American Scientist

Airplanes and birds have been running into one another since the earliest days of powered flight.

After their historic first powered flight on December 17, 1903, at Kitty Hawk, North Carolina, Orville and Wilbur Wright continued their testing and training flights over Huffman Prairie in Dayton, Ohio. On September 7, 1905, Wilbur was piloting and recorded that he had tangled with a flock of birds (probably red-winged blackbirds), killing one, but with no ill effects on pilot or plane.

The earliest fatal airplane crash attributed to a bird strike took place seven years later, on April 3, 1912. Calbraith (Cal) Perry Rodgers, piloting a Wright Flyer over Long Beach, California, ran into a flock of seagulls, crashed the biplane into the surf, and was killed.

The inexorable growth of worldwide air travel has escalated bird and aircraft incidents, and all along engineers have been striving to ensure the safety of crew and passengers in the event of a bird collision. Higher-speed jet propulsion itself has also greatly increased the seriousness of bird strike damage, giving birds less time to avoid an approaching aircraft, with the resulting higher-speed impact causing much greater damage to both the craft and the birds.

Bird Ingestion Engine Damage

As aircraft manufacturer Boeing pointed out in a 2011 publication, bird strikes occur at various wing and fuselage locations but they usually inflict most damage to the jet engines, composed as they are of intricate high-speed rotating parts. Airplane damage and the effect on flight from bird strikes are closely correlated with kinetic energy derived from the mass (determined by the bird species) and the velocity squared. (A 20 percent increase in speed raises the kinetic energy by 44 percent.)

A popular misconception is that a jet engine is a huge vacuum cleaner, sucking in birds from everywhere. The reality is that birds are only drawn into the engine if there is direct alignment between the flight paths of the engine and the bird. But nonetheless, in industry parlance, a bird-engine impact is called a “bird ingestion,” and the bird, enmeshed in engine internal components, is pretty much “digested” in the process.

All commercial jet engines must comply with bird ingestion regulations established by regulatory authorities such as the U.S. Federal Aviation Administration (FAA) and the European Aviation Safety Agency (EASA). These regulations involve certification testing of commercial jet engines for bird ingestion, calling for demonstrations of an engine’s ability to ingest birds in small, medium, and large categories at takeoff power, and still maintain a specified level of performance.

Not being able to meet these regulations can have serious consequences for an engine company. For instance, while Rolls Royce was in the final stages of developing their early RB211 turbofan engine, the engine failed certification-required bird-ingestion tests, leading to the bankruptcy of the company in 1971.

Most jet-engine bird encounters occur during takeoff. Stuart Frost, a retired Pratt and Whitney engineer, gave me a firsthand account of an engine bird strike he experienced while traveling on a flight from Dublin to London on December 7, 1985. He was sitting near the front, with a good view of the Pratt and Whitney JT8D-9A left engine. After liftoff from Dublin Airport, the aircraft, with 117 passengers, encountered a flock of 20 to 30 black-headed gulls (weighing about 0.5 kilograms each) near the end of the runway. Several bangs were heard and the Boeing 737 aircraft yawed and buffeted. Frost heard a loud explosive noise from the left engine, as gulls were ingested. As the engine’s fan blades broke, the engine almost immediately stopped, which forced two of the three engine mounts to fail. The now thrustless engine hung from the wing by one remaining mount and two thrust reverser hydraulic lines. In the short time it took for all this to happen, Frost remembers thinking, “This is going to hurt!” However, with remaining thrust from the right engine (which had also ingested gulls), the pilot and copilot managed to make an emergency landing on an adjacent runway, with the left engine barely hanging from the wing.

Perhaps the most famous recent airline bird-strike incident is the “Miracle on the Hudson.” On January 15, 2009, US Airways flight 1549, an Airbus A320-214 with 150 passengers, took off from New York’s La Guardia Airport, bound for Charlotte, North Carolina. About two minutes into the flight at an altitude of about 0.85 kilometers, it struck a flock of migrating Canada geese just northeast of the George Washington Bridge. Each of the two jet engines (which were manufactured by CFM International, model CFM56, and produced about 30,000 pounds of thrust) ingested at least two 4-kilogram geese, damaging the engines to the point that they could not maintain thrust for sustained flight. The crew successfully made a water landing in the Hudson River with no loss of life. The largely intact wreck of the Airbus is on display at the Carolinas Aviation Museum in Charlotte, North Carolina, complete with its two bird-ingested-disabled jet engines. The captain, Chesley B. Sullenberger III, was further honored in a 2016 movie that dramatized the incident and the subsequent investigation.

Dual bird ingestion incidents in twin-engine jets are proving to be not so rare an occurrence. Two such incidents happened in 2008—to a Boeing 737 at Rome’s Ciampino Airport and to an Airbus A320 at Bourgas, Bulgaria—and another in 2009 to a Boeing 737 in Ireland.

What’s the Risk?

A bird-strike event has been estimated to occur about once in every 2,000 flights, depending on the time of year and flight location. But many events are not reported. The number of strikes annually reported to the FAA increased 7.4-fold from 1,847 in 1990 to 13,795 in 2015, reflecting similar trends in other parts of the world.

A 2016 report by Richard Dolbeer of the USDA’s National Wildlife Research Center and his colleagues gave statistics for the United States (which includes U.S. registered aircraft in foreign countries) for 1990 to 2015. During these 25 years, 17,494 jet engines were hit in 16,694 bird-strike events, but in many cases the impact didn’t lead to engine damage. Out of these reported incidents, 4,516 engines were damaged in 4,370 bird-strike events (4,227 events with one engine damaged, 141 with two engines damaged, 1 with three engines damaged, and 1 with all four engines damaged).

The number of bird-airplane strikes annually reported increased 7.4-fold from 1990 to 2015.

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Considering that there are approximately 20,000 aircraft worldwide involved in commercial flight, and that these statistics are only for U.S. aircraft, it’s obvious that for commercial jet engines, bird ingestion is a significant concern.

Current statistics show that incidents of bird ingestion by commercial aircraft jet engines are increasing and are expected to be a continuing challenge over the next decade. The main factor contributing to this avian threat is increased air traffic, especially because most aircraft are now powered with quieter turbofan jet engines—great for urban noise reduction, but not for giving birds more notice to avoid collisions. Another factor is increasing populations of large birds. The 2016 report from Dolbeer and his colleagues noted that the resident Canada goose population in North America increased from 1 million to 3.9 million from 1990 to 2014, and during that same period, the snow goose population went from 2.6 million to 5.5 million birds. A 2002 report by Dolbeer and his colleagues finds that from the 1970s to the 2000s, factors such as increased pesticide regulation and expanded wildlife refuges led to rising bird populations; in addition, species such as Canada geese became more adapted to urban environments.

Which Birds?

The Smithsonian Institution’s National Museum of Natural History in Washington, D.C., has for decades played a key role in systematically gathering and analyzing data on birds struck by aircraft. It houses the Feather Identification Laboratory, which can associate a bird type, if not a specific species, with the remains (known as snarge) of birds that have collided with aircraft, after they are sent in from agencies, airports, airlines, and other owners of aircraft struck by birds.

The Feather ID Lab was founded by the late forensic ornithologist Roxie Laybourne, who began the work in the 1960s and continued until her death in 2003. She pioneered many of the methods used in feather identification, to determine from just these remains not only the bird, but even its gender, age, and migratory status. As biologist Thor Hanson notes in his 2011 book on feathers, for an animal body covering, nothing competes with feathers for sheer diversity of form and function, and hence, for identification. Feathers are also very durable and may be all that is left of an impacted or jet engine–ingested bird.

Laybourne’s Feather ID Lab activities have had a central role in systematically gathering data on birds and bird strikes. The FAA, and other organizations such as the U.S. Air Force, can use the data on bird types involved in collisions to make inferences regarding how the behavior, dietary preferences, and migratory patterns of these species can be used to reduce hazards at airports.

Knowing the exact species provides guidance regarding the size, behavior, and ecology of the bird in question and is key to tracking species trends as well as focusing preventive measures. Species identifications provide the baseline data needed to plan habitat management on airfields (to make them less attractive as feeding and nesting areas) and to build avoidance programs, and have been used to assist engineers in designing jet engines that are more resilient to bird-strike events.

Currently, forensic ornithologist Carla Dove, who was trained by Laybourne, and a team of four, process about 9,000 bird-strike cases each year in the Feather ID Lab. This number is 30 times higher than in Laybourne’s day—and it continues to grow.

Bird-Ingestion Engine Testing

FAA certification of any new commercial transport jet engine requires the engine manufacturer to demonstrate resilience testing of bird ingestion at takeoff power. The FAA has multiple requirements, which include the ability of the engine to survive a strike from one large bird (such as a Canada goose) weighing 2 to 4 kilograms, depending on the engine size. After impact, the engine must not catch fire and must be able to be safely shut down after 15 seconds with no throttle movement.

Engines must also be able to withstand a strike from multiple medium-size flocking birds (such as gulls) weighing 0.7 to 1.3 kilograms. And the engine must continue to deliver at least 75 percent of its normal takeoff thrust for 2 minutes with no throttle movement, followed by about a 20-minute run-on period at various power levels that are consistent with the aircraft’s return to airport and landing.

An additional testing standard is impact from one large flocking bird (such as a snow goose) weighing 2 to 3 kilograms. On this test, the engine must continue to deliver at least 50 percent of normal takeoff thrust for 1 minute followed by the 20-minute run-on period.

Almost all commercial aircraft are powered by turbofan jet engines, so named for a fan located inside a cylindrical duct that is mounted on the front of the engine. Air drawn into the fan is divided, so that some flows into the jet engine itself and the rest bypasses the engine. The lower-velocity bypassed air and the higher-velocity engine air combine downstream to produce thrust, and this arrangement produces a larger mass of airflow at a lower average velocity than other jet engines, to reduce noise and increase the engine’s efficiency.

However, this engine layout means that when a bird is ingested, the fan blades are the first to be hit, so they are key points in engine testing. The single large bird test targets the most critical area of the fan blade, as determined by the fan designer. The multiple medium flocking bird test distributes birds to critical areas of the fan (also as determined by the fan designer) so that at least one bird must enter the engine core. The single large flocking bird test targets a 50 percent span of the fan blades.

During these tests, an engine is mounted in a stationary stand. The test engine is configured only with the instrumentation necessary for engine controls, such as controls for rotor speeds, fuel flow burner pressure, and exhaust gas temperature.

A special multi-barrel air gun, specific to each engine manufacturer, is mounted in front of the engine. The air gun fires commercial domesticated poultry livestock (such as chickens and ducks) that have been euthanized, which are of the sizes listed in the various FAA standards. With the engine running at takeoff thrust, the bird cannon can be accurately aimed at the fan. Birds are then fired at a velocity that simulates takeoff aircraft speed, which can typically be around 170 knots. High-speed video cameras focused on the engine inlet record the test and the engine performance.

Better Engineering; More Avoidance?

The approaches that the civil aviation community has taken to minimize the occurrence, or the effect, of jet-engine bird ingestion are rather fragmented. Excluding birds from an entire airport environment is nearly impossible. Fences, loud noise generators, dogs, birds of prey, predator effigies, lasers, flares, and elimination of food sources (such as nearby dumps and landfills) have all been employed, with limited success. There is still a self-evident need for more enlightened actions or regulations to prevent birds and aircraft from occupying the same air space at the same time.

Engine manufacturers are repeatedly called upon to make their engines strong enough to endure the destructive effects of bird ingestion. For instance, currently the FAA is issuing an NPRM (notice of proposed rulemaking) for a fourth, new, engine qualifying test, “Medium Flocking Bird Test at Climb Condition,” to address the 2009 “Miracle on the Hudson” incident that was caused by migrating Canada geese.

The FAA is considering a fourth engine-qualifying test to address the 2009 Hudson crash.

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Making an engine more resistant to bird strikes involves significant effort, and cost, for a manufacturer. It can involve strengthening fan blades (without significantly increasing engine weight), designing engine fuel controls to account for strike-induced performance changes, or installing fan and engine casings capable of containing a possible rotating blade failure.

In 2013, at an international gas turbine conference in San Antonio, I helped organize a panel on bird ingestion. The panel members included an expert on bird detection and surveillance, a representative of the World Birdstrike Association, flight safety experts from Boeing and GE Aviation, and a retired airline pilot.

My impression from our panel was that approaches to solving bird-strike issues depend on the focus of the entity involved. For instance, manufacturers were most intent on meeting regulatory requirements, whereas other participants concentrated more on their particular areas of expertise.

The lead-off panelist was Captain Paul Eschenfelder, a retired Delta Airlines pilot, who gave an overall review of recent bird-strike accidents around the world. He went over gaps in bird-strike mitigation regulations and emphasized the lack of a systematic approach to this problem. For example: You are a pilot of an airliner, with 200 passengers sitting behind you, all waiting for you to take off. The control tower informs you are clear to take off—and adds the general warning that there are birds at the end of the runway. What do you do? Eschenfelder’s example is buttressed by a similar remark made elsewhere by Captain Sullenberger of the Hudson bird-strike incident, regarding such a general warning: “But that’s like saying ‘Be careful out there!’ It’s not useful. It’s not effective.”

It’s possible that some technique or some kind of mechanism or device, could use an avian behavioral trait to make birds want to avoid the airspace around an airport. For example, many small lakes in private complexes have long had the issue of being populated by large flocks of Canada geese that foul the lakes and their shores. Some lake management companies have taken to stringing barely visible wires near the shores, about 0.5 meter above the water level. At two small lakes we have at the Storrs campus of the University of Connecticut, where such wires have been strung, the geese have disappeared, presumably not wanting to run the risk of hitting the wires on landing or takeoff. Could some variant of this approach be effective at airports?

Not all airports are the same, so it’s doubtful that one solution will fit all locations. Perhaps what is needed is a call for proposals for inventive bird-strike solutions from an agency such as the Department of Defense’s Defense Advanced Research Project Agency (DARPA). That agency has been responsible for funding projects that resulted in the Global Positioning System, the internet, and many other ideas that were far out at the time, and certainly has a track record that might yield a new approach to the avoidance of aviation bird strikes. With increasingly congested flight routes and crowded wildlife, the need for solutions will only continue to grow.