Attrition: Hail, Birds And Other Flying Hazards


July 14, 2013: On April 23rd a hailstorm in Afghanistan, at Kandahar airport, caused over a million dollars in damage to eighty helicopters. The large hail, up to 25mm (or an inch) in diameter and weighing up to 100 gr (several ounces), damaged rotors, electronics, and windows. The hail came down at over 200 kilometers an hour. Such hail storms are common in areas away from oceans and often do major damage to crops and domestic animals (as well as any people who cannot take shelter). Some hailstones weigh nearly a kilogram (2.2 pounds) and can kill people and cattle and do serious damage to aircraft and vehicles on the ground. 

Such hailstorms are also one of the rare, but regular, hazards of aircraft in flight. Hail will melt as it gets closer to the ground but for an aircraft several kilometers up, the hail is “fresh” and capable of doing notable (although very rarely fatal) damage to an aircraft. Every year aircraft, especially in North America, suffer millions of dollars in damage from hail. This is because interior North America is one of the main hailstorm producing areas on the planet and also has a very high concentration of air traffic.

Hail is not the only natural hazard in the air. A more common problem is bird strikes. In rare cases these can even bring down a helicopter. For example, two years ago a U.S. Marine Corps collided with a red-tailed hawk, weighing about 1.4 kg (3 pounds). The bird hit the top of the main rotor mast on a marine AH-1W helicopter gunship. The hawk impact damaged the pitch change link, which caused vibrations that quickly led to the transmission and rotor blades breaking away from the helicopter. The chopper then fell to earth, killing the two man crew. The AH-1 has since been modified to better protect the pitch change link, one of several highly vulnerable (to damage) components on a helicopter. Normally, the pitch change link would not be hit by ground fire. No one thought the risk of a bird strike up there was worth doing anything about.

Aircraft bird strikes are a widespread, if little publicized, problem. There are about 5,000 incidents a year. These often just mean replacing windows or canopies, or wherever the bird hit. Most of the incidents involve near misses or collisions on non-critical portions of the aircraft. But in about one percent of the incidents the damage is severe and some aircraft are lost. On average, 40-50 people a year die because of aircraft bird strikes. This is far more than are lost to hail (in the air or on the ground).

Nearly all the fatal bird strikes are to aircraft with gas turbine engines (which birds fly into). This often wrecks, or severely damages, the engine when the high speed fan is damaged. Multiple engine aircraft usually can survive this if they still have one or more working engines. But sometimes single or two engine aircraft lose all engine power and go down with heavy loss. One exception was the "Miracle On The Hudson" in January 2009, when Airbus 320 over New York City lost both engines to bird strikes. Exceptional work by the crew managed to bring the aircraft down, intact, on the Hudson River. Hail caused a similar (although not as life-threatening) incident in 2006, where a B-727 jet was climbing after takeoff from Calgary (in central Canada) when it ran into a massive hail storm that did extensive (but not disabling) damage to the aircraft exterior. With most of the lights out and the cockpit windows obscured by cracks caused by numerous fast moving hail stones, the aircraft turned around and landed safely.

Lightning used to be a common hazard, causing a fatal crash every 3-4 years. Most heavily used aircraft are still hit by lightning at least once a year. But in the last three decades aircraft designers and manufacturers have made aircraft much more resistant to fatal side effects of lightning strikes and there have been few fatal accidents from lightning. Birds and hail have proved more difficult to deal with. 




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