Comments about technological history, system fractures, and human resilience from James R. Chiles, the author of Inviting Disaster: Lessons from the Edge of Technology (HarperBusiness 2001; paperback 2002) and The God Machine: From Boomerangs to Black Hawks, the Story of the Helicopter (Random House, 2007, paperback 2008)

Saturday, February 25, 2012

Collisions and Copters

Thinking of a tragic incident in Arizona, earlier this week – the collision of two USMC helicopters at Marine Corps Air Station Yuma. One was an AH-1W Super Cobra and the other a "Yankee" model UH-1 Huey. Both were twin-engined. There were no survivors. This AP photo shows the ground-impact zone:
It may be surprising that people inside a helicopter ever survive a midair collision at altitude, considering the machine's dependence on a large and complex main rotor for lift. 

Even an impact limited to the tail can send the helicopter out of control; if the impact shears off the tail boom, the helicopter instantly goes into a dive (because the center of gravity shifts forward) and also into a spin (because of the uncompensated torque from the main rotor.)

Readers may recall reading news about a Feb. 19 collision near Antioch, California, involving a Beechcraft Bonanza single-engine fixed-wing airplane and a Robinson R22 two-seat helicopter. The Beechcraft made an emergency landing 20 miles away; the Robinson crash-landed. No one died in that incident and injuries were minor. Here's the Robbie, which was flown by a remarkably level-headed pilot: (photo, AP/Contra Costa Times)
Rarely do things turn out so well after mid-air collisions, which constitute on average between one and two percent of all aircraft crashes. 

When two USMC helicopters, a CH-46 and a Cobra, collided during maneuvers at Camp Lejeune, NC, in 1996, it killed 14 Marines and left a pilot and co-pilot injured. One issue: whether the night vision goggles in use were suitable.

Survivability depends a good deal on what the crash does to the main rotor systems. The pilot of the Robinson R22 collision was spared a free-fall from altitude because the landing skids took the principal impact of the Beechcraft, rather than the main or tail rotor. 

In the case of two AS-350 AStar news-copters that collided over Phoenix in 2007 while covering a car chase, the main rotors overlapped as one helicopter came up behind the other. The cameramen and pilot-reporters all died. Here's a computer animation of that crash, from two angles. One lesson: pilots shouldn't be TV reporters too. Both pilots were watching a new development in the pursuit, off to their right-hand side. The camera operators were concentrating on the monitors.

There are very few cases in which two helicopters collided in flight and people survived in both aircraft. That's because at least one of the helicopters is likely to sustain catastrophic failure of the main rotor. 

Following are top-of-the-head observations about safety and crashworthiness in helicopters:

Head on a swivel: Pilots' expression for a constant visual sweep, inside and outside the cockpit. Most midair collisions were avoidable. They happened in the daytime, in VFR conditions, when the see-and-avoid principle was in effect.

Power management: Pilots must know how much power reserve is available; otherwise the helicopter may begin settling, or turn out of control.

Nothing loose that's going to fall out: It's not unusual when operating a light helicopter to remove the doors before flight. What a view! That's how I trained in Minnesota when researching The God Machine. But my instructor made sure that nothing was loose to fly out the door, where Murphy's Law would send it into the tail rotor. 

Helmet and flight suit: One for impact, the other for fire. Many people have died after a main rotor blade crashed into the passenger cabin during a hard landing. A helmet can make a difference. When I rode in the front seat of a Cobra at Whiteman Air Force Base in Missouri for a magazine article, the commander made it clear we weren't going anywhere until I had put on a borrowed helmet, gloves, and a flame-resistant flight suit. 

He also showed me the location of the compact hammer I'd need to get out if it crashed, explaining that the Nomex wouldn't hold back flames for long. Here's what the front seat looks like:
Energy-absorbing seats and landing gear: Such machinery aims to absorb most of the energy from the crash, before it can injure people on board. Here's a paper on crash seats for crop-spraying pilots.

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