Drone risk in sharper focus

If the reported drone collision with an airplane over California in January (which did not damage the Cessna 188 involved) was a nearly silent precursor, another reported drone strike over London on April 17 was a shot heard around the world.

A British Airways Airbus A320 on final approach to London’s Heathrow Airport reported striking what the crew believed to be a drone, though a post-flight inspection found no damage significant enough to delay the following flight. The Telegraph, citing a government minister, reported April 21 that there was no damage done to the airliner at all, and the investigation had not ruled out the possibility that a "plastic bag" had been struck. If it was a drone, and if the drone hit one of the engines, rather than the nose of the airliner, it might well have ended with a failed engine and emergency landing, or possibly worse. The details of a worst-case scenario remain largely unknown.

U.S. government agencies and academic institutions are working hard to model the risk posed by drones of various sizes in a scientifically sound way, and testing traffic management and related systems designed to prevent such collisions in the first place. That work is well underway, and a technological traffic management solution was tested for the first time on a large scale April 19.

As drones proliferate virtually everywhere—a boom driven by advancing technology that puts a quadcopter in easy reach of virtually any consumer—efforts to educate those consumers have taken on a sense of urgency. Answering critical questions about risk, mitigation, and regulation has also been a priority for many in and out of government, here and abroad, as drones proliferate.

“We’re confident that UAS are enough of a threat to manned aircraft that they should remain a safe distance from airports and seaports,” said retired U.S. Air Force Maj. Gen. James Poss, leader of a collaboration between the FAA and various universities studying a drone safety and management.

“Keep in mind,” Poss wrote in an email exchange, “the bulk of our research is devoted to making sure collisions never happen.”

Poss is the executive director of the Alliance for System Safety of UAS through Research Excellence (ASSURE), which is also the FAA Center of Excellence for Unmanned Aircraft Systems and is working in conjunction with NASA, private industry, and a long list of research institutions to manage the influx of unmanned aircraft into the National Airspace System. Funded through a 2014 congressional appropriation, ASSURE is collaborating with the NASA Unmanned Aircraft System Traffic Management program on technology to prevent collisions such as the suspected drone strike by the British Airways flight. (There had by April 19 been no official confirmation that the object that the A320 struck was actually a drone, and the incident remained under investigation.)

The A320 was approaching Heathrow at about 1,700 feet when it struck the object, the Associated Press reported. The airliner’s altitude and flight track data combined with the reported altitude of the collision nearly pinpoints the location of the suspected drone midair: about five nautical miles from the end of Heathrow’s Runway 27L, as the Airbus was making its final descent at 160 knots, well above the 400-foot altitude limit required of recreational drone operators in both Britain and the United States.

Less than 24 hours after the Airbus hit the suspected drone, DJI, the world’s leading manufacturer of consumer drones, sent a press release touting the built-in geofencing feature, “which restricts flights near airports and other sensitive locations.” DJI products do not, however, disallow flights near every airport: nontowered airports, seaplane bases, and heliports or helicopter landing pads, are to date excluded from DJI’s built-in no-fly zones. Operators may override the system’s altitude limits, also. This may have been the case in London, though it’s unclear if the details of what the A320 hit will ever be known with certainty. The drone pilot, if there was in fact a drone, faces stiff penalties if his or her identity becomes known to authorities.

The collision left the airliner undamaged. Part of the ASSURE research effort is focused on understanding exactly what that worst-case scenario would look like, if it does happen.

‘What I count on’

Tom Aldag, director of research and development at the Wichita State University National Institute for Aviation Research, one of the many ASSURE collaborators, is right in the thick of that. He also flies a Cessna 182 and occasionally patrols pipelines, so, “I’m down in the weeds” with more than a purely academic interest in the topic, he said in a telephone interview.

“It is a concern,” Aldag said, noting that while commercial drone operators are probably “going to do exactly what they’re supposed to do, the rogue operator, we can never do anything about. They’re going to do what they’re going to do.”

Aldag hopes that the public will take safety to heart and utilize the many available resources to learn the rules and where not to fly. (This includes the Know Before You Fly campaign,  which is also a collaborative effort involving the FAA and various groups including AOPA, as well as private sector efforts by AirMap and others).

He said research teams from four universities are meanwhile working to validate a computer model that simulates collisions between the most common consumer drones (a quadcopter and a small fixed-wing model) and narrow-body airliners, business jets, and a turbofan engine. Validating the model (and the simulated collisions it supports) requires setting up real-world collisions, which can become prohibitively expensive, so the team has worked its way up from a metal plate sensor previously used to measure the energy in birdshot, and will continue to compare simulated and actual collisions with actual aircraft parts and components. They’re still working on finding an affordable turbofan that can be destroyed for science.

“The really cool thing about the approach that we’re taking, especially if we can get these tests and make sure that the models are accurate, we’ll be able to translate it into general aviation, ag, helicopter … different scenarios,” Aldag said. “That’s the real value of simulation versus testing … testing gives you (just) one data point.”

Even when Aldag’s team reports its initial simulation results to the FAA in September, further development will be required to model the effects of a drone collision on GA aircraft. One element of the equation not yet included, Aldag said, is the lithium polymer battery that powers nearly all consumer drones. Lithium polymer batteries are well-known for volatility if damaged, and can burn at temperatures exceeding 1,400 degrees Fahrenheit, a condition known as thermal runaway, and a potential failure scenario that has bedeviled electric vehicle makers for years. A rapidly combusting LiPo battery would be an obviously unwelcome addition to any aircraft cockpit or cabin, should a drone penetrate the windshield.

“That battery really could misbehave once it gets damaged,” Aldag agreed. “We’ve proposed to look at that next year.”

Aldag is meanwhile flying his Cessna without being gripped by concern over the potential for hitting a drone. He checks notams for drone activity (commercial operators are required to file notams, though hobbyists are not), and keeps an eye out, though he also suspects that the chance of seeing a drone, particularly a small quadcopter, in time to take effective evasive action is vanishingly small.

“I don’t want to sound cavalier,” Aldag said of his relative lack of concern. “There’s still a lot of airspace, there’s still a lot of UAS operators who are doing what they are supposed to do. That’s what I count on.”

Managing traffic

Two days after the Heathrow incident, dozens of drones took to the air at six test sites around the country, the first multi-site, real-world test of the technology being developed under the NASA UTM program (in collaboration and coordination with ASSURE). The test was coordinated through the NASA Ames Research Center in California, and included both actual flights and flights that were designed to be aborted as the drone operator submitted flight plan details to a system created to vet drone flight plans for safety and potential conflicts. Once airborne, the drones flying at FAA test sites in Alaska, Maryland, Nevada, New York, Texas, and Virginia were to continue testing a variety of apps and displays that will eventually become central to a traffic management system that tracks the location of drones using cellular and Automatic Dependent Surveillance-Broadcast (ADS-B) technology, and helps pilots and controllers maintain safe separation.

"NASA is building UTM and ASSURE is following up with FAA research to prove that UTM is a safe and viable airspace management system,” Poss wrote. “ASSURE will prove that on the range and in labs first, then test it out in real world low altitude hubs dedicated to realistic, market based scenarios. Our first hubs will be for precision agriculture and maritime UAS operations, followed by disaster response/news gathering, linear infrastructure inspection (oil pipes, power lines, highways, railroads, etc.) and others.”

Poss said that another line of defense is also being actively developed: technology that allows a remote system or the drone itself to detect and avoid other aircraft. ASSURE is “also just about to launch a major research effort to detect rogue UAS activity around airports. Preventing collisions is the key. Tom is correct; if everyone follows the rules the risk is minimal.”

Calculating risk

The April 17 collision may have been Britain’s first, but it was most likely not the world’s first. Listed in an FAA spreadsheet released in March containing the latest 582 drone sighting reports collected by the agency from pilots and ground observers, a pilot flying a Cessna 188 Jan. 2 over Modesto, California, reported hitting a drone, though no damage was done to the Cessna. (The pilot told officials he believed the drone hit one of the aircraft’s tires.) A media report from Costa Rica published in November described a drone collision with a Cessna 172 flown by a student pilot over San José, a strike that damaged the Cessna’s wing, though the airplane landed safely. That collision was reported two months after that country’s aviation authorities released new regulations limiting drones to 400 feet agl, and imposing licensing and training requirements for operators.

Three collisions in six months (assuming that all three involved a drone) suggests the actual risk may be higher than calculated by researchers at George Mason University who looked at bird strike data and applied some statistical modeling to calculate risk. They concluded the risk of a drone collision resulting in damage or injury is extremely small:

“We find in general that small UAS under 2kg pose a negligible risk to the safety of the national airspace,” the authors wrote. “One damaging incident will occur no more than every 1.87 million years of 2kg UAS flight time. We further estimate that 6.12x10⁻⁸ collisions that cause an injury or fatality to passengers on board an aircraft will occur every 100,000 hours of 2kg UAS flight time, or once every 187 million years of operation. This appears to be an acceptable risk to the airspace.”

The paper, published in March, states that no known collisions had yet taken place between manned and unmanned aircraft, which appears to be a possibly mistaken notion based on reports from Costa Rica and California. The London incident followed about a month later.

The actual risk posed by misguided drones has proved challenging to calculate.

Dallas Brooks, director of the Raspet Flight Research Laboratory at Mississippi State University and the ASSURE associate director of research, told attendees at the International Drone Conference and Exposition in Las Vegas in September that computer models have been developed and refined for airspace that is covered by radar, a comparatively straightforward task (particularly for the minds at the Massachusetts Institute of Technology), but that researchers continue to search for data and information about the many flights (including many general aviation flights) that take place outside of radar coverage by virtue of location or altitude.

Brooks said in September that he thought it was likely that drones large enough to pose a risk to other aircraft will, within a few years, have the technology on board (or on the ground) to detect and avoid other aircraft.

“Hopefully we’re going to get to that intersection,” Brooks said.