What Really Happens When a Drone Strikes an Airplane

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Drones strikes are not like bird strikes—they're much, much worse.

What Really Happens When a Drone Strikes an Airplane

The skies are getting crowded.

The FAA says that reports of near-misses between drones and planes have surged since 2014, with as many as 650 cases as of August 2016. Last month an airliner narrowly avoided hitting a drone near London's tallest skyscraper. Dubai airport has been repeatedly shut down by drone activity, and low-flying drones are increasingly disrupting firefighting aircraft in the western U.S.

Sooner or later, those near-messes are going to become hits. So experts from the U.S. and U.K. are recreating these deadly scenarios before they wreck real-world consequences, hoping to nip this 21st century problem in the bud. New studies by the FAA and its European counterpart, the EASA, are looking at computer simulations and running physical tests to understand the problem and prevent disaster.

MECHANICAL GEESE FROM HELL
What actually happens when an aircraft runs into drone? While some continue to believe drones are no more of a danger than one stray goose, new studies are showing that drones are more like mechanical geese from hell when it comes to commercial and low-altitude helicopters.

"Impacts from drones are not the same as impact of birds," says Javid Bayandor, associate professor and director of the Crashworthiness for Aerospace Structures and Hybrids (CRASH) Lab at Virginia Tech.

Bayandor's simulations show exactly what happens when drones and birds meet a choppy death by jet engine. An airliner's engine would dice up the smallest commercial drones without much of a problem. But as these mechanical birds grow bigger, things get more dangerous.

Drones are made of materials much denser and stiffer than soft tissues and muscles of geese and other airborne animals. Simulations show that when a bird goes into an engine, it's essentially liquidized by spinning blades, like a macabre episode of "Will It Blend." Not so with a drone.

"Birds can disintegrate relatively easily...you get something like a very viscous bulk of fluid on the other side" says Bayandor. "A drone can be like a rock going through the engine."

That can mean immediate damage, leaving an engine blade deformed, broken, or completely fragmented, as shown in the above animation. Even where there is no initial damage, the sheer momentum of a larger drone can cause the engine to become unbalanced. This unbalance can escalate, and the blades may start hitting the casing that contains them. The engine ricochets back and forth inside its casing and the damage grows into a real problem.

Luckily, engines are designed to withstand some damage from stray objects, and the high-speed fragments thrown out by such events would likely be contained within the engine. "In general, the casing is designed so that it can also act as a shield between fuselage and engine," says Bayandor.

However this protection is not a given. Modern engines are not designed to gobble up drones, and models have already shown some unexpected results, such as the dramatic difference that the location and angle of impact can make.

But it's not just engines we need to worry about.

BREAK ON THROUGH
Across the Atlantic at Cranfield University in the U.K., Professor Ian Horsfall spends a lot of time "throwing things at other things" using different kinds of exotic cannons. He leads the Impact and Armor Group, and recently began simulating the aerial battle of drones versus aircraft.

To test a drone impact with an airliner during take-off or landing—the most likely times for such a disaster, since it's when planes are closest to the ground—Horsfall developed a 4-inch-caliber cannon with a ten-foot barrel, powered by compressed air and capable of firing projectiles at 200 mph. While some other tests fired real dead birds–leading to an aviation urban myth all its own–Horsfall's project used blocks of gelatin as stand-ins, like the ballistic gelatin used to test the effect of gunshots on flesh.

To simulate drone impact, Horsfall created a projectile which mimics the physical properties of a drone, with components of the same size and weight (as seen in the top GIF). There are four grape-sized steel cylinders instead of motors, nylon blocks standing in for circuit boards, a camera, and drone batteries. These components were then encased in a Styrofoam block to recreate the similar weight and structure of an average commercial drone.

Tests showed that mock drones won't damage an airliner's windshield. However, the radome, the circular cover over the radar at the aircraft's nose, isn't so lucky. While simulated birds of the same weight just bounce off, a drone can become embedded or even tear right through the radome. It is the hardness of some of the components rather than the weight that matters.

To test a drone impact with an airliner during take-off or landing—the most likely times for such a disaster, since it's when planes are closest to the ground—Horsfall developed a 4-inch-caliber cannon with a ten-foot barrel, powered by compressed air and capable of firing projectiles at 200 mph. While some other tests fired real dead birds–leading to an aviation urban myth all its own–Horsfall's project used blocks of gelatin as stand-ins, like the ballistic gelatin used to test the effect of gunshots on flesh.

To simulate drone impact, Horsfall created a projectile which mimics the physical properties of a drone, with components of the same size and weight (as seen in the top GIF). There are four grape-sized steel cylinders instead of motors, nylon blocks standing in for circuit boards, a camera, and drone batteries. These components were then encased in a Styrofoam block to recreate the similar weight and structure of an average commercial drone.

Tests showed that mock drones won't damage an airliner's windshield. However, the radome, the circular cover over the radar at the aircraft's nose, isn't so lucky. While simulated birds of the same weight just bounce off, a drone can become embedded or even tear right through the radome. It is the hardness of some of the components rather than the weight that matters.

FIRE DANGER
The other big issue is a battery's annoying tendency to burst into flames when damaged. To recreate this effect, Horsfall used a gun designed to fired a chisel blade at a drone battery. After a few seconds the damaged battery heated up and soon started burning fiercely. If the battery was lodged inside a radome or embedded in a plane, a fire could be the catastrophic icing on a tragic cake.

While most attention remains focused on airliners, thanks to the dramatic near-misses at the airport, other aircraft are in greater danger of drone strikes. Horsfall says helicopters and light aircraft have far more to fear from drones because their windshields are not as strong and are more likely to be flying at the same altitude as drones, especially when fighting fires. While airliner windshields are unlikely to be broken by bird impact, there have been cases where helicopters have been lost because of a run-in with a flock of geese. Drones will only make matters worse.

Bayandor and his CRASH Team will be presenting two papers giving the full results of their research – including a detailed study based on their review of a hundred and fifty different types of commercial drones – at the AIAA in January. Meanwhile, Horsfall is planning to upgrade his laboratory set-up, including a dedicated range for testing drone and bird strikes full time.

Many new measures, such as flight regulations, restrictions on drones, and sense-and-avoid systems are needed to ensure safety of shared airspace. But accidents will happen, and Bayandor says that drone impacts will need to be a part of future engine design, just like bird and hail strikes.

"Drones are here to stay," says Bayandor. "This is just the beginning."


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Interesting but still with quite a bit of hyperbole. What absolutely surprises me is that these academics have just completely failed to come up with the basic experiment that we're all waiting for. Take a dead Phantom 3 (carefully euthanized) and shoot it out of the cannon at an old engine. Of which there are thousands floating around. None of this 'grape sized metal ball' simulation that, even at first glance, seems to be very different from the P3 motor.

I'm sure DJI would be happy to send these guys a whole box of dead Phantoms / Inspires. These simulations are just that. Reality is often very different.
 
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The aircraft industry does FAA mandated "Bird Strike" certifications on engines. A 'calibrated' bird carcas I believe procured from the UK used to be used and launched into a test engine running at cruise power.

There was a tale going around years ago that the birds were shipped to the US frozen and some new guy did not read the instructions to THAW them first, with comical / expensive consequenses.
 

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