On 18 October 1999, a Boeing 767-300 encountered a flock of wood pigeons, at 450 feet agl after take off from London Gatwick, and the ingestion of one caused sufficient distress to the left engine for it to be shut down and an air turn back made; it was subsequently concluded that the degree of damage caused was inconsistent with the applicable requirements of engine certification.
The following is an extract from the official AAIB report:
"The aircraft had just taken off and climbed to about 450 feet agl when there was a 'bang' and the No 2 engine ran down. This was accompanied by severe vibration and a number of passengers saw flames coming from the right engine at that time. The pilots, who had both seen a flock of birds disappear under the aircraft just before the incident, believed they had suffered a birdstrike and shutdown the No 2 engine. The commander elected to return immediately and subsequently made an uneventful overweight landing back at Gatwick.
Subsequent Inspection of the No 2 engine found that one of the fan blades had fractured and had released the outer third (9 inches) of the blade which had become embedded in the fan duct acoustic lining, just ahead of the outlet guide vanes. Bird remains were evident in the engine and the fan blades adjacent to the one which had fractured exhibited typical soft body impact damage, with minor curling of their tips. The fan tip-path abradable lining had been heavily gouged and a segment, extending over some 20° of arc, was missing. There were several holes and tears in the acoustic lining. There was no evidence that any damaging debris had not been contained by the nacelle.
Metallurgical examination of the fractured blade parts revealed no evidence of a pre-existing crack, nor of any other material defect which might have rendered the blade particularly vulnerable to the bird impact. In order to meet the relevant Airworthiness Certification Requirements before entry into service, this engine type had to demonstrate its ability to withstand ingestion of a volley of eight medium sized birds (1.5 lbs each) at take off power with an acceptably small reduction in thrust, the minimum acceptable residual thrust being 75%. During the associated Certification test on this engine type, the engine absorbed the required volley of birds satisfactorily and therefore passed this test. Indeed, the test engine was apparently then used for further Certification tests without significant rectification work having being required. Although the pigeon which was struck may have been marginally above the 1.5 lbs weight for the 'medium bird' specified in Requirements, it would not have been expected to have had the capability of causing a fan blade to fracture. In service experience of ingestion of similar sized and considerably larger birds has shown that this engine type has continued to operate normally for the remainder of the flights affected. Previous ingestions of considerably larger single birds have been sustained by engines of this type and they have subsequently continued to operate almost normally. The RB 211 engine type has therefore generally been regarded as being more robust than required with respect to bird ingestion."