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 Randall Watt, Project Engineer at AEDC,
loads a sabot carrying a thawed chicken carcass
into the gun for a birdstrike test. |
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.
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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.
A proposed Memorandum of Understanding on Aircraft-Wildlife
Collisions developed by the Federal Aviation Administration,
USAF, U.S. Army Corps of Engineers, U.S. Environmental
Protection Agency, U.S. Fish and Wildlife Services, and
the U.S. Department of Agriculture's Wildlife Services
Division, seeks to alleviate this problem.
Reference Aerospace Engineering
Online
Frank Bokulich
Aerospace Engineering March 2000
Images of RAAF F-111 bird strike damaged aircraft to follow
soon - David de Botton - Webmaster-Australia
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