Bird strike at Heathrow
A Boeing 747-136 operating a scheduled passenger service from Heathrow to New York (JFK) had normal engine start procedure for all four engines and the initial part of the take-off run was normal. At a speed between V1and VR, the commander saw a large bird ahead of the aircraft flying from right to left. He called "Rotate" at the correct speed, at which point the bird was on the left side of the aircraft, but it then appeared to veer back and upward towards the aircraft. All three crew members felt a pronounced "thump" at about 100ft agl, at which time the speed was estimated to be 165KIAS. Almost immediately, the exhaust temperature for the No. 2 engine rose rapidly, exceeding the maximum limit, and the crew noted that the other engine parameters for the No. 2 engine also indicated damage to the engine. None of the other engines showed any unusual indications and the FO encountered no significant handling problems in controlling the aircraft. The crew of another aircraft which had been cleared for take-off later, saw debris fall from the inboard left engine of the incident aircraft and informed ATC whilst electing to remain at the holding point.
The flight data recorder (FDR) and cockpit voice recorder (CVR) were removed from the aircraft and replayed by the AAIB at Farnborough. The optical disk from the quick access recorder (QAR), which recorded a larger selection of engine parameters than the FDR, was also removed and the data recovered by the operator. The CVR, being of 30 minutes duration, had recorded over the period of the incident take-off but covered the subsequent landing and taxi to the stand. The recorded data added detail to the accurate description of events provided by the operating crew.
Aircraft and engine examination
Initial examination by the AAIB after the aircraft had returned to the stand showed that the left inner (No. 2) engine had suffered severe damage to the fan;two adjacent fan blades had lost substantial portions of their outer length and all the blades had some hard object damage. It was also observed that the complete intake assembly, fan cowls, jet pipe and exhaust cone had separated from the powerplant assembly; these components, together with fragments of fan blade and some feathered bird remains were retrieved from the western end of Runway 27R (Plate 7). The feathers were sent to the Birdstrike Avoidance Unit of the Central Science laboratory, where they were identified as having come from a Grey Heron.
Plate 7 Disposition of the engine and nacelle debris at the western end of runway 27R
The current engine certification requirements specify the tolerance to the ingestion of small (85g), medium (680g) and large (1.8kg) birds which the engine must demonstrate. Typical Grey Herons fall into the weight range of 1.3kg to 1.6kg and as such are regarded as large birds. The Pratt & Whitney JT9D engine's performance in this incident indicated that the basic engine would probably have met current requirements for this type of engine. There are also airworthiness requirements which specify that the loads generated by the damage resulting from a large bird strike will not cause failure of the engine,mountings and separation of the engine from the aircraft; these were also met. There do not, however, appear to be any requirements that nacelle parts should remain attached to the engine during a violent event such as this incident.
More detailed examinations of the engine were conducted at Heathrow and at the engine overhaul facility used by the operator. These revealed that the outer half of the first of the two fan blades and the outer two-thirds of the second had separated. Blades either side of these had suffered leading edge tearing and the relatively large radius bending typical of bird-strike damage. The blade stubs remaining in the fan disc were removed and their remaining moment weights established. This showed that the moment weight imbalance resulting from the initial outer blade separations had been 5 per cent greater than the loss of one entire blade, including its root end fitting. All the fan blades had suffered hard object damage, mostly confined to the outer third and tip zones, and the abradable liner of the fan tip path had completely eroded away. Examination of the fan stream straightener vanes showed that the birdstrike had occurred just below the horizontal diameter on the inboard (right) side of the engine. There was evidence in both the intake and fan stream ducts that separated fragments of fan blade had spiralled round their outer peripheries. In the fan stream duct the fragments had torn and distorted the acoustic lining before damaging the straightener vanes. In the intake duct, abrasion paths spiralled forward from the fan plane and there was evidence that two of the larger fragments had penetrated the structure and emerged through the intake outer skin. In addition to the fan damage, hard object damage was found on blades of all compressor stages and the outer lining of the gas path was eroded through the core engine. Both the high and low pressure turbines had suffered tip rubbing. Six of the eight bolts attaching the main gearbox to the diffuser case had sheared, only the two aft bolts on the right side remaining intact. The overall damage to the engine was sufficient to render it beyond economic repair.
Attachment of powerplant components
Examination of the nacelle components which had separated showed that all of the bolts attaching the intake assembly to the fan case front flange had failed due to overload, as had those attaching the jet pipe and exhaust cone. All assemblies had sustained damage as a result of striking the ground but there was additional damage on both the jet pipe and exhaust cone which indicated that contact of the cone on the jet pipe may have caused the separation of the latter from the engine.
During the time that the Boeing 747 has been in service there have been several instances of major nacelle components separating after birdstrikes. Of these, most have been tail cones although at least two involved the separaration of intake ducts as well. Since both these intake separations were the result of the imbalance caused by the loss of most of two adjacent, or nearly adjacent,fan blades, it would appear that this should be the criterion to which the attachment of nacelle components to the engine should be designed.
Whilst, in this incident involving ingestion of a single bird, only one engine was damaged and the aircraft made a successful recovery to Heathrow, the investigation has highlighted not only the potential secondary damage which could arise from large nacelle parts falling from damaged engines onto busy areas adjacent to Heathrow, but also the increasing hazard of multiple birdstrikes on aircraft taking off or landing at the airport.
Safety recommendations
Whilst the primary cause of this incident was the ingestion by the right engine of a single "large" bird which had approached the western area of runway 27R undetected, it is apparent that there is a growing population of large birds adjacent to Heathrow which poses an increasing hazard to aircraft. It is evident that the scale of this hazard needs to be quantified and improved measures devised to control the bird population in the short term and to further protect public transport aircraft using the airport. An effective automated airport system to detect large birds about to transit across the airport could enable the pilots of departing or arriving aircraft to be alerted to potential multiple birdstrike conflicts. Such a future development could reduce the risk of a related large scale accident in the congested vicinity of the airport as a result of associated multiple engine damage, and also reduce the commercial costs of damage arising from lesser birdstrike incidents. As a result of such concerns and considerations the following safety recommendations are made:
Recommendation 98-58. In view of the apparent incidence of large bird formation (e.g. Canada geese) transit flying over London Heathrow Airport, with the attendant increasing risk of multiple birdstrike occurrence involving departing or arriving public transport aircraft, it is recommended that Heathrow Airport Limited and the CAA should set up a working group, in conjunction with airport operators, to review available technology to determine if a radar-based large flock detection system, or an alternative automated system could more effectively alert pilots and ATC to potential multiple birdstrike encounters.
Recommendation 98-59. In order to reduce the risk of multiple large birdstrike encounters, involving bird formations over-flying London Heathrow Airport conflicting with departing or arriving public transport aircraft,Heathrow Airport Limited should seek maximum cooperation with the relevant local authority bodies and associated land owners to expedite effective management of the associated large bird habitat and population around Heathrow Airport. Similar co-operative initiatives should be actively promoted by the CAA around other affected major airports in the UK.
Recommendation 99-18. The CAA should expand the remit of its sponsored current study by the Central Science Laboratory Birdstrike Avoidance Team of the habitat, population and transit behaviour of flocking large bird species around Heathrow Airport to include the formulation of recommendations on the best means of managing and reducing the associated hazard of multiple birdstrike encounters involving departing or arriving public transport aircraft.
In addition, since the detachment of large heavy engine nacelle parts from aircraft in flight poses a serious hazard to populated areas being overflown,the following safety recommendations are made:
Recommendation 98-60. In order to reduce the likelihood of large intake assemblies suffering in-flight detachment from Pratt & Whitney JT9 powerplants on Boeing 747 aircraft as a result of bird-ingestion damage-induced vibration effects, the aircraft manufacturer should review the reduction in the number of intake attachment bolts (from 74 to 37) which was introduced by Service Bulletin 747-71-2065 to ease intake interchange. (Whilst service experience has shown that this reduced number of attachment bolts clearly has been sufficient for normal operating conditions it is demonstrably not reliably capable of retaining the intake assembly against the forces resulting from the damage resultant from a collision with a large bird. As a result of this incident, the operator has, in agreement with the manufacturer, increased the number of intake attachment bolts and fitted increased strength bolts to attach the exhaust cone to the engine in order to reduce the incidence of cone- and subsequent jet-pipe-separation.)
Recommendation 98-61. The current engine certification requirements,FAR 33.77 and JAR-E 800, should be amended to require that large nacelle components remain attached to the engine up to the limit of forces on the engine and nacelle which result in the detachment of the entire nacelle from its pylon or wing attachments.
Terry Ford
Further reading
AAIB Bulletin 9/99.
