the end, the flight crew gave their accounts without lawyers present.

Jefferson Morley is an investigative reporter in Washington who blogs at JFKFacts.org and tweets @jeffersonmorley.