Downed helicopter in Abbottabad: What causes chopper accidents?

Answers to your questions about the news.
May 6 2011 5:07 PM

Black Hawk Down

Why do helicopters seem to crash all the time?

1_123125_123073_2279751_2292779_110506_explainer_heli_tn
A Black Hawk helicopter

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?

Katy Waldman Katy Waldman

Katy Waldman is a Slate staff writer. 

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.

Advertisement

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.)

TODAY IN SLATE

Foreigners

More Than Scottish Pride

Scotland’s referendum isn’t about nationalism. It’s about a system that failed, and a new generation looking to take a chance on itself. 

iOS 8 Comes Out Today. Do Not Put It on Your iPhone 4S.

Why Greenland’s “Dark Snow” Should Worry You

Three Talented Actresses in Three Terrible New Shows

The Human Need to Find Connections in Everything

It’s the source of creativity and delusions. It can harm us more than it helps us.

Jurisprudence

Happy Constitution Day!

Too bad it’s almost certainly unconstitutional.

The Ungodly Horror of Having a Bug Crawl Into Your Ear and Scratch Away at Your Eardrum

My Father Was James Brown. I Watched Him Beat My Mother. Then I Married Someone Like Him.

  News & Politics
Weigel
Sept. 17 2014 12:02 PM Here It Is: The Flimsiest Campaign Attack Ad of 2014, Which Won’t Stop Running
  Business
Business Insider
Sept. 17 2014 1:36 PM Nate Silver Versus Princeton Professor: Who Has the Right Models?
  Life
Outward
Sept. 17 2014 1:59 PM Ask a Homo: Secret Ally Codes 
  Double X
The XX Factor
Sept. 17 2014 1:26 PM Hey CBS, Rihanna Is Exactly Who I Want to See on My TV Before NFL Games
  Slate Plus
Slate Fare
Sept. 17 2014 9:37 AM Is Slate Too Liberal?  A members-only open thread.
  Arts
Brow Beat
Sept. 17 2014 1:01 PM A Rare, Very Unusual Interview With Michael Jackson, Animated
  Technology
Future Tense
Sept. 17 2014 12:35 PM IOS 8 Comes Out Today. Do Not Put It on Your iPhone 4S.
  Health & Science
Bad Astronomy
Sept. 17 2014 11:18 AM A Bridge Across the Sky
  Sports
Sports Nut
Sept. 15 2014 9:05 PM Giving Up on Goodell How the NFL lost the trust of its most loyal reporters.