Attrition: And Then There Were Two

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July 15, 2014: In early July the U.S. Air Force released the results of an investigation into the loss of a HH-60G (a variant of an older UH-60 model helicopter (used for rescuing downed pilots). This occurred during a February night training exercise in eastern Britain (Salthouse on the Norfolk coast). Two HH-60Gs were travelling over a marsh, one behind the other, with the pilots using night vision equipment to navigate. A flock of geese went by the lead helicopter, which paid them no mind and three of the birds hit the second HH-60G, knocking the two pilots unconscious. One of the birds also hit the front of the helicopter disabling the automatic flight control system. The loss of the pilots and the flight control system caused the low flying HH-60G to crash, killing all four people on board.

It took months to collect the components of the crashed helicopter, reassemble them and sort out exactly what happened and when. It was also discovered that the crews of both helicopter had been told that minimal bird activity was expected over the marsh. This was common for the area, especially during colder months. Local wildlife officials had not reported recent activity by large birds.

British and American investigators concluded that it was a freak accident and there was nothing that could be done to prevent rare occurrences like this happening again. That’s because the odds of a situation in which birds knock both pilots unconscious and simultaneously disable the automatic flight control system are extremely high. The probability of that happening again is very low, as are hundreds of other possible situations that could occur in the future but cannot realistically be coped with in advance.

In reality bird strikes on military helicopters are quite rare but not unknown and rarely fatal. These collisions occur 50-100 times a year. Most of them cause no injuries although there is often damage to the helicopter usually because the bird got sucked into the gas turbine engine and did damage there. Since helicopters are often not moving very fast when hit, this reduces bird strike damage and injuries.

Even so in the past twenty years eight people aboard helicopters were injured because of bird strikes and six were killed. Navy helicopters, naturally, were most often hit by seabirds. Inland, army and air force helicopters sometimes collided with bats, which do little damage because of their small size. Pilots have come to know that bird strikes are a seasonal thing. A quarter of the year (September thru November) accounted for 42 percent of the annual bird strikes while December and February each averaged about five percent of the bird strikes. The HH-60G incident occurred during the latter (low activity) period.

Aircraft bird strikes are a widespread, if little publicized, problem. There are about 5,000 incidents a year, mostly to fixed-wing commercial aircraft. These often just mean replacing windows or canopies, or wherever the bird hit. Most of the incidents involve near misses or collisions with 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.

Nearly all the fatal bird strikes are to aircraft with gas turbine engines. Birds get sucked in and damage the fast moving turbine. This can often result in more damage as the turbine flies apart and no longer provides power. 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. One exception was the "Miracle On The Hudson" in January 2009, when an 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 with no loss to passengers or crew.

There have been two fatal helicopter collisions and the other one occurred in 2011 when a U.S. Marine Corps AH-1 helicopter collided with a red-tailed hawk. While the bird only weighed about 1.4 kg (3 pounds) it hit the top of the main rotor mast on the 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-1s in service were 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 expected a bird strike up there either. But with bird strikes you have to expect the unexpected.

 


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