No American soldiers were killed in Sunday’s raid on Osama Bin Laden’s Abbottabad compound, but a helicopter sustained damage after a hard landing and the SEALs blew it up to keep the technology secret. This high-profile chopper failure is hardly unique. During the Iran hostage crisis, troops abandoned a rescue mission because three of the eight Sea Stallion transport copters proved defective. Why do so many helicopters seem to fail?

Because chopper aviation is highly complex. Setting aside the dangers of flying through a war zone, operating a helicopter means navigating in multiple directions—straight up and down, as well as backward, forward, and to the sides. Helicopters fly at lower altitudes than airplanes, which means that pilots must often maneuver in tight spaces and have to contend with obstructions such as phone lines, trees, and buildings. Generally, they also need to improvise landings, whereas fixed-wing crafts often travel by predetermined routes and use paved runways. In 2005, the U.K.-based Civil Aviation Authority published a set of industry accident reports for helicopters. “Struck power cables in a low cloud,” read one. “Entered fog and crashed into woodland,” said another. Operational errors caused by weather conditions and high-risk flight environments are by far the leading causes of chopper mishaps, especially as maintenance technologies continue to improve.

If there is mechanical trouble, though, the engine is one of the more common culprits. Helicopters may come equipped with single-turbine engines, twin-turbine engines, or reciprocating (piston-driven) engines; the general failure rate for choppers with piston engines is 7.04 per 100,000 flying hours, as opposed to between 2 and 2.3 for the other two types. This may be because intricate reciprocating mechanisms are heavier and have more moving parts that can break down.

Rarer mechanical problems include electrical misfires, which can result in erratic navigation readings, and errors in the flight control system. (A malfunctioning hydraulic pump, for instance, might leave the pilot unable to operate the “cyclic” or the “collective,” two steering instruments that allow the helicopter to change direction.) Rotor blades also need regular inspections: They sometimes crack due to corrosion.

Of course no mode of transportation is immune from the occasional freak accident: One copter transporting workers to an oil rig in the North Sea was struck by lightning in 1995, and another chopper crashed when a bird entered the cockpit and got jammed in the pilot’s anti-torque pedals.

Given the complexities of helicopter aviation, chopper safety statistics aren’t as dire as you might expect. According to the National Transportation Safety Board and the Federal Aviation Administration, there were 140 civil helicopter accidents in 2008, out of a total of 3,813,000 hours flown. * This puts the rotorcraft accident rate at 3.67 per 100,000 hours flown. Flying a helicopter is actually much safer than flying a noncommercial plane: The 2008 accident rate for “general” aviation (all civil flights except for scheduled passenger and cargo airlines) was 6.86.

Still, the accident rate for commercial airlines in 2008 was lower—around .147 per 100,000 hours flown. Furthermore, between 2001 and 2005, helicopter fatality statistics went essentially flat, while those for other types of aircraft declined. In 2005, the International Helicopter Safety Team, a group of aviation community stakeholders, made it their mission to reduce the chopper accident rate at least 80 percent by 2016. How? By spotlighting “human factors” (like decision-making) and by training pilots to monitor flight data in real time.

That human focus might not have averted the conspicuous military accidents mentioned above. The Abbottabad helicopter was designed to evade Pakistani radar, and experts speculate that it was weighed down by its additional stealth features. A retired special operations aviator told the Army Times that the rotors may have been unable to produce enough lift to counterbalance both the chopper’s heavy frame and the downward push of the whipped-up air, especially with the high walls of the compound preventing air from exiting the rotor. During Operation Eagle Claw, the ill-fated Iran hostage rescue mission of 1980, one of the elite RH-53D choppers was grounded by a dust storm, another had a cracked rotor blade, and yet another suffered from a broken hydraulics system. A fourth unlucky copter did succumb to human error, though. Performing a routine but complicated maneuver at Desert One, the pilot misjudged his distance from a transport aircraft and collided into it.

Got a question about today’s news? Ask the Explainer.

Explainer thanks James Crouse and Stan Rose, director of safety at the Helicopter Association International.



Correction: Due to an error in the original research material, the original figure in this sentence was 381,300. (Return to corrected sentence.)