DRONE VS. JET ENGINE
For decades, aerospace engineers have designed engines to mitigate foreign object damage, including bird strikes. But how well might they fare against drones?
Written by Kayt Sukel
DRONE VS. JET ENGINE
For decades, aerospace engineers have designed engines to mitigate foreign object damage, including bird strikes. But how well might they fare against drones?
Written by Kayt Sukel
DRONE VS. JET ENGINE
For decades, aerospace engineers have designed engines to mitigate foreign object damage, including bird strikes. But how well might they fare against drones? Written by Kayt Sukel
THE OLD STORY GOES SOMETHING LIKE THIS: A lone Canadian moose wandered onto a hill near the glide path of a busy airport. Thanks to gray and gloomy weather conditions, a jet pilot approached the runway guided only by instruments. Then just before touchdown—BOOM!—the right engine unexpectedly blew. Luckily, the pilot was able to maintain control and land the aircraft safely. But when the ground crew started to investigate the engine failure, they discovered quite a surprise. A large antler, not to mention an appalling mess, stuck amid some broken turbine blades.
Whether this particular story is just an old aviation yarn is anyone’s guess. Yet, most flight experts have heard some version of it and, even if they could not verify its authenticity, agreed it was entirely plausible.
“It’s possible, though I’ve never seen it. The suction power of these engines is quite high and if the engine was of the right size,” said Gerardo Olivares, senior research scientist and director of Crash Dynamics and Virtual Engineering and Flight at Wichita State University.
Regardless of the truth behind this particular tale, the ingestion of foreign objects, moose or otherwise, remains a stark problem for gas turbine engines. Jet engines regularly experience foreign object damage (FOD) caused by loose concrete, taxiway lights, unsecured tools, and other random debris. A 2024 American Airlines flight even made headlines when a wayward cargo container was sucked clean through the Boeing 787 engine on the tarmac at O’Hare International Airport, causing catastrophic damage.
Bird strikes happen regularly, but sometimes to a more destructive degree, which is what happened with the “Miracle on the Hudson.” Shortly after takeoff from LaGuardia Airport on January 15, 2009, US Airways Flight 1549 struck a flock of birds, causing it to lose all engine power and crash into the Hudson River. Photo: Jerritt Clark/Getty Images
But perhaps the most dangerous type of engine ingestion is the wildlife strike, defined by the Federal Aviation Administration (FAA) as the ingestion of birds, coyotes, deer, or bats. Bird strikes, in particular, have been responsible for high profile events including 2009’s “Miracle on the Hudson” crash, where a flock of birds took out both Airbus 320 engines mere minutes after take-off from LaGuardia Airport, and the more recent Jeju Air Flight 2216, which crash landed in Muan, South Korea, after encountering a large number of migratory ducks in flight.
Over the past few decades, aircraft and engine manufacturers have worked hard to design engines that can withstand such strikes. Case in point: while nearly 20,000 wildlife strikes were reported to the FAA in 2023 alone, only 3.6 percent of them caused engine damage. Yet, the world’s airspace is getting more and more crowded by the day. With increasing numbers of larger, more powerful unmanned aerial vehicles (UAVs) taking to the air, it raises an important question: what might happen to a gas turbine engine if it ingests a drone?
Trial by Cannon
Companies that design and build gas turbine engines have to follow strict FAA guidelines to ensure aircraft safety, explained Robert Mazzawy, principal engineer at Trebor Systems who previously spent 36 years as an engineer with Pratt & Whitney. And that involves shooting a large air cannon that can propel objects at speeds of 200-250 knots (230-288 miles per hour) and a few dozen chicken carcasses. Those chickens (thawed, not frozen) traveling at speed mimic the real-world impact of a bird, or flock of birds, that might make contact with the plane’s fuselage, wing, or, perhaps most importantly, gas turbine engine.
“The big requirement for the test is that, even after ingesting a bird or even a flock of birds, the engine can either recover or be safely shut down,” he said.
These six examples show the effective plastic strain in an engine fan from different UAS ingestion simulations. A: UAS 70 percent of nominal model; B: UAS nominal model (DJI phantom 3 standard unscaled); C: UAS 130 percent of nominal model; D: UAS 150 percent of nominal model; E: UAS 200 percent of nominal model; F: UAS 300 percent of nominal model). Chart: Kiran D’Souza
Engineers aim the cannon, sometimes called a “chicken gun,” loaded with one or more 4-to-8-pound chickens, at the engine core as well as other critical points defined by the engineering structures group. They then measure the resulting damage.
“You will typically get some damage with the smaller birds. It’s not catastrophic, but it can affect the flow capacity of the turbine fan,” Mazzawy explained. “When birds enter the core and strike the blades, they actually become fluidized so you get this circumferential extent of fluid being ingested into the engine, which, more than likely, will result in an engine surge.”
With decades of data—as well as new testing underway—aerospace engineers have been able to help engines recover from such a surge with at least three-quarters of the normal thrust generated by an undamaged engine. In most cases, the engine will suck, squeeze, bang, blow, and, after a brief shutdown and restart, continue on its merry way.
“You will typically get some damage with the smaller birds. It’s not catastrophic, but it can affect the flow capacity of the turbine fan. When birds enter the core and strike the blades, they actually become fluidized so you get this circumferential extent of fluid being ingested into the engine, which, more than likely, will result in an engine surge.”
—Robert Mazzawy, principal engineer at Trebor Systems
The Hard Body Problem
After decades of research, and the use of high-speed cameras and sensors measuring chicken gun testing, aerospace engineers can now successfully simulate potential damage due to wildlife strikes. But as the number of UAVs increases and their flight capabilities expand, the FAA’s Alliance for System Safety of UAS through Research Excellence (ASSURE) sponsored a project to find out what might happen if a drone were to be taken into the engine, explained Kiran D’Souza, associate director of the Gas Turbine Lab at The Ohio State University.
“The emphasis is avoiding any incidents,” he said. “They want to keep UAVs out of the airspace where we have manned aircraft. But, if you are talking about safety issues or fires, it’s possible you might want UAVs surveying. It’s important to understand what might happen in a worst-case scenario.”
To that end, D’Souza, Olivares, and colleagues ran a simulation to examine what might happen if the hard body of different sized drones were to be ingested. The group found hard bodies can do significant localized damage in the form of cracks or breaks, which “could lead to liberation of a whole blade tip or the whole blade,” D’Souza said. “There are definitely extra dangers with hard bodies, even when we are talking about the same mass of a bird. And it’s certain components of the UAVs that are most dangerous. The harder parts like the motor, the camera, and the battery.”
A simulation shows how a drone might come apart during an ingestion event at takeoff. Video: Kiran D’Souza
Validation experiments and corresponding simulations performed with titanium plates that feature a similar profile as a blade with a UAS in engine ingestion conditions. Video: Kiran D’Souza
Birds make impact at high velocity and they “liquefy,” so “everything behaves almost like a fluid” as it travels through the engine, Olivares said. Since drone parts are metal, as you might expect, they behave like metal, he added.
“The type of damage that is introduced is much larger,” he said. “After looking at multiple configurations with simulation, it looks as those these components are more likely to destroy the engine. Not that it will explode, but it probably won’t be operative after an ingestion event.”
The research group has also done a real-world test, using the typical cannon loaded with a drone (they used a UAS model created by the National Institute for Aviation Research), but D’Souza said those results are still being analyzed. Yet, the simulation results demonstrate that it remains important that FAA regulations keep UAVs out of manned airspace.
“It’s important that people see the danger,” D’Souza said. “Because hobbyists may not be aware of the regulations or break the rules so they can go high up in the air. They shouldn’t be doing these things. There’s a reason why we have these regulations and protocols to keep UAVs away from airports and flight zones.”
“With UAVs being a newer industry, they have more flexibility in making design changes that can help in the case of an ingestion. They could potentially use lighter materials in the future for batteries or motors.”
—Kiran D’Souza, associate director of the Gas Turbine Lab at Ohio State University
FRANK HASELBACH Senior Vice President, Propulsion Engineering, Airbus
Engineering Deterrence
Certainly, increased UAVs are not the only potential “hard body” that engines could foreseeably ingest. The development of Advanced Air Mobility (AAM) aircraft, including “air taxis” that leverage electric vertical takeoff and landing (eVTOL), will also crowd future airspace. Helicopters aren’t going anywhere either. And, of course, we still must also think about birds and other wildlife.
When asked what Airbus engineers are doing to help avoid jet engine ingestion incidents, Frank Haselbach, senior vice president of Propulsion Engineering, said his team is working on improving the design of engine installation as well as the use of sensors to minimize ingestion risks. Yet, the onus should not be fully on aerospace original equipment manufacturers, he said.
“Airports are responsible for keeping taxiways and runways clean, while maintenance teams must ensure that no tools or equipment—such as those that could obstruct an engine’s air intake—are left behind,” he said. “Furthermore, even if an aircraft were to detect a foreign object on the runway or a bird in flight, its design doesn’t allow for immediate evasive maneuvers.”
Kurt Leslie, a 737 pilot for United Airlines, said he has encountered at least three bird strikes “that [he] knows of.” And he agreed that adding more engineering through sensors or alerts to an already busy flight deck may not do much good.
“Luckily, I’ve never had a bird go into the engine. But strikes happen so fast and, by the time a sensor picked up a bird, the opportunity to change our flight path, our maneuverability, is basically zero,” he said. “But if there was some kind of frequency that might deter birds or tell them to move out of the way, that might work.”
Unfortunately, the effort involved with trying to make engines or aircraft more resistant to bird strikes likely won’t be cost-effective over the long term, said Lee Langston, professor emeritus at the University of Connecticut’s School of Mechanical, Aerospace, and Manufacturing Engineering, and an expert on bird strikes.
“I don’t envision anyone replacing or even doing much to change gas turbines. They’ve proven to be the most effective way to generate thrust over the years,” he said. Yet, there may be ways for aerospace engineers to work with their civil engineering counterparts to alter airports in ways that help to deter birds, and perhaps, even UAVs, in the future, Langston added. It will just take some good, old-fashioned collaboration and problem solving.
“Engineers are pretty ingenious when it comes to problems,” he said. “We like solving them. We know that problems are more or less an opportunity.”
All that said, the greatest opportunity to prevent damage from drone ingestion may come down to UAV fliers and manufacturers.
“With UAVs being a newer industry, they have more flexibility in making design changes that can help in the case of an ingestion. They could potentially use lighter materials in the future for batteries or motors,” D’Souza said. “But the big thing is not to fly UAVs in areas where you aren’t supposed to fly. The first and best level of protection is keeping UAVs out of the airspace and UAV users following all FAA safety protocols. If you don’t, you could be putting other people’s lives at risk.”
"NO GUTS, NO GLORY"
“Most of the stuff we did for [Mythbusters] met three basic criteria. One: it’s funny. Two: it’s technically challenging. And three: there’s some kind of destructive element or mischief involved. The chicken gun kind of fit all of those bills.”
Jamie Hyneman, Owner of M5 Industries, Inc.
Kayt Sukel is a technology writer and author in Houston.
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