Birdstrikes remain a concern for pilots & crew

One of the safety concerns plaguing the aircraft industry since its inception has been birdstrikes, or bird ingestion. Flocks of birds seem harmless enough when admiring them from the ground, but experiencing them from the cockpit of an aircraft can prove lethal. A 12 lb Canadian goose struck by an aircraft travelling 150 mph at liftoff generates an impact force similar to that created by a 1000-lb weight dropping from a height of 10 ft. More than 300 people have been killed by birdstrikes since the first fatality was recorded in 1912.

Since 1960, aircraft-bird collisions destroyed 20 U.S.-registered commercial aircraft and, since 1985, 23 U.S. military aircraft. The most common impact areas on the aircraft include the engine inlet, the nose, the canopy, and the wing. U.S. Air Force (USAF) officials have reported more than 2500 strikes annually, not including strikes to commercial and U.S. civil aircraft.

An F-16 canopy undergoes birdstrike evaluation.

In response to the birdstrikes, the USAF developed procedures to prepare pilots for such an event. These, in addition to further impact testing and analysis by USAF engineers at the Arnold Engineering Development Center (AEDC), have improved pilots' chances of returning from such an occurrence.

The birdstrike testing and training procedures proved very useful to now Colonel Craig Christen while a student on a test flight in a F-111 fighter-bomber in 1985. "I was at the Air Force test pilot school deployed to Cannon Air Force Base, NM, for an F-111 orientation flight," Christen recalled. "We were flying a simulated ground-attack mission where we were going to enter a low-level run using autopilot, then switch to hands-on operation to enter a simulated bomb drop. I was in the pilot's seat and the instructor was in the systems operator's seat. We were flying at about 500 kts and had gone through canyons using terrain-masking techniques on autopilot. Just as we entered bomb range, we switched to manual flight. All of a sudden, the front windshield canopy crashed in, crazed over (cracked), and bowed in onto the head-up display, and a small piece of the windshield inner laminate layer fell onto my lap. At that point my forward visibility was zero."

A hawk had impacted the left windshield at about 510 kts at an altitude of 100 ft above the ground. "At that speed, I never saw the bird," said Christen.

The leading edge of a Gulfstream V wing is prepared for bird impact testing at AEDC.

Fortunately, the last topic covered during the prebrief was birdstrike procedures. Using the procedures, Christen pulled back on the stick to increase altitude, informed the instructor pilot that he was alright, and then returned safely to base.

Christen also credited his safe return to the design, testing, and analysis activities of engineers at AEDC. "Without AEDC's validation and developmental testing on aircraft canopies, the bird would have penetrated the windshield," Christen added. "At that point, I would have become blinded and disoriented, or even been killed, by the sudden inflow of high-pressure air and flying objects."

Relatively low-tech compared to other testing equipment at the center, the AEDC's chicken gun has enabled aircraft manufacturers to make aircraft safer from birdstrikes. Using high-pressure air, technicians fire 4-lb chicken carcasses from a 60-ft long launch tube at speeds exceeding 900 mph to simulate a direct birdstrike. Videotapes and high-speed motion picture cameras operating at speeds up to 10,000 frames/s provide visual data on what actually occurs during a simulated birdstrike. This information is used to determine the ability of the test article to withstand the impact and the damage caused during the strike. Manufacturers use the data to redesign and build components before flight, providing substantial savings to the aircraft programs.

Birdstrikes have occurred for as long as aviation has existed, but they have become more evident to the military as aircraft began flying more low-altitude, high-speed profiles. According to Randall Watt, Project Engineer at AEDC, the need for the chicken gun surfaced during the Vietnam War, when aircraft such as the F-111 were equipped with terrain-following radar, which allowed pilots to fly at high speeds at only a few hundred feet above ground. It was during these flights that pilots encountered thousands of birdstrikes each year.

The chicken gun concept was devised when the Air Force tasked the Aeronautical Systems Division at Wright-Patterson Air Force Base, OH, to address the bird impact hazard. Engineers at Wright-Patterson teamed with those working at AEDC to develop a bird-impact test. Working together the team developed the idea for the chicken gun. The mechanism was constructed using scrap hardware, including an old 8-in Naval gun. The first shot was fired September 14, 1972, at an F-111 escape module.

Since then, the chicken gun has fired over 1300 shots, testing windshields and canopies of aircraft such as the A-7, A-10, F-15, F-16, F/A-18, F-111, T-37, B-1, C-130, and the Gulfstream V. The latest test using the chicken gun facility was performed on the Raytheon T-6A Texan II. The test involved launching a chicken at 270 kts at the leading edge of the wing.

However, even with testing tools such as the chicken gun, birdstrikes have been occurring more often. In 1995, 24 people aboard an E-3B Sentry AWACS were killed when 35 Canadian geese were ingested by the aircraft's engine during takeoff at Elmendorf Air Force Base, AL. The following year 34 fatalities were recorded when a Belgian C-130 Hercules collided with a flock of about 500-600 starlings. The U.S. Bird Strike Committee reported that increasing fowl populations, such as Canadian geese, pelicans, starlings, and blackbirds, will increase the number of birstrikes, causing an estimated $149 million in damage between 1999 and 2008.