Consultation expired with comments. The Proposed Special Condition has been updated based on the comments received and the corresponding Comment-Response Document (CRD) has been completed by EASA.
Details
Identification of issue
CS-E 800(a) states the objective that the Engine will respond in a safe manner following specified encounters with birds, as part of compliance with CS-E 540.
CS-E 800(c), for “large flocking bird”, states the bird speed and bird mass, and the acceptance criteria of CS-E800(c)(2) applies, i.e., the ingestion at 90% sea level static Rated Take-off Thrust must not cause more than a sustained 50% thrust lost, as well as not to cause the engine to be shut down during the test, or cause a Hazardous Engine Effect.
CS-E 800(d)(1), for “medium and small birds”, states that for a medium bird mass, as per Table A, the critical ingestion parameters that affect power loss and damage must be determined by analysis or component test or both, and they must include but are not limited to the effects of the bird speed, the critical target location and the first stage rotor speed.
As well, it states that when two or more birds are specified for the test, the largest must be aimed at the Engine core primary flow path and a second bird must be aimed at the most critical exposed location on the first stage rotor blades.
CS-E 800(d)(3), for “Medium and small birds”, requests for a medium bird mass, as per Table A, additional integrity assessment, done by appropriate tests or analysis or both, for the case of birds fired at the most critical locations of the first stage rotor.
In both cases for “Medium and small birds”, the acceptance criteria of CS-E 800(d)(2) applies, i.e., the ingestion must not cause more than a sustained 25% thrust lost, as well as not to cause the engine to be shut down during the test.
The associated acceptable means of compliance, as defined in AMC E 800, provides certain guidance for the definition of critical impact parameters, and examples for the effects of bird mass, bird velocity, fan speed, impact location or fan blade geometry.
Two known field events of medium bird ingestions and one event of large flocking bird ingestion have caused the fracture of the fan blade near the root, followed by an IFSD, and in two events an under-cowl fire. In both cases the birds are assumed by the Applicant to have been presented to the engines in a 90-degree (yaw) orientation to the engine centre line, claimed by the Applicant to be a more adverse orientation for fan blade fracture near the root when compared to an orientation aligned axially with the centre line of the engine, as historically interpreted from CS-E 800.
The Certification Specifications CS-E 800(c), for “Large Flocking Bird”, and CS-E 800(d), for “Medium and small birds”, do not explicitly mention bird orientation. It has been commonly accepted that the rule refers to a benchmark test with axial orientation of the bird to the engine centreline.
It is assumed that the rate of bird strikes with a yaw orientation is significantly lower than the rate of bird strikes with axial orientation, and considering that a yaw bird orientation results in a more severe impact, it might be acceptable that this impact results in higher thrust loss than the ones specified by CS-E 800(c) and (d).
However, the compliance with this benchmark test is intended to provide sufficient robustness to cover the natural random variability of birds encountered in service, therefore increasingly severe conditions related to Engine operating conditions, bird speed, bird mass and orientation would be expected to cause a gradual degradation of the engine behaviour and not an immediate IFSD, including the fracture of the fan blade near the root.
