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Aircraft Certification for Bird Strike Risk
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Although the great majority of reported bird strikes have little or no effect on continued safe flight, a small number of encounters, usually with flocks of birds and especially flocks of large birds, can damage aircraft or their engines so badly that they cannot continue to fly.
Current aircraft certification standards therefore include requirements to demonstrate both airframe and engine resistance to bird impact. The standards which apply are those in place at the time of introduction of a new aircraft type or engine. Experience of Accident and Incidents has led to progressively tougher requirements although, as with most certification standards, grandfather rights are applied so that new requirements are not retrospectively applied to in-service aircraft and engines. The Standards established by both the FAA and EASA are essentially similar but are not yet fully harmonised. However, new aircraft and engine types have to meet both so the more demanding of each applies in each instance. Assurance that certification standards have been met is achieved by various means including ground testing using dead birds, of specified weights and quantities, at representative impact speeds.
Bird Impact Forces
For any given impact, the most important determinant of damage potential is the speed of impact. This is because the kinetic energy, which has to be absorbed by the airframe or within the engine, is the product of mass and the square of the speed. Clearly, the speed of the aircraft, rather than that of the bird, makes up nearly all of the closing speed of impact so that, except for very small aircraft, aircraft speeds are directly proportional to the damage potential for collision with a particular object. Civil aircraft speeds are generally at their lowest where most birds are found - near the ground - but increase progressively with altitude until the bird hazard disappears at somewhere above FL200. What has been convincingly demonstrated from incident data analysis is that, although the number of recorded bird impacts reduces rapidly with altitude, the greater the altitude, the greater the proportion of bird strikes which produce major damage.
Apart from speed, a number of factors have been identified as influencing the damage a bird impact can cause. These are all considered during the design of both aircraft and engines in an attempt to understand the robustness of structures and engines to bird impact from first principles as well as to prepare to meet certification standards. They include, with the most common simplifying assumptions shown in parenthesis:
- Bird weight
- Bird density
- Bird rigidity (deformation by 50% of its shape)
- Angle of impact (90 degrees)
- Impact surface shape (flat)
- Impact surface rigidity (no deformity)
It is also important to understand that the kinetic energy which is absorbed by an airframe during an impact is 'converted' into an effective force on that airframe based upon the distance over which the impact is 'delivered'. This notional distance is the product of the various simplifying assumptions listed above. The only additional assumption required to calculate impact force is that mass = weight.
Structural damage is, therefore, proportional to impact force rather than the quantity of kinetic energy absorbed. The forces are large. A 6.8kg14.991 lbs
0.0068 tonnes goose impacting an aircraft doing 200 kts370.4 km/h
102.8 m/s can be assumed to exert a force of over 16 tonnes. The effect of proportionality from the square of the airspeed is illustrated by the fact that the same 6.8kg14.991 lbs
0.0068 tonnes bird hitting an aircraft doing 250 kts463 km/h
128.5 m/s can be assumed to exert a force of nearly 26 tonnes and hitting an aircraft doing 280 kts518.56 km/h
143.92 m/s one of over 32 tonnes. Whilst these figures are approximations, their order of magnitude and their concentration over a very small area means that there is little prospect of 'hardening' any engine or airframe to completely resist such a force and certification standards tend to address the containment of the effects of bird impacts.
Engine Certification Standards
Current standards, for both multiple and single bird engine ingestions into a single fixed wing aircraft engine, exist in equivalent form in 14 CFR Part 33-77 and in EASA Airworthiness Code CS-E 800 ’Bird Strike and Ingestion’. The basic requirements for engine ingestion were revised in 2000 to take account of both evidence of an increase in the size of birds impacting aircraft and issues raised by the development of very large inlet, high by pass ratio, engines. The requirements, to be demonstrated by testing, are, in outline, now as follows:
- That at a typical initial climb speed and take off thrust, ingestion of a single bird of maximum weight between 1.8kg3.968 lbs
0.0018 tonnes and 3.65kg8.047 lbs
0.00365 tonnes dependent upon engine inlet area shall not cause an engine to catch fire, suffer uncontained failure or become impossible to shut down and shall enable at least 50% thrust to be obtained for at least 14 minutes after ingestion. These requirements to be met with no thrust lever movement on an affected engine until at least 15 seconds have elapsed post impact.
- That at a typical initial climb speed and take off thrust, ingestion of a single bird of maximum weight 1.35kg2.976 lbs
0.00135 tonnes shall not cause a sustained thrust or power loss of more than 25%, shall not require engine shut down within 5 minutes and shall not result in hazardous engine condition.
- That at a typical initial climb speed and take off thrust, simultaneous ingestion of up to 7 medium sized birds of various sizes between weight 0.35kg0.772 lbs
3.5e-4 tonnes and weight 1.15kg2.535 lbs
0.00115 tonnes, with the number and size depending upon the engine inlet area, shall not cause the engine to suddenly and completely fail and it shall continue to deliver usable but slowly decreasing minimum thrust over a period of 20 minutes after ingestion. [Engines with inlet sizes of less than 0.2 m2 (300 square inches) only have to meet the standard for a single bird of this weight]
- That at a typical initial climb speed and take off thrust, simultaneous ingestion of up to 16 small sized birds of weight 0.85kg1.874 lbs
8.5e-4 tonnes, with the number dependent upon the engine inlet area, shall not cause the engine to suddenly and completely fail and it shall continue to deliver usable but slowly decreasing minimum thrust over a period of 20 minutes after ingestion. [Direct testing to this standard may not be required if the medium bird multiple standard is demonstrated or if this bird size can pass the inlet guide vanes into the rotor blades]
Airframe Certification Standards
Current standards for the impact of a single bird with a large aircraft airframe exist in both 14 CFR Part 25-571 and in EASA CS-25.631 as design requirements for which means of compliance are provided. This is that an aeroplane must be capable of continued safe flight and landing after hitting a 1.8 kg3.968 lbs
0.0018 tonnes bird at the more critical of:
- Vc (cruise speed) at mean sea level or
- 85% of Vc at 8000 feet altitude.
The FAA (only) has an additional requirement under 14 CFR Part 25-631 that an aeroplane must be capable of continued safe flight and a subsequent normal landing after the empennage structure has been impacted by an 3.6 kg7.937 lbs
0.0036 tonnes bird at cruise speed (Vc) at mean sea level.
In addition, both EASA CS-25 and 14 CFR Part 25 require that:
- Windshield integrity after single bird impact requires that the inner ply must be non-splintering and the panes directly in front of the pilots must withstand, without penetration, a 1.8 kg3.968 lbs
0.0018 tonnes bird at cruise speed at mean sea level
- Pitot Tubes must be far enough apart to preclude damage from a single bird impact
Under EASA CS-23.775 and 14 CFR Part 23.775, smaller aircraft are required only to have limited windshield integrity - a demonstrated single bird impact resistance of up to 0.91 kg2.006 lbs
9.1e-4 tonnes at maximum approach flap speed and at least one pane with sufficient forward vision remaining to allow continued safe flight.
Under 14 CFR Part 29-631, Helicopters are required only to have a structure which will ensure that continued safe flight and landing is possible after impact with a single bird of up to 1 kg2.205 lbs
1.0e-3 tonnes weight at the lesser of Vne and Vh at 8000 feet above mean sea level.
A number of concerns have been quite widely voiced about the contribution of certification to the mitigation of the risk of hazardous bird strikes:
- The case of bird ingestion into more than one engine at the same time is not addressed directly and it is clearly extremely difficult to meaningfully estimate the probability of such an occurrence. However, it has been observed that, since some of the current standards only require that a damaged engine can be safely shut down, this circumstance should be more fully considered when determining the acceptable outcome of ingestion into single engines, especially for the twin engine case.
- It has been noted that the potential effects of bird strikes on modern electronic flight control systems and flight deck instrument displays have not yet been fully assessed.
Both EASA and the FAA have indicated that these matters remain under review at varying levels of priority.
Concerns have also been expressed in the past about the risk of injury and damage to persons and objects on the ground from falling engine nacelle debris consequent upon the engine vibration which often follows a major bird strike. After nacelle debris fell from an aircraft departing London Heathrow in 1997, this subject was discussed and a related Safety Recommendation was made. See B741, London Heathrow UK, 1997 (BS)
The other issue, which has been raised in relation to certification to protect against unacceptable outcomes of aircraft bird impact, is the apparent absence of a co-ordinated approach from the standpoints of aircraft certification, aircraft operational matters (like speed and vertical profile) and the management of bird prevalence (the dichotomy between the strongest risk management options within airport perimeters and arguably the greatest risks to aircraft safety beyond the airport perimeter).