Description Of Bird Strike Engineering Essay

“A bird strike (sometimes bird strike, bird hit, or BASH – Bird Aircraft Strike Hazard) is a collision between an airborne animal (usually a bird) and a man-made vehicle, especially aircraft. It is a common threat to flight safety, and has caused a number of accidents with human casualties. Major accidents involving civil aircraft are quite low and it has been estimated that there is only about 1 accident resulting in human death in one billion (109) flying hours. The majority of bird strikes (65%) cause little damage to the aircraft but result in a great number of bird fatalities. Most accidents occur when the bird hits the windscreen or flies into the engines. These cause annual damages that have been estimated at $400 million within the United States of America alone and up to $1.2 billion to commercial aircraft worldwide”. (Wikipedia.com)

Bird strike incidents are the serious problem for flight safety. It is known that during the period of 1912 to 2004 there were more than 45 fatal bird strike accidents killing more than 200 people. The total number of aircrafts destroyed due to the bird strikes in that period was 90. Statistics shows that the major threat(more than 70% of accidents) to Airlines and Executive jets is engine ingestion.

That’s why numerical investigations of the bird strike are very important and should be analysed during design stage of a development process. Basing on the results of the analysis the design of the engine must be changed to exclude any serious effects of the bird strike incident.

Because of significance of the problem, the bird strike analysis is widely covered in scientific literature.

LITERATURE REVIEW

Event description

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View of fan blades of JT-8D Jet engine after a bird strike.

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 m (19,685 ft) to 9,000 m (29,528 ft) above the ground. Bar-headed geese have been seen flying as high as 10,175 m (33,383 ft) above sea level. An aircraft over the Cote d’Ivoire collided with a Rupp ell’s Vulture at the astonishing altitude of 11,300 m (37,073 ft), the current record avian height. The majority of bird collisions occur near or on 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 900 m (2,953 ft) and 61% occur at less than 30 m (100 ft).

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A hawk stuck in the nosecone of a C-130

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 impact. The energy of the impact increases with the square of the speed difference. Hence a low-speed impact of a small bird on a car windshield causes relatively little damage. High speed impacts, as with jet aircraft, can cause considerable damage and even catastrophic failure to the vehicle. The energy of a 5 kg (11 lb) bird moving at a relative velocity of 275 km/h (171 mph) approximately equals the energy of a 100 kg (220 lb) weight dropped from a height of 15 metres (49 ft). However, according to the FAA only 15% of strikes (ICAO 11%) actually result in damage to the aircraft.

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Inside of a jet engine after a bird strike

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

Remains of the bird, termed snarge, are sent to identification centres where forensic techniques may be used to identify the species involved. These samples need to be taken carefully by trained personnel to ensure proper analysis and reduce the risks of zoo noses.

Sacramento International Airport has had more bird strikes (1,300 collisions between birds and jets between 1990 and 2007, causing an estimated $1.6million in damage) than any other California airport. Sacramento International Airport has the most bird strikes of any airport in the west and sixth among airports in the US, according to the FAA, as it is located along the Pacific Flyway, a major bird migration path.

Species

The animals most frequently involved in bird strikes are large birds with big populations, particularly geese and gulls in the United States. In parts of the US, Canada Geese and migratory Snow Geese populations have risen significantly while feral Canada Geese and Greylag Geese have increased in parts of Europe increasing the risk of these large birds to aircraft. In other parts of the world, large birds of prey such as Gyps vultures and Milvus kites are often involved. In the US reported strikes are divided between waterfowl (32%), gulls (28%), and raptors (17%) (Data from the BSC USA). The Smithsonian Institution’s Feather Identification Laboratory has identified turkey vultures as the most damaging birds, followed by Canada geese and white pelicans, all very large birds. In terms of frequency, the laboratory most commonly finds Mourning Doves and Horned Larks involved in the strike.

The largest numbers of strikes happen during the spring and fall migrations. Bird strikes above 500 feet altitude are about 7 times more common at night than during the day during the bird migration season.

THREE WAYS TO CONTROL THE PPROBLEM

1); 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 (4 lb). The engine does not have to survive the ingestion, just be safely shut down. This is a ‘stand alone’ requirement, i.e., the engine must pass the test, not the aircraft. Multiple strikes (due to 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, as in the January 15, 2009 forced ditching of US Airways Flight 1549..

Modern jet aircraft structures must be able to withstand one 1.8 kg (4 lb) collision; the empennage (tail) must withstand one 3.6 kg (8 lb) bird collision. Cockpit windows on jet aircraft must be able to withstand one 1.8 kg (4 lb) bird collision without yielding or sapling.

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 gelatine, to ease testing. Currently testing is mainly conducted with computer simulation, although final testing usually involves some physical experiments.

Aircraft Forward Lighting can play an important role in enhancing the detect ability of birds to aircraft. Vision is the primary sensory pathway serving the animal in detection of approaching objects (e.g., trees, buildings, other birds, and predators) and adjustment of flight path relative to an object’s approach. In a very basic sense, once a threat is identified, the animal can utilize its high aerodynamic capabilities to avoid collision. Recent experimental findings suggest that birds will use similar strategies in response to aircraft approach

2): Bird management

To reduce bird strikes on takeoff and landing, airports engage in bird management and control. There is no single solution that works for all situations. Birds have been noted for their adaptability and control methods may not remain effective for long.

This includes changes to habitat around the airport to reduce its attractiveness to birds. Vegetation which produces seeds, grasses which are favoured by geese, manmade food, a favourite of gulls, all should be removed from the airport area. Trees and tall structures which serve as roosts at night for flocking birds or perches for raptors should be removed or modified to discourage bird use.

Other approaches try to scare away the birds using frightening devices, for example sounds, lights, pyrotechnics, radio-controlled airplanes, decoy animals/corpses, lasers, dogs etc. Firearms are also occasionally employed.

A successful approach has been the utilization of dogs, particularly Border collies, to scare away birds and wildlife. Another alternative is bird capture and relocation.

Falcons are sometimes used to harass the bird population, as for example on J. F. Kennedy. At Manchester Airport in England the usual type of falcon used for this is a peregrine falcon /lanner falcon hybrid, as its flight range covers the airport.

An airport in New Zealand uses electrified mats to reduce the number of worms that attracted large numbers of sea gulls.

3): Flight path

Pilots have very little training in wildlife avoidance nor is training required by any regulatory agency. However, they should not takeoff or land in the presence of wildlife, 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 3,000 feet as rapidly as possible as most bird strikes occur below 3,000 feet. Additionally pilots should slow their aircraft when confronted with birds. The energy that must be dissipated in the collision is approximately the relative Kinetic energy (Ek) of the bird, defined by the equation E_{k} = frac{1}{2} m v^{2}where m is the mass and v is the relative velocity (the sum of the velocities of the bird and the plane). 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 Aviation Hazard Advisory System uses a Bird Avoidance Model based on data from the Smithsonian Institution, historical patterns of bird strikes and radar tracking of bird activity. This model has been extremely successful. Prior to flight USAF pilots check for bird activity on their proposed low level route or bombing range. If bird activity is forecast to be high, the route is changed to one of lower threat. In the first year this BAM model was required as a pre-flight tool, the USAF Air Combat Command experienced a 70% drop in bird strikes to its mission aircraft.

TNO, a Dutch R&D Institute, has developed the successful ROBIN (Radar Observation of Bird Intensity) for the Royal Netherlands Air force. ROBIN is a near real-time monitoring system for flight movements of birds. ROBIN identifies flocks of birds within the signals of large radar systems. This information is used to give Air Force pilots warning during landing and take-off. Years of observation of bird migration with ROBIN have also provided a better insight into bird migration behaviour, which has had an influence on averting collisions with birds, and therefore on flight safety. Since the implementation of the ROBIN system at the Royal Netherlands Air force the number of collisions between birds and aircraft in the vicinity of military airbases has decreased by more than 50%.

There are no civil aviation counterparts to the above military strategies. Some experimentation with small portable radar units has taken place at some airports. However, no standard has been adopted for radar warning nor has any governmental policy regarding warnings been implemented.

INCIDENTS

The FAA estimates the problem costs US aviation 600 million dollars annually and has resulted in over 200 worldwide deaths since 1988. In the United Kingdom the Central Science Laboratory estimates that, worldwide, the cost of bird strikes to airlines is around US$1.2 billion annually. This cost includes direct repair cost and lost revenue opportunities while the damaged aircraft is out of service. Estimating that 80% of bird strikes are unreported, there were 4,300 bird strikes listed by the US Air Force and 5,900 by US civil aircraft in 2003.

The first reported bird strike was by Orville Wright in 1905, and according to their diaries Orville … flew 4,751 meters in 4 minutes 45 seconds, four complete circles. Twice passed over fence into Beard’s cornfield. Chased flock of birds for two rounds and killed one which fell on top of the upper surface and after a time fell off when swinging a sharp curve.

The first recorded bird strike fatality was reported in 1912 when aero-pioneer Cal Rodgers collided with a gull which became jammed in his aircraft control cables. He crashed at Long Beach, California, was pinned under the wreckage and drowned.

The greatest loss of life directly linked to a bird strike was on Oct.4,1960, when Eastern Airlines Flight 735, a L-188 Electra flying from Boston, flew through a flock of common starlings during take off, damaging all four engines. The plane crashed shortly after take-off into Boston, with 62 fatalities out of 72 passengers. Subsequently, minimum bird ingestion standards for jet engines were developed by the FAA.

On 22 September 1995, a U.S. Air Force E-3 Sentry AWACS aircraft (Callsign Yukla 27, serial number 77-0354), crashed shortly after take off from Elmendorf AFB, AK. The plane lost power to both port side engines after these engines ingested several Canada Geese during takeoff. The aircraft went down in a heavily wooded area about two miles northeast of the runway, killing all 24 crew members on board.

The Space Shuttle-DISCOVERY also hit a bird (a vulture) during the take-off of STS-114 on July 26, 2005, although the collision occurred early during take off and at low speeds, with no obvious damage to the shuttle.

NASA also lost an astronaut, Theodore Freeman, to a bird strike. He was killed when a goose shattered the plexi glass cockpit of his T-38 Talon, resulting in shards being ingested by the engines, leading to a fatal crash.

Aircraft continue to be lost on a routine basis to bird strikes. In the fall of 2006 the USAF lost a twin engine T-38 trainer to a bird strike (ducks) and in the October 2007 the US Navy lost a T-45 jet trainer in a collision with a bird.

In the summer of 2007, Delta Air Lines suffered an incident in Rome, Italy, as one of its Boeing 767 aircraft, on takeoff, ingested yellow legged gulls into both engines. Although the aircraft returned to Rome safely, both engines were damaged and had to be changed. United Air Lines suffered a twin engine bird ingestion by a Boeing 767 on departure from Chicago’s O’Hare Field in the spring of 2007. One engine caught fire and bird remains were found in the other engine.

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On April 29, 2007, a Thomson fly Boeing 757 from Manchester Airport, UK to Lanzarote Airport, Spain suffered a bird strike when at least one bird, supposedly a heron, was ingested by the starboard engine. The plane landed safely back at Manchester Airport a while later. The incident was captured by a plane spotter, as well as the emergency call picked up by a plane spotter’s radio. The video was later published.

On November 10, 2008, a Ryan Air flight FR4102 Boeing 737 from Frankfurt to Rome made an emergency landing at Ciampino after multiple bird strikes put both engines out of commission. After touchdown, the left main landing gear collapsed, and the aircraft briefly veered off the runway before the crew regained control. Passengers and crew were evacuated through the starboard emergency exits. Three passengers and two crew members were injured, none seriously.

On January 4, 2009, a bird strike is suspected in the crash of a PHI S-76 helicopter in Louisiana. While the final report has not been published, early reports point to a bird impacting the windscreen and retarding the throttles, leading to the death of 7 of the 8 persons on board.

On January 15, 2009, US Airways Flight 1549 from LaGuardia Airport to Charlotte/Douglas International Airport ditched into the Hudson River after experiencing a loss of both turbines. It is suspected that the engine failure was caused by running into a flock of geese at an altitude of about 975 m (3,200 feet), shortly after takeoff. All 150 passengers and 5 crew members were safely evacuated after a successful water landing. The NTSB has yet to publish a report on this incident.

On September 18th, 2009, American Eagle Airlines Flight 5183 from Dallas Texas to Lawton Oklahoma, collided with over 100 pigeons during takeoff on runway 31L. The takeoff was aborted and the aircraft sustained minor damage. 34 whole birds were recovered, hundreds of body parts were also recovered. The aircraft returned safely to the gate with no injuries.

DESCRIPTION OF SPECIFIC PROBLEM.

BIRD STRIKE IN AEROENGINE.

VIRGIN EXPRESS BOEING 737-43Q

Bird strike in engine no.1 during take-off. Incredible how much damage a little bird can do to a fan blade. The X on the damaged fan blades are marked and indicating the blades to removed.

The airplane was out of service for 3 days.

ABSTRACT

Bird ingestion is a costly and difficult engine test to perform. The capability to model bird ingestion effects is, therefore, critical to the success of any competitive jet engine program. LS-DYNA has been used in the design and analysis of fan blades for bird strikes.Comparisons of analysis and test results from bird-strikes on fan blades are also presented.

INTRODUCTION

Certification of jet engines requires the demonstration of the ability to ingest birds into an engine and meet federal Aviation Administration regulations for airworthiness. Specifically, the medium bird requirement is to ingest several 1.5 pounds or 2.5 pounds into an engine and maintain specific thrust levels for various time durations. The large bird requirement is to ingest a 6 pounds or 8 pounds bird and demonstrate safe shutdown.

To predict component damage from birds requires both a structural model of the fan blade and a method to generate the distribution of impact loads onto the structure. The finite element method is a desireable approach because of its ability to handle complex geometries, material nonlinearities and the component-projectile dynamic interaction.

APPROACH

Using LS-DYNA the bird and blade are modeled independently and their interaction is defined with an analytical contact algorithm. The bird is modeled as an ellipsoid of solid elements with material properties similar to water. The hydrodynamic or fluid-like behaviour for the bird is required because it undergoes large deflections and segmentation during its interaction with the blade. The blade is modeled with plate elements with a thickness defined at each nodal point. An elastic-plastic-strain-rate dependent material model is also defined for the blade.

FIGURE A: BLADE AND BIRD FINITE ELEMENT MESH

Figure A shows a typical bird and fan blade finite element model.

The analysis is completed in two phases. The first phase is an implicit solution of the blade where the blade root nodes are fixed and a body force load representing a prescribed angular velocity is applied to the blade. This results in the proper blade deformation and stress for the start of the transient analysis. In the second or transient phase of the analysis, the blade root nodes are prescribed to rotate with the angular velocity from the first phase and the entire blade is given the same initial angular velocity. For shrouded blades like that shown in Figure A, the shroud nodes are constrained to move only radially during the implicit solution and they are prescribed to rotate with the blade angular velocity during the transient analysis. These boundary conditions simulate the constraints imposed by the neighbouring blade shrouds. A the start of the transient analysis, the bird model is given an initial velocity based on the aircraft speed. During the transient analysis, the bird and blade interaction is achieved via the defined contact algorithm. The blade nodes are defined in a slave set to contact the master surface blade elements. The transient analysis is then performed in LS-DYNA explicit.

DESIGN

In order to use the analysis in the design of new blades, analytical thresholds for failure were determined. This was achieved by comparing analytical results of tests where failure or cracking of the blade occurred with results from tests where no cracks or failures occurred. For all these analyses, a consistent modelling approach was used, i.e., identical bird material models, blade material models, mesh densities, and contact algorithms. Analysis maximum strains were documented at the blade lead edge and at the shroud hard point(the location on the blade airfoil in front of the shroud) for comparison with test results to determine the blade analytical failure threshold. New blade designs are then analysed with the same consistent modelling approach to assure this maximum strain threshold is not exceeded.

DISCUSSION OF RESULT

Using the above procedures, an analysis of a bird-strike on a fan blade was completed for comparison with test results. In the test, a 2.5 ponds bird with a velocity of 180 knots was shot at a fan blade in-board of the shroud. The fan was 94 inches and the blades had a speed of 3862rpm. The test resulted in a cracked blade at the shroud hard point that is visible in Figure B. The LS-DYNA analysis of the blade resulted in the deformed shape shown in Figure C. The shroud hard point strain in the analysis exceeded the material analytical failure threshold, which is indicative of some type of failure. In addition, the lead edge line strains correlated well with the strains calculated from a grid placed on the blade prior to test.

The LS-DYNA analysis is readily extended beyond a single fan blade to include an entire set or multiple fan blade. Figure D shows a deformed blade set from a 2.5 ponds birdshot. Figure E shows the LS-DYNA deformed shape plots from an analysis of that test event. The total extent of damage including the number of blades damaged in the test correlates well with the analysis results.

CONCLUSIONS

These design and analysis procedures using LS-DYNA for bird strike on fan blades have been documented as part of standard work procedures and they are being used in the design of new fan blades. In addition, efforts are underway to incorporate the threshold strain into the LS-DYNA material model to evaluate propagation.

Figure B. Deformed Blade with crack Figure C. Analysis Deformed Blade

FIGURE.D. DEFORMED BLADE SET FROM TEST

FIGURE.E. DEFORMED BLADE SET FROM ANALYSIS.