Things That Go Bump in the Flight

April 26, 2005 — -- "What the heck was that?"

Almost everyone has had that phrase crackle through their heads at one time or another during a commercial airline flight. Sometimes it's caused by a strange noise that you can't recall hearing before, and sometimes it's an unexpected lurch or motion. The problem is that neither the seatback safety cards nor the flight attendant briefings explain such things, and you're left to wonder if you're the only one on the airplane who heard (or felt) "it," and worse, whether there's a problem.

The truth is that while we've managed to make airline flying closer to 100 percent safe than virtually any other form of transportation, it still involves racing down a runway that isn't perfectly level and lifting off on the shoulder of air molecules that aren't perfectly smooth in a very large and complex machine full of subsystems that use hydraulics, pneumatics, electrical motors and incredibly powerful jet engines -- all of which produce additional noises and motions that are transferred to the aluminum tube in which you're riding.

Yet it's all normal.

It would take a book to explain all the noises, but over the next few months I'll tell you in detail about a number of them, starting this time with the heaviest structure we move around radically on a big airliner in flight: the landing gear -- basically the wheels that support the aircraft on the ground.

Big airplanes can fly fast because they use hydraulic pistons to fold their wheels into the belly after takeoff to greatly reduce what we call drag, the same force you feel when you put your hand out the window of a moving car. But the process is noisy and bumpy, starting with the muffled bump you hear as the airplane lifts off the runway and the heavy wheel assemblies -- which are mounted on compressible "struts" that act like shock absorbers -- extend to their limits when the weight of the plane is lifted off of them.

When the pilot flying up in the cockpit sees on his "rate of climb" instrument that the aircraft is safely away from the ground and moving up, he orders "Positive rate, gear up." The other pilot lifts the gear handle and pressurized hydraulic fluid surges into the gear lines to first unlock the mechanical devices physically holding the landing gear "struts" in the extended position. That makes a noise. Then large hydraulic pistons begin pulling the heavy gear sideways and in (on most big jets) causing the unsettling sideways lurch you may feel if you're sitting over or close to the wings.