Spiral looks birds in the eye
The US Airways that ended up in the Hudson River has stimulated general discussion on the hazard presented by bird collisions in air transport.
The probability of colliding with a bird during a flight is actually quite high. In commercial aviation around 2,500 bird collisions are reported each year – and in Finnair, around fifty. Of these, most are rated as “single bird – no damage” incidents. The bigger, harder material usually triumphs.
Only around one tenth of bird collisions cause some kind of damage to the aircraft. Typical types of damage are small indentations on the leading edge of the aircraft wings, a damaged windscreen wiper or cracks in the radar dome on the nose of the aircraft. Landing gear wiring may also damage, because collisions happen at low altitude, close to the airport where the landing gear is already down. Ninety per cent of bird collisions happen below 700 metres.
Due to its large frontal surface area and the suction of the jet turbine, the engine is the most common impact location, generally accounting for more than half of the collisions. Smaller birds generally don’t cause major damage, because the centrifugal force and high by-pass ratio of the engine prevent small birds for reaching the core of the engine.
In the case of larger birds, the front part of the engine has been designed to withstand the impact of even a four-kilo bird or a flock of seven one-kilo birds at a typical climb speed.
In connection with engine certification, this is checked by firing so-called standard chickens, i.e. solid masses resembling meat, into engines running in a test rig. After impact, the engine must be able to generate thrust enough for a period that will allow the aircraft to return to the departure airport.
As a special feature introduced to avoid bird collisions, the centre of the engine’s intake opening is painted with a white spiral pattern, which helps the birds to see it more easily. The aircraft’s other structures are also designed to withstand bird collisions so that flight safety is not jeopardised.
For example, the several centimetres thick windscreen will withstand a collision with a two-kilo bird at cruising speeds. The tubes that provide information to the flight instruments, moreover, are situated on separate sides of the fuselage, which means that a bird cannot block all of the sensors.
To prevent bird collisions, Finnair uses landing lights below 3 kilometres altitude whenever possible. On seeing the lights from afar, birds know to stay well clear. A fast moving aircraft, on the other hand, seldom has time enough to react and avoid birds, but the, relatively, low air speeds at low altitudes significantly reduce the damaging effect of collisions. When air speed is reduced by a half, the force of an impact is declines by 75 per cent.
Bird hazards are also encountered on the ground. At airports and in their direct proximity, bird populations are controlled by keeping grass – which provides hiding, feeding and nesting places for birds – short. Marshlands, which attract water birds, are kept dry. Landfill sites are located far enough to reduce the possibility of bird impacts caused by gulls.
In addition, various grids and spiked mats are installed on airport buildings and structures to prevent birds from roosting. The environment is therefore not attractive to birds, but should they happen to end up in this hostile environment, they are scared off by loud bird management bangs, generated with small gas cannons.
Statistically it was highly improbable for the US Airways aircraft to collide with so many large birds. According to the investigations, the incident involved a flock of Canada geese, which can weigh more than six kilos.
Multiple hits by heavy birds damaged both engines. Without thrust the aircraft became a glider. At the time of the incident, the aircraft was flying at a speed of 370 km/h at an altitude of one kilometre. The 10 kilometre glide to the landing place took three minutes.
During this time, the crew had time to consider and discuss with air traffic control about a landing at Teterboro Airport, tried to restart the engines, warned the cabin, and read the emergency checklist. At the same time, the pilots flew manually, which requires, among other things, constant monitoring of air speed. This work was disrupted by many audio and text warnings as well as fumes arising from the shredded bird remains in the air-conditioning system.
In addition, the pilots had to be aware of the buildings and bridges that lay ahead. After the aircraft stopped on the river, the cabin crew carried out a model evacuation. The rescue operation was also a great success.
Even though many safety features planned for bird collisions fell short of expectations, the passengers and crew survived the incident due to experienced, skilled professionals. In the aviation industry, something new is always learned from events like this, and safety systems will be developed further.
Filed under: Safety | Tagged: bird collision, engine
