Understanding Air Safety in the Jet Age/Wildlife Encounters/Bird Strikes

Bird strikes are a significant threat to flight safety, and have caused a number of accidents with human casualties. There are over 13,000 bird strikes annually in the US alone. However, the number of major accidents involving civil aircraft is quite low and it has been estimated that there is only about one accident resulting in human death in one billion flying hours. The majority of bird strikes (65%) cause little damage to the aircraft; however the collision is usually fatal to the bird(s) involved.

Most accidents occur when a bird collides with the windscreen or is sucked into the engine of a jet aircraft. Strikes to the propellers can also cause major problems. Bird strikes happen most often during takeoff or landing, or during low altitude flight. However, bird strikes have also been reported at high altitudes, some as high as 6,000 to 9,000m above the ground. For example, an aircraft over the Ivory Coast collided with a Rüppell's vulture at the altitude of 11,300m. The majority of bird collisions occur near or at airports (90%, according to the ICAO) during takeoff, landing and associated phases. According to the FAA wildlife hazard management manual for 2005, less than 8% of strikes occur above 900m and 61% occur at less than 30m.

The point of impact is usually any forward-facing edge of the vehicle such as a wing leading edge, nose cone, jet engine cowling or engine inlet.

Jet engine ingestion is extremely serious due to the rotation speed of the engine fan and engine design. As the bird strikes a fan blade, that blade can be displaced into another blade and so forth, causing a cascading failure. Jet engines are particularly vulnerable during the takeoff phase when the engine is turning at a very high speed and the plane is at a low altitude where birds are more commonly found.

The force of the impact on an aircraft depends on the weight of the animal and the speed difference and direction at the point of impact. The energy of the impact increases with the square of the speed difference. High-speed impacts, as with jet aircraft, can cause considerable damage and even catastrophic failure to the vehicle. The energy of a 5 kg bird moving at a relative velocity of 275 km/h approximately equals the energy of a 100 kg weight dropped from a height of 15 m. However, according to the FAA only 15% of strikes (ICAO 11%) actually result in damage to the aircraft.

Bird strikes can damage vehicle components, or injure passengers. Flocks of birds are especially dangerous and can lead to multiple strikes, with corresponding damage. Depending on the damage, aircraft at low altitudes or during take-off and landing often cannot recover in time.

Countermeasures
There are three approaches to reduce the effect of bird strikes. The vehicles can be designed to be more bird resistant, the birds can be moved out of the way of the vehicle, or the vehicle can be moved out of the way of the birds.

Vehicle design
Most large commercial jet engines include design features that ensure they can shut-down after "ingesting" a bird weighing up to 1.8 kg. The engine does not have to survive the ingestion, just be safely shut down. This is a 'stand-alone' requirement, i.e., the engine, not the aircraft, must pass the test. Multiple strikes (from hitting a bird flock) on twin-engine jet aircraft are very serious events because they can disable multiple aircraft systems, requiring emergency action to land the aircraft.

Modern jet aircraft structures must be able to withstand one 1.8 kg collision; the empennage (tail) must withstand one 3.6 kg bird collision. Cockpit windows on jet aircraft must be able to withstand one 1.8 kg bird collision without yielding or spalling.

At first, bird strike testing by manufacturers involved firing a bird carcass from a gas cannon and sabot system into the tested unit. The carcass was soon replaced with suitable density blocks, often gelatin, to ease testing. Current testing is mainly conducted with computer simulation, although final testing usually involves some physical experiments.

Flight path
Pilots should not take off or land in the presence of wildlife and should avoid migratory routes, wildlife reserves, estuaries and other sites where birds may congregate. When operating in the presence of bird flocks, pilots should seek to climb above 3000 ft as rapidly as possible as most bird strikes occur below 3000 ft. Additionally, pilots should slow down their aircraft when confronted with birds. The energy that must be dissipated in the collision is approximately the relative kinetic energy ($$E_{k}$$) of the bird, defined by the equation $$E_{k} = \frac{1}{2} m v^{2}$$ where $$m$$ is the mass of the bird and $$v$$ is the relative velocity (the difference of the velocities of the bird and the plane, resulting in a lower absolute value if they are flying in the same direction and higher absolute value if they are flying in opposite directions). Therefore, the speed of the aircraft is much more important than the size of the bird when it comes to reducing energy transfer in a collision. The same can be said for jet engines: the slower the rotation of the engine, the less energy which will be imparted onto the engine at collision.

The body density of the bird is also a parameter that influences the amount of damage caused.

The US Military Avian Hazard Advisory System (AHAS) uses near real-time data from the 148 CONUS based National Weather Service Next Generation Weather Radar (NEXRAD or WSR 88-D) system to provide current bird hazard conditions for published military low-level routes, ranges, and military operating areas (MOAs). Additionally, AHAS incorporates weather forecast data with the Bird Avoidance Model (BAM) to predict soaring bird activity within the next 24 hours and then defaults to the BAM for planning purposes when activity is scheduled outside the 24-hour window. The BAM is a static historical hazard model based on many years of bird distribution data from Christmas Bird Counts (CBC), Breeding Bird Surveys (BBS), and National Wildlife Refuge Data. The BAM also incorporates potentially hazardous bird attractions such as landfills and golf courses. AHAS is now an integral part of military low-level mission planning, aircrew being able to access the current bird hazard conditions. AHAS will provide relative risk assessments for the planned mission and give aircrew the opportunity to select a less hazardous route should the planned route be rated severe or moderate. Prior to 2003, the US Air Force BASH Team bird strike database indicated that approximately 25% of all strikes were associated with low-level routes and bombing ranges. More importantly, these strikes accounted for more than 50% of all of the reported damage costs. After a decade of using AHAS for avoiding routes with severe ratings, the strike percentage associated with low-level flight operations has been reduced to 12% and associated costs cut in half.

Avian radar is an important tool for aiding in bird strike mitigation as part of overall safety management systems at civilian and military airfields. Properly designed and equipped avian radars can track thousands of birds simultaneously in real-time, night and day, through 360° of coverage, out to ranges of 10 km and beyond for flocks, updating every target's position (longitude, latitude, altitude), speed, heading, and size every 2–3 seconds. Data from these systems can be used to generate information products ranging from real-time threat alerts to historical analyses of bird activity patterns in both time and space. The United States Federal Aviation Administration (FAA) and the United States Department of Defense (DOD) have conducted extensive science-based field testing and validation of commercial avian radar systems for civil and military applications, respectively. The FAA used evaluations of commercial 3D avian radar systems developed and marketed by Accipiter Radar as the basis for FAA Advisory Circular 150/5220-25 and a guidance letter on using Airport Improvement Program funds to acquire avian radar systems at Part 139 airports. Similarly, the DOD-sponsored Integration and Validation of Avian Radars (IVAR) project evaluated the functional and performance characteristics of Accipiter® avian radars under operational conditions at Navy, Marine Corps, and Air Force airfields. Accipiter avian radar systems operating at Seattle-Tacoma International Airport, Chicago O'Hare International Airport, and Marine Corps Air Station Cherry Point made significant contributions to the evaluations carried out in the aforementioned FAA and DoD initiatives. Additional scientific and technical papers on avian radar systems are listed below,  and on the Accipiter Radar web site.

A US company, DeTect, in 2003, developed the only production model bird radar in operational use for real-time, tactical bird-aircraft strike avoidance by air traffic controllers. These systems are operational at both commercial airports and military airfields. The system has widely used technology available for bird–aircraft strike hazard (BASH) management and for real-time detection, tracking and alerting of hazardous bird activity at commercial airports, military airfields, and military training and bombing ranges. After extensive evaluation and on-site testing, MERLIN technology was chosen by NASA and was ultimately used for detecting and tracking dangerous vulture activity during the 22 space shuttle launches from 2006 to the conclusion of the program in 2011. The US Air Force has contracted DeTect since 2003 to provide the Avian Hazard Advisory System (AHAS)previously mentioned.