It's a Bird, It's a Plane... It's a Bird Striking a Plane

Ever seen those silly chicken cannon skits on CBC's Royal Canadian Air Farce? Few people know that a similar device is used for serious tests. The tests and related research ultimately make air travel safer for Canadians and save money for airlines and aircraft manufacturers.

There are few places in the world like NRC's specialized flight impact simulation facility. Although some companies have flight impact simulators of their own, many manufacturers prefer to rely on NRC for bird strike expertise, accurate calibration and specialized facilities, all of which are required to certify aircraft to meet strict international aviation certification standards.

"You'd be amazed how destructive a small bird can be when it hits an airplane," says Ron Gould, a technical officer with the NRC Institute for Aerospace Research. Cockpit windshields, aircraft wings or the leading edges of tail sections (empennage), propellers and turbine engines are the areas most likely to sustain bird strike damage.

How can birds cause so much destruction? Think back to physics class – energy equals half the mass multiplied by the velocity squared (E = ½ MV²). With the exception of geese and swans, wild birds typically weigh less than 3.6 kg (8 lbs), but the velocity involved in these collisions is high, upwards of 350 knots (648.2 km/h).

Manufacturers must have their aircraft certified to Transport Canada, Joint Aviation Authorities (Europe's JAA) or United States Federal Aviation Administration (FAA) specifications before delivering aircraft to their customers. If an aircraft component fails, then it is back to the drawing board to improve the design until that component can sustain a bird strike.

Commercial and military aircraft – helicopters, prop planes and jets – have been tested by NRC. They have ranged from small commuter planes, such as Bombardier's Dash-8s, to huge Boeing and Airbus passenger planes. Birds typically hit aircraft during takeoff or landing, rather than when an aircraft is traveling faster at higher altitudes. Collisions tend to occur below 1.5 km (5 000 ft), near airports where speed is restricted to a maximum of 350 knots (648.2 km/h). Remarkable damage is suffered even at relatively low speeds.

NRC has three flight impact simulators or bird guns, pneumatic devices ranging from 5.5 to 21.3 metres long. Tests for bird strikes have been as 'slow' as 87.8 km/h up to more than Mach 1.4 (1 674 km/h). NRC's simulators use compressed air pressure to launch projectiles. Pressure can be as high as 150 pounds per square inch (psi), but most work is done below 50 psi.

A mere 35 psi is needed to launch a small 1.8 kg (4 lb) projectile at 350 knots. To mitigate costly repairs caused by such collisions, cockpit windows can be up to 10 plies thick, alternating between glass and sticky plastic layers, some are tempered to provide structural strength, while others have invisible heating /defogging elements and outer layers can be replaced without affecting the entire window. Even back in the late 1970s, the six windows for a large transport aircraft cockpit cost $5 000 USD each. That explains why companies would prefer to refurbish rather than replace windows.

How have things changed?

Now flight impact simulators can be used for testing more than just bird strikes. New digital cockpit voice and flight data recorders must be able to survive water immersion and fire, and investigators need to be able to access the recorder's data even after it is subjected to a 3 500 G impact (3 500 x gravitational force). These 'black boxes' can be tested for that kind of impact by being launched from NRC's largest cannon.

Since NRC began flight impact studies in the 1960s, researchers have tried many different synthetic projectiles, including modeling clay, cellulose gelatins, and even ground meat. Unfortunately, these are not satisfactory substitutes for a bird strike – only real birds will do. Synthetic versions are used for calibration to the desired velocity and bird carcasses are used just for the actual certification tests.

These birds are ultimately keeping air travelers safe. Thanks to the data generated by NRC's expert bird gun operators, manufacturers have been able to better understand the effects of bird strikes on aircraft, and on passenger and pilot safety. This knowledge has helped manufacturers design aircraft and parts that are lighter and able to withstand severe impact.

Next-generation flight impact studies

NRC-IAR researchers know that while full scale testing provides physical validation of how aircraft respond to bird impacts, this can only be done after the aircraft structure is designed, or at least a prototype is built. If problems arise, then it may be necessary to redesign. Any delays late in the development and production processes can be very costly to manufacturers, but clearly safety cannot be compromised.

Accordingly, researchers want to find an effective way to help manufacturers out earlier in the design process. NRC-IAR's Aeroacoustic and Structural Dynamics Group is working on computer modeling of bird impact that may prevent costly redesigns and physical tests, sparing the need for bird guns. Drs. David Zimcik and Manouchehr Nejad Ensan are collaborating with industry and external research institutes to develop the capability of using advanced numerical simulation methods to model the structural response of aircraft structures subjected to bird strikes.

Using experimental data from NRC's flight impact tests in Ottawa and some computer-aided-design tools, these researchers are combining their own modeling and simulation R&D. Material modeling of the bird is crucial. The model must represent both the solid and fluid matter of a real bird as well as the effects of a potential high velocity impact. The model's interface between the bird and the aircraft structure is also very important because of the large deformations that take place. Early feedback about this analytical simulation project is very positive. If the computer simulations are successful, then the bird guns may finally retire after nearly half a century of service.

The last 50 years of aircraft bird strike research has not only greatly improved the safety of air travel for Canadians, but has helped save money for the aerospace industry.

Bird gun illustration

NRC's flight impact simulators or bird guns have three parts: a storage reservoir (tank at the back) pressurized to the test pressure; a step chamber (half of the reservoir's pressure); and a barrel with sabot arrestor. The sabot, which holds the projectile, must withstand the launch, protect the projectile and crush the muzzle arrestor so that nothing but the projectile continues on to the target. Adjusting the thickness of the sabot lining helps various sizes of projectiles to fit in the barrel. The firing mechanism uses a double diaphragm method. Each diaphragm is made of very thin plastic material (Mylar).

Early flight impact research

Air travel became more common after the Second World War, resulting in more frequent collisions with birds. Sometimes the result was tragic. For example, in the early 1960s more than 60 people died in Boston when an airplane flew into a flock of starlings, lost three turbine engines and crashed.

To address this problem, NRC formed an Associate Committee on Bird Hazards to Aircraft in 1963, bringing together government, university and industry experts from different disciplines. The Committee studied the growing number of bird-aircraft collisions and recommended ways to minimize these dangerous and costly encounters.

Quickly realizing it would be too expensive to make aircraft completely 'birdproof,' the Committee advised airport authorities to make the grounds surrounding airports less attractive to wild birds. The suggestions included draining nearby ponds, stopping agricultural use of airfields, moving dump sites away from airports, and using noisemakers or falcons to deter birds.

In the mid-1960s, researchers at NRC's Division of Mechanical Engineering built NRC's first bird gun. Surprisingly, it is still in service after being retrofitted with a new barrel. In 1967, NRC engineers designed a much larger bird gun for a unique Canadian facility dedicated to this work. NRC's Flight Impact Simulator Facility, located next to the Ottawa airport, featured a 10-inch gun, modeled after a smaller British bird gun built in 1961 by the Royal Aeronautical Establishment.

Longtime NRC employee, Paul MacLean, father of the Canadian astronaut Steve MacLean, commissioned this 10-inch flight impact simulator which turned out to be Canada's 'hardest working bird gun.' Up to 13 tests were carried out in a single day and a single research project required firing that bird gun on 110 consecutive 747 aircraft windshields as part of a program to develop a lighter windshield design.