Table of Contents
5.2.2 Bird Strikes
The term “bird strike” refers to the process of a bird entering an aircraft engine, including the impact of the bird against inlet and engine components.
Damages to engines caused by bird strikes can vary greatly (Fig. "Typical bird strike damage"). Usually, the compressor blading is affected. The allowable amount of damage caused by bird strikes should be specified in the certification and rating regulations for each engine type. Flocks of smaller birds up to about 80 grams must generally be tolerated by the engine without any unallowable damage or changes to operating behavior. The blading must not be permanently deformed or suffer impact nicks that would reduce the dynamic strength considerably. Even if larger birds are ingested, at the very least it should be guaranteed that a controlled shutdown of the engine is possible. The engine will usually have to create a minimum amount of power over several minutes after the bird strike. During this period, it must be ensured that no consequential damages such as blade failures occur due to dynamic fatigue, etc. If blades are seriously deformed, vibrations will most likely occur due to stalls and flutter (Fig. "Consequential damage", Ref. 126.96.36.199-16). In case of a blade failure, under no circumstances may fragments escape from the engine or put unallowably high stress on the engine suspension or nacelle. Blade failures with extreme imbalances can cause titanium fires in the high-pressure compressor, causing serious and extensive damage in the core engine resulting in engine failure within a few seconds.
Heavy bird strikes will often result in damage to the nose cone and inlet housing struts, depending on the design of the engine. Powerful axial forces caused by the stiffness of the struck parts and/or large imbalances and overload the main bearings and make them fail.
Bird strikes are one of the most common causes of engine damage. Especially military aircraft that operate at relatively low altitudes are prone to bird strike damage. This can be exacerbated if flight paths are in the area of migratory bird routes. Bird strikes are one of the leading causes of single-jet fighter aircraft crashes.
An important parameter for the stressing of the engine components by the ingested bird(s) is the flight speed, which is usually equal to the speed at which the bird strikes the engine, since the bird`s airspeed is negligible compared with the speed of the aircraft.
The sensitivity of engines to bird strikes also depends on a large number of constructive factors inside the engine. These include not only the location of the engine on the aircraft, but also the engine materials and the design of engine components.
Certification of engines for civilian and military applications includes verifications (through testing and design analysis) of acceptable behavior in case of bird strikes (see Chapter 5.1.1). This verification is generally conducted by using an air-pressure cannon to fire actual birds into the engine at the required speed, while the engine is running at a specified performance level. The engine must then show a specified operating behavior depending on the size and number of the birds. For example, the performance may only decrease a certain percentage, or in extreme cases with large birds engine failure must be controllable and any engine part fragments must be contained.
The procedures used in these bird strike tests was developed over a long period of time. It was discovered, for example, that birds cannot be simulated by plastic bags filled with water of equal weight, since the air pockets inside birds and their unique shape are less damaging. For this reason, during the engine development stage a special substitute mass is used that has sufficient air pockets to realistically simulate the shock waves created in the foreign object and engine component by the impact.
These tests are very elaborate and a determining step of the certification process that must often be repeated after constructive changes. In this manner, the use of fiber-reinforced synthetic materials in large fan blades failed due to the poor bird strike behavior of this material combined with the design standards at the time.