Chapter 2 — Wildlife-strike Prevention: The System Safety Approach

Using a comprehensive process that focuses on the accident to be prevented.


The following script is not from a Hollywood work of fiction—it’s real, and presents some of what happened in June 1993. As the early morning sun warmed the air, making it feel heavy after a recent rainfall, a Canadian Airlines B-737 moved along the taxiway at Calgary International Airport in preparation for takeoff. We pick up the conversation between the pilots:

“ We’ve got a full load this morning: 110 passengers and 12,400 lbs of fuel.”

“ Yeah, this flight’s always a full one. Let’s see if we can save a few minutes and get runway 28 for takeoff.”

A moment later:

“ ATC says there’ll be no problem with getting runway 28.”

“ Great. Have you noticed the number of gulls around these days?”

“ Yeah, look at all those over there on Foxtrot.”

With the before-takeoff checks completed and takeoff clearance received, the pilot turned on the landing lights and moved the thrust levers forward. The aircraft accelerated down the runway, a beautiful view of the Rocky Mountain foothills filling the windscreen.

“ V1.”

“ Roger.”

“ Rotate.”

The aircraft lifted and then seemed to pause nose-high before the crew felt the main wheels lift from the pavement. Then the normal routine was suddenly shattered… by gulls.

The pilot-in-command later described flying into a “sea of white.” He remembered loud noises, strange smells, a muffled curse, and then—as suddenly as the sky had gone white—there was clear blue sky ahead. But the aircraft was sinking; it had taken numerous serious strikes.

Both pilots watched the needle on the vertical speed indicator slowly climb. Once a positive-climb rate was established, they raised the landing gear. Then it was time to assess their situation. The left engine was surging and torching flames from its exhaust. At 400 ft AGL, they eased the left thrust lever back. The airspeed stabilized in the climb. They slowly accelerated, raised the flaps and felt the drag reduce. They banked the aircraft and started a slow, climbing turn to return to the airport. They leveled-off near 2000 ft AGL but there were increasing vibrations on the right engine—the engine that was keeping them in the air. There was no choice but to ease back on its power lever and track downwind to the landing runway.

Once around, the pilot lowered the nose and turned the aircraft to line up with the runway. The crew could see the crash trucks moving into position. Moments later, it was over—they had landed safely. The pilots cleared the runway and shut down the left engine before taxiing to the ramp.

In hindsight, it could be argued that there was a greater than normal probability of a near-catastrophe occurring that day. But it’s a fair bet that the pilots of Canadian Airlines Flight 661 didn’t know that. They probably didn’t know there was a major gull-breeding area northeast of the airport, and that the gulls flew daily to feed at landfill sites to the southeast and northwest. Nor could they have known that the unusually wet summer had led to a number of changes in the airport wildlifemanagement program. The pilots didn’t know that during the two hours preceding their takeoff, the airport duty manager and ATS providers had detected several large flocks of gulls in the vicinity of runways 28 and 34. The ATIS (Automatic Terminal Information Service) recording heard by the crew of Canadian Airlines Flight 661 did not include the bird-activity warning that had been on the ATIS just an hour before their departure. They would not have known that ATS providers and airport operational staff were often hampered in detecting stationary and low-flying birds because of tall grass and a lack of colour contrast in the airport’s undulating terrain. And almost certainly the crew did not know their aircraft was at greater risk of a bird strike because they were the morning’s first departure from runway 28, a runway where birds often settled. The application of a System Safety Approach might have ensured the crew of Flight 661 knew all these things.

The System Safety Approach

The only way to prevent wildlife strikes is through the careful application of a System Safety Approach—an approach that systematically and proactively involves all stakeholders.

System safety is outcome-based: to prevent an accident, the approach is used to identify all of the complex, interwoven events that can lead to an accident. Within the system, specific responsibilities are distributed among various stakeholders— responsibilities that are closely linked. As long as all stakeholders fulfill their roles, the system remains intact, and safety is ensured.

From its origins in aerospace engineering after the Second World War, system safety has been adopted by numerous and diverse industries—accepted as a best-business practice and an assurance of demonstrated due diligence in industries where failure can lead to catastrophic losses.

Wildlife strikes: a dynamic risk-management challenge

The risk of a bird or mammal strike is greatest when an aircraft is operating on the ground or in the lower altitudes, where wildlife hazards are most prevalent. The risks

Damage to an MD-11
January 2001 in Portland, Oregon. Damage to an MD-11 following rejected takeoff due to bird strike. The aircraft struck a gull at V1 (167 kts). Other damage included a detached cowl and destruction of one engine.

are most serious when an aircraft is taking-off and climbing, as was the case in the Calgary occurrence. According to data from Boeing and the U.S. NTSB (National Transportation Safety Board), 50 percent of all high-speed RTOs (rejected takeoffs at speeds in excess of 120 kts) are caused by bird strikes.

When an aircraft operates at its maximum operating weight, carrying the many tons of fuel it will burn on the flight, it is close to the edge of the certified performance envelope. A bird strike on the takeoff roll just before or at V1 forces the flight crew to make a split-second decision to reject or continue the takeoff. If the decision is to reject the takeoff, the brakes, wheels and tires will be fully tested as the crew carries out swift actions to bring the aircraft to a halt before the end of the runway. If the decision is to continue the takeoff, the crew’s skills will be challenged to get airborne and climb in an aircraft that has suffered an undetermined degree of engine and airframe damage. Even when airborne, they will be operating a heavy aircraft at critical speeds, where options for maneuvering may be limited by the presence of obstructions and proximity of terrain. From a management perspective, risk is extremely high.

Applying the risk-management formula

Introduction to risk management

As Figure 2.1 indicates, there are three components associated with wildlife-strike risk management:

  1. reducing the overall exposure to wildlife hazards,
  2. reducing the probability of striking wildlife, and
  3. reducing the severity of a wildlife strike.

When all three components are managed effectively, system safety is optimized.

Reduce exposure

The key to successful risk management—and the aim of strategic risk-management activities—is to reduce exposure to hazards whenever possible. Simply operating at altitudes where most birds do not normally fly often reduces exposure to most bird species. But some aircraft can’t operate at high altitude, and all must takeoff and land. As they do, particularly over water or near urban areas, exposure to wildlife-strike hazards generally increases. The challenge is to reduce an aircraft’s exposure when operating on the ground and in lower altitudes.

Birds and mammals will always be found where their physical needs are best met. If their food sources are limited and nesting locations scarce, birds and mammals will seek more hospitable habitats. For this reason, municipal authorities play an important role in reducing exposure to wildlife-strike hazards. These authorities generally influence the location and nature of landfill sites and other waste-disposal

Risk-management Formula
Figure 2.1 – The Risk-management Formula

facilities, influencing the activities of many hazardous bird species. By the same token, local commercial interests must also contribute to a solution; agricultural practices and food-service outlets, for example, may attract birds that otherwise would not inhabit the area. Finally, the manner in which airport operators manage habitats on airfields is critical in determining which birds and mammals will be found on or near airport lands.

Reduce probability

No matter how successful municipal authorities, local business leaders and airport operators are in discouraging hazardous wildlife species from frequenting lands near an airport, some will inevitably feed or loaf there. And so the task is to reduce the probability of strikes, which can be achieved through detection, deterrence and avoidance. These activities are tactical, and supplement previously described strategic efforts that are aimed at exposure reduction.

Airport staff, ATS providers and pilots all play an important role in the timely detection and reporting of wildlife activity in proximity to airports. Airport staff regularly and systematically patrol airport land. ATS providers scan the airport area with binoculars to locate signs of wildlife activity.

Once birds and mammals are detected, wildlife-management personnel are dispatched to directly intervene and initiate some form of active management. Other deterrent methods are more passive and may include electronic distress signals, propane cannons and chemical deterrents.

  Exposure Probability Severity
Airports X X X
Air Traffic Service Providers   X  
Airlines   X X
Engine Manufacturers     X
Aircraft Manufacturers     X
Pilots X X X
Regulatory Bodies X X X

Table 2.1 – Summary of Responsibility for Risk Management of Bird Strikes

Not surprisingly, pilots play a significant role in reducing the probability of an occurrence. Through careful observation, pilots can initiate early action to avoid collisions with wildlife. By communicating observed locations, types and numbers of birds and mammals to ATS providers, they reduce the probability of strikes to other aircraft.

By operating with landing lights illuminated, pilots provide birds and mammals with increased opportunity to see and avoid the aircraft. When operating conditions permit, pilots can plan their arrival and departure routes to avoid large concentrations of birds.

The success of these tactics relies on timely and accurate communication, a critical aspect of the system safety approach. Birds and mammals detected by pilots, airport staff and ATS providers remain a threat until the information is passed to those who can initiate some management action to prevent a strike.

Reduce severity

Although exposure- and probability-reduction efforts are bound to produce dramatic results, wildlife-strikes will inevitably occur. With that in mind, the risk-management formula’s third component is reducing the severity of strike damage.

Aircraft-engine manufacturers are now developing power plants that will better withstand the impact of one or more birds. Aircraft manufacturers are producing some windscreens and other parts to deflect birds, or to absorb the energy of their impact. Airline-training programs hone pilots’ skills to ensure wildlife strikes are managed confidently and competently. Pilots employ numerous defences when preparing themselves for the unexpected, including:

  • availing themselves of updated information on local wildlife activity,
  • heightening their awareness during high-risk flight profiles,
  • remaining proficient in emergency procedures,

James Reason’s Accident Trajectory
Figure 2.2
 James Reason’s Accident Trajectory (Reason 1997)

  • applying heat to windscreens to make the surface more pliable in the event of a strike, and
  • protecting themselves from impact debris through the use of aircraft visors and, in the case of helicopter or military pilots, by wearing helmets with visors extended.

Case Study: Amending regulations to limit airspeed

In the past, the development of air regulations has been reactive; most aviationindustry rules have been drafted in response to accidents. However, Notice of Proposed Amendment (NPA) 2002-022 to Canadian Aviation Regulation 602.32 marked a turning point for Transport Canada. The amendment was among the department’s first attempts to apply data-driven risk-management procedures to develop air-safety regulations proactively.

Presented on February 26, 2002, the NPA proposed to eliminate the ATC practice of allowing aircraft departing from Canadian airports to exceed 250 kias (knots indicated air speed) below 10,000 feet MSL. The NPA responded to data that demonstrated populations of large flocking-bird species—which tend to migrate at relatively high altitudes—are increasing in Canada, as are damaging bird-strike events at higher altitudes.

To provide additional data in support of the amendment, TC contracted a risk analysis of the ATC practice. The resulting study, Risk Analysis of High Speed Aircraft Departures Below 10,000 Feet, was the world’s first holistic, system-safety examination of risks associated with high-speed departures.

The TC study applied a rigorous data-driven, risk-analysis process and determined that high-speed departures are not safe, and that NPA 2002–022 was appropriate and effective risk mitigation. Furthermore, the study demonstrated that any organization that would endorse high-speed air operations below 10,000 feet MSL, which would subject airframes and engines to conditions that exceed certification standards, would face potential liability.

The TC Canadian Aviation Regulatory Committee reviewed all data, concluded that there was a requirement to manage risks associated with collisions between large flocking birds and aircraft operating at the altitudes in question, and decided to enact NPA 2002-022 (see Appendix D).


The management of bird-strike risk is multi-tiered. As Table 2.1 illustrates, airport authorities are on the leading edge of the risk curve, poised to minimize the presence of birds and take measures to keep them away from aircraft. Airlines and aircraft manufacturers reside mainly at the other end of the risk curve, minimizing the effects once a bird or mammal strike occurs. Air-traffic service providers are positioned near the centre of the curve, detecting and communicating so that others can reduce the risk of a wildlife strike. Finally, pilots—who along with other crew members and passengers stand to benefit or lose the most—take actions that affect all three components of the risk-management curve.

Breaches in the defences

Risk-management specialists refer to aviation as a tightly coupled industry operating in a high-consequence environment. This implies that a change in risk-management procedures by one stakeholder can dramatically alter the effectiveness of riskmanagement initiatives by others. The bottom line is that, as a defence against wildlife strikes, risk management is a finely balanced activity involving many stakeholders in the aviation community. As experience proves, it’s a balance that can be easily tipped.

In James Reason’s famous Swiss-cheese model, reproduced in Figure 2.2 (Reason 1997), defences are depicted as walls separating the source of danger in the background from an accident in the foreground. Breaches of these defences are presented as holes in the walls. The model illustrates the porous nature of well established defences; even those employing the latest technological advances are not impervious. The reasons for most breaches can be understood by examining how people and organizations function in tightly coupled operations. As the source of danger we’re concerned with is wildlife strikes, the walls can be thought of as defences designed to minimize exposure to wildlife, and to reduce the probability and severity of a wildlife strike.

Imagine a situation in which an airport operator changes long-standing wildlifemanagement practices, or diverts funds to another part of the operation. Altering the risk-management framework could create holes in the defences of others, such as those critical to airlines operating from the airport. These new holes might align with other previously harmless airport practices. An immediate increase in wildlife strikes might be the result.

To illustrate the delicate interweaving of wildlife-strike lines of defence, let’s examine a few examples which, while fictitious, could just as easily be fact. As you read, note how everyday policy and business decisions can affect the exposure, probability and severity of wildlife risks felt elsewhere. Note also that in many cases other stakeholders could effectively address the risks if they were aware of the change—and of the need to do things differently.

Cases of increased exposure

  • An aircraft operator with routes concentrated over land introduces several new routes along the coast. Because of high-traffic densities, the short-flight segments are operated at low altitudes. The result is increased exposure to soaring-bird species that pose a high risk to aircraft. Though predictable, this risk might remain unidentified in the rush to introduce the new service.
  • The completion of a new runway aligned 90 degrees to existing parallel runways results in the regular departure of trans-oceanic aircraft over wetlands far removed from previous aircraft operations. The result is increased exposure of passenger-carrying aircraft—operating near the performance envelope—to large flocks of waterfowl.
  • A new in-flight catering business opens near the airport. The operation’s garbage attracts gulls from sites several kilometers away. The result is increased exposure to flocking birds.

Cases of increased probability

  • Landing lights for a specific aircraft type are in short supply, so an airline temporarily rescinds its longstanding policy that requires illuminated landing lights on aircraft operating below 10,000 feet. The airline saves money, so the change in policy is extended to all aircraft types in the company’s fleet. The result is increased probability of wildlife strikes to all aircraft types operated by the company.
  • At an inland airport, unusually heavy rain dilutes chemicals used at the airport to kill worms that attract hazardous bird species. The probability of bird strikes will increase until new and effective spray schedules are drafted and implemented.
  • An advanced-technology engine is introduced. Its intake is significantly larger and the engine is quieter than those of previous generations. Both the increased size and reduced noise combine to increase the probability of bird strikes against aircraft equipped with these engines.

Cases of increased severity

  • As a cost saving measure, a helicopter company no longer requires its pilots to use helmets and visors. Consequently, the use of this equipment gradually erodes. Pilots are therefore at greater risk of injury in the event of a bird strike. Passengers and crew are also at heightened risk, as a pilot may not be able to safely land an aircraft.
  • Higher airspeeds are permitted while flying below 10,000 feet, despite power plants and airframes designed to withstand only bird strikes occurring at lower speeds. More severe damage could result from the increased energy of bird strikes.


  • Change is an essential part of aviation. The system-safety challenge is to identify safety consequences before changes are introduced—consequences that can affect all stakeholders in the safety-management formula. The best-engineered system will not prevent an accident if decisions and actions unwittingly undermine risk-management efforts. Information must be shared to ensure that defences remain strong.
  • Local, national and international bird-strike committees are central to riskmanagement efforts in the aviation industry, particularly in sharing information, knowledge and experience.
  • System safety demands the exchange of accurate, relevant data regarding aircraft operations, airport operations, bird movements and bird strikes, so that riskmanagement strategies can be continuously improved.

The best-laid plans of airlines, pilots, airport operators, aircraft manufacturers, investors, scientists and policy makers can easily go awry because of a few wayward birds. This is most likely to happen when aviation-industry activities are uncoordinated, and fail to target the effective management of bird-strike risks.

The Calgary occurrence in June 1993 was not just a case of bad luck. It illustrated— with near-fatal consequences—how easily a well-defended operation can be breached. In a number of other cases, the end results were much more tragic.  

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