Bird strike –
a tough test for aircraft engines
JAR regulations guarantee safety as far as possible
Fox-holes, trained birds of prey, firecrackers: All of the preventive measures taken at the airports cannot stop it: aircraft and birds may collide during take-off and landing. This unfortunate incident, known as –bird strike,– can be critical for aircraft parts like the cockpit glazing and especially for the engines and their first compressor rotor, the so-called fan blades. Regulations of the Joint Aviation Requirements (JAR, an agreement signed by all European civil aviation authorities) for the development and the subsequent operational certification of the engines ensure a maximum of passenger safety, even when there is a major bird strike. According to current regulations, safe continuation of the flight must be guaranteed following a single bird strike. However, despite all safety precautions, a multitude of bird strikes caused by a flock of bigger birds can in some seldom cases trigger a failure in one or even more engines.
Testing of the crucial engine regions
Bird strikes and their possible consequences for the engines are tested comprehensively by engine manufacturers. The so-called –large bird ingestion test,– for example, involves shooting a bird body weighing at least 3.65 kg at a speed of about 360 km/h into an engine with a minimum intake area of 3.9 m². Depending on the test procedure and the diameter of an engine's air intake the size and weight of the birds shot into the engine by a gas cannon can vary. During the –medium and small bird ingestion tests,– several birds are aimed simultaneously at critical regions of the fan blades (e.g., blade tips) and at the intake of the core engine. The impact of birds can also be simulated by using gelatine blocks. All test objects are shot into the running engine by a gas cannon with a speed of up to 500 kilometres per hour, the maximum speed assumed in most international regulations. This value is much lower than the usual cruise speed of jet powered aircraft, assuming that most bird strikes happen during takeoff and approach to the airport, where aircraft are much slower.
Even during the flight, instrument readings inform the cockpit crew of any loss of performance by the engines which are a consequence of damage from a bird strike. Such an event will immediately be reported and in some cases the pilots return the aircraft to the airport. After a landing, so-called bird strike checks will be carried out. These inspections are routinely performed by the personnel of Lufthansa Technik. The affected engines are thoroughly examined with a borescope for any internal damage.
Consequences of bird strikes – quickly detected with borescopics
A borescope is the technical application of the endoscope known from medical science. This equipment helps Lufthansa Technik's specialists to gain insight in even the most remote areas of an engine without having to disassemble it. Mounted at the end of a long solid tube or a flexible hose, the borescope's optical array provides sharp images from inside the engine's cavities, for example the compressor stages. If a birdstrike has occurred to the aircraft, remains of the birds can often be found in here, which can help the technicians to determine the severity of the impact. Afterwards, every single blade of the fan and the compressor stages is also inspected with the borescope. The equipment helps to detect cracks or other damage that might have occurred on impact to single engine blades or even the whole compressor rotor. Depending on the type and extent of the damage determined, it may be necessary to replace the engine afterwards. If the inspection has been carried out only on suspicion of a bird strike and no damage is found, the specialists of Lufthansa Technik can release the engine into service again. In this case the borescopics equipment saves a lot of money because it helps to avoid an unnecessary disassembly.
Lufthansa Technik's years of experience show that the consequences of bird strikes mostly have economical effects to increased repair costs from the replacement and repair of damaged components.