Things That Go Bump in the Flight

"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.

When the wheel assemblies are horizontal and in the belly (the nose gear retracts forward or backward under the cockpit), they're forced into a set of uplocks with a very solid and startling "thunk" that you'll also feel and hear, and then the gear doors (designed to make the aircraft smooth on the bottom) close with yet another series of sounds ending in a double "thunk," like the closing of two doors on a very large, very heavy car. If the rushing wind sounds you heard on takeoff suddenly stop, that confirms it was the gear doors you just heard closing.

On landing, the situation is reversed, but because you've become used to the background noise of air rushing by and engines doing their job, the sudden sound when the pilots put the gear "down" can really rattle you, even though it's normal, routine -- and, of course, rather necessary.

If you're sitting way up in first class or in the back, you may not hear or feel the gear come out, but if you're over the wings and you're not expecting it and the gear doors open with the "thunk" and the sudden noise of rushing wind below, it can cause you to jump. The hydraulic pressure then releases the uplocks and lets the gear extend until it engages the downlocks. Every one of those steps involve loud "thunks," bumps and sideways motions.

And, of course, there's the landing itself. Sometimes we give you a "grease job," kissing the wheels on with such little downward motion at the moment of contact that you're not even sure we're on the ground. Sometimes, however, the landing is an arrival -- perfectly safe (airliners are designed to handle amazingly hard landings that almost never happen), but a startling "boom" or series of "booms" as the tires thud onto the runway and the big bird settles down onto its shock-absorber struts.

But we have one more kicker for you: Reverse thrust, which rattles the cabin, shakes the engines on their mounts, and adds to the impressive deceleration process as the airplane and you slow from 150 mph (approximately) to a taxi speed of 10 or 15 mph.

Next time we'll talk about turbulence in flight, but I have one postscript: For those of you who've wondered about that barking dog sound on Airbus A-320, A-319 and A-318 aircraft ("Ruff, ruff, ruff") most audible in the mid-section and, frankly, funny -- that's an electrical hydraulic pump in the wheel well area beneath the floor, not an upset pup in the cargo compartment.

John J. Nance, ABC News' aviation analyst, is a veteran 13,000-flight-hour airline captain, a former U.S. Air Force pilot and a lieutenant colonel in the Air Force Reserves. He is also a New York Times best-selling author of 17 books, a licensed attorney, a professional speaker, and a founding board member of the National Patient Safety Foundation. A native Texan, he now lives in Tacoma, Wash.