San Andreas, the latest entry into the catalog of absurd natural disaster movies, opens in theaters today. Dwayne “The Rock” Johnson plays Ray, a search-and-rescue helicopter pilot who’s airborne when Los Angeles freeway overpasses start collapsing underneath him. He immediately goes AWOL to save his family, and heads north to his daughter in San Francisco.
We won’t give away the (entirely predictable yet 100% entertaining) storyline, except that there’s one scene in particular that piqued our interest. In flight, the helicopter engine finally succumbs to the massive debris damage it sustained getting out of the Southland. Ray calmly tells his passenger:
“The engine died. I’m going to autorotate down.”
“…We’re going to crash.”
The scene that follows is wild, as Ray tilts his chopper up to keep the blades spinning, then plunges down dramatically, then up again, undulating his way down to a conveniently placed department store parking lot in Nowhereville, Central California.
So just what is autorotation?
If Ray had been flying an airplane when his engine quit, he would have had enough lift under the wings to stay airborne and glide to a landing, although he would have had to point the nose down, sacrificing altitude for speed, and land as quickly as practical.
Helicopters, of course, use blades spinning atop a pole to generate lift. The spinning replaces the need for forward motion, which is why a helicopter can hover while the blades clatter furiously above. In normal flight, pilots simply tilt forward, and the blades pull them that way.
When a helicopter engine quits, lift will disappear as soon as the inertia drains from the blade rotation. The solution is autorotation. The pilot gets airflow through the rotor by descending quickly, angling the rotor blades so that the wind turns them. By the time the helicopter gets close enough to the ground, the rotor is spinning much faster than it normally would. Only then, while the rotor is spinning fast enough, can the pilot change the blade’s angle to generate lift again, slowing the helicopter into what’s known as a flare. Flare too low, and the helicopter plummets into the ground. Too high and the blades could stop spinning fast enough to generate lift while still in the air, and the helicopter lands hard. Do it just right and the helicopter levels off and uses the lift to settle gently on the ground below. (Ray runs into some outside trouble in the parking lot, but we’re confident The Rock would have set his chopper down like a feather.)
Here’s a pro from the California Department of Forestry and Fire Protection practicing autorotation; it doesn’t look like much, until he flares at the end.
Read about more autorotation and other dead stick landings here.
You probably won’t be shocked to hear that the maneuver is much—much—less dramatic in real life than it is in San Andreas. It may look—and feel—scary as hell, but no helicopter pilot gets behind the stick solo without mastering it.
San Andreas is playing at the National Air and Space Museum’s Udvar-Hazy Center IMAX theater.