The module's last hazard has feathers. Birds share exactly the airspace where the aircraft is slowest and lowest, and the system's answer runs through three hands: the aeronautical information services map the risk, the aerodromes manage it, and the commander reports it. This lesson follows that chain, from why a single bird can defeat an airliner to the two reports the rules put on the commander's desk.
Why birds are a hazard
ICAO's Airport Services Manual, Doc 9137, calls the underlying condition a wildlife hazard: the presence of wildlife, birds and other animals, wild and domestic, that could result in damage to aircraft. A strike is a collision between wildlife and an aircraft; a near miss is the potential of one. The damage catalogue is the syllabus's own list: birds wreck probes and sensors, feeding the instruments the bad data the instrumentation course priced; they are ingested by engines, the mechanism that forced an Airbus into an emergency landing on the Hudson River in 2009 after a flock of geese went into both engines; they shatter and craze windscreens and degrade the crew's vision at exactly the heights where seeing matters most; and they dent and penetrate airframes, a 1962 swan strike at 6,000 ft taking off a Viscount's horizontal stabiliser and driving the design criteria that followed.
Two statistics organise the risk. First, the manual's planning geometry rests on the figure that 95 per cent of bird strikes occur below 2,000 ft: the hazard lives on and around the aerodrome, in the take-off, initial climb and approach. Second, strike databases worldwide show a consistent relationship between wildlife mass and the percentage of damage, and strikes involving flocks are more likely to damage the aircraft than strikes with single birds of the same species: the larger the bird and the greater its tendency to be struck in groups, the greater the risk. That is why the geese-and-gulls end of the spectrum, big and gregarious, dominates the accident record.
Where birds gather
Birds go where the food is, and the syllabus names the table: rubbish (garbage) facilities, open sewage treatment works, recently ploughed land, and their natural habitats. Doc 9137 fills in the mechanics. Landfills and garbage dumps are so rich a food source that some species travel tens of kilometres to reach one, gulls, kites and vultures soaring in the thermals over the composting waste, and their commuting lines to and from the dump cross aerodromes and flight paths. Sewage settling ponds pull the same crowds, and the closer the facility sits to the aerodrome the greater the risk, which is why the guidance wants no new ones built nearby. A freshly ploughed or tilled field is a banquet of exposed seeds, invertebrates and small prey. And water is a primary requirement: a water body offers drinking, resting, escape from predators and feeding all at once, with wetlands drawing the water birds whose size and flocking behaviour make them the higher-hazard species.