Cockpit Confidential (3 page)

And there is. Your arm flies—heck, even a brick can be made to fly if you stick enough air under it—but it's not particularly
good
at it. The wings of a jetliner need to be very, very good at it. Wings achieve optimum economy during cruise flight. That occurs at high altitudes and just shy of the sound barrier for most jets. But they also need to be efficient at low altitudes and speeds. Getting all of this right is grist for the engineers and their wind tunnels. The lateral cross section of a wing, around which the air does its thing, is called an airfoil, and it's meticulously sculpted. Not only in profile, but also spanwise—the shape and thickness changing from front to back, and from root to tip, in accordance with the type of aerodynamic calculations neither you nor I could fully understand.

Wings are augmented with an array of supplemental components—namely flaps, slats, and spoilers. Flaps trail backward and downward, enhancing the airfoil's curvature for safe, stable flight at lower speeds. (Airliners take off and land with flaps extended, though exact settings will vary.) There are inboard and outboard subsets of flaps, which themselves can be segmented horizontally. Slats roll forward from a wing's forward edge and perform a similar function. Spoilers are rectangular planks that spring from the wing's upper surface. A raised spoiler interrupts airflow across the wing, destroying lift while adding copious amounts of drag. In flight they are used to increase rates of descent; on touchdown they assist in deceleration.

I remember one of my first times on an airplane, in a window seat on a 727, just behind the wing, and how the entire wing seemed to disassemble itself during descent. Big, triple-slotted flaps came barreling down, the spoilers fluttering and waving, the slats dropping into position. Magically, almost, you could see right
through
the very center of the wing, like through the bones of some skeletonized animal, with houses and trees appearing where the sections had slid apart.

You've probably noticed that a jetliner's wings are angled rearward. When a wing cuts through the sky, molecules of air accelerate across its camber. As this acceleration reaches the speed of sound, a shock wave builds along the surface, potentially killing lift. Sweeping the wings backward induces a more agreeable, spanwise flow. On faster planes you'll find a sweep greater than 40 degrees; the slowest have almost no sweep at all. Angling the wings upward from the root, meanwhile, counteracts a lateral rolling and swerving tendency known as yaw. This tilt, most easily seen from a nose-on perspective, is called dihedral. The Soviets, ever the good contrarians, used to apply an opposite version called anhedral, canting their wings downward.

The wing is everything. A plane is built around its wings the way a car is built around a chassis or a bicycle around a frame. Great big wings produce great big amounts of lift—enough to get a max weight 747, at nearly a million pounds, off the ground once it hits about 170 knots.

In his essay “A Supposedly Fun Thing I'll Never Do Again,” David Foster Wallace is on a cruise ship, where he's repeatedly perplexed by mention of “knots,” unable to figure out what they are. I figure he was bluffing. Wallace was a math whiz, and the answer is easy enough: a knot, used both at sea and in the air, is a mile per hour. Except it's a nautical mile, not a statute one. Nautical miles are slightly longer (6,082 feet versus 5,280). Thus a hundred knots is slightly faster than a hundred miles per hour. Origins of the word itself go back to when lengths of knotted rope were tossed from a ship to figure distances. A nautical mile represents 1/60 of a degree of longitude along the equator. With 60 miles to each degree, we compute 360 degrees and 21,600 nautical miles of equatorial Earthly circumference.

When a bird needs to maneuver, it does so by twisting its wings and tail, something pioneer aviators emulated by incorporating wing-bending in early aircraft. But airplanes today are made from aluminum and high-strength composites, not wood, fabric, or feathers. Operated hydraulically, electrically, and/or manually via cables, various moveable contrivances are fitted that help us climb, descend, and turn.

Atop the rear fuselage is the tail, or vertical stabilizer, which functions exactly as its presence suggests—by keeping the plane straight. Hinged to the tail's back edge is the rudder. The rudder complements but does not control turns; its function is chiefly one of stability, tempering a plane's side-to-side swerve, or yaw. Some rudders are divided into sections that move together or separately, depending on airspeed. Pilots move the rudder by means of foot pedals, though an apparatus known as a yaw damper does most of the work automatically.

Beneath the tail, or occasionally attached to it, are two small wings. These are the horizontal stabilizers, the moveable rear portions of which are called elevators. The elevators command a plane's nose-up/nose-down pitch, as directed by the forward or aft motion of the pilot's control column, or joystick.

Ailerons, located at the trailing edges of the wings, are responsible for turns. Pilots steer via the control wheel or stick, which directs the ailerons up or down. They are interconnected and apply opposing forces: when the aileron on the left goes up, the one on the right goes down. A raised aileron reduces lift on that side, dropping the respective wing, while a lowered one causes the reverse. The smallest twitch of an aileron provides a good deal of turn, so you won't always spot them moving. It might look as though a plane is banked without anything having budged, but in fact the ailerons have done their thing, if ever so slightly. Large planes have two ailerons per wing, inboard and outboard, working in pairs or independently, depending on speed. Ailerons are often linked to the aforementioned spoilers, which partially deploy to aid turning.

So as you can see, even a simple maneuver might require a whole choreography of moving parts. But before you picture a hapless pilot kicking his feet and grasping madly for levers, keep in mind that individual pieces are interconnected. A single input to the steering wheel or column will cause any combo of movements outside.

Adding to the confusion, rudders, elevators, and ailerons are equipped with smaller tabs that operate independently from the main surfaces. These “trim” tabs fine-tune the motions of pitch, roll, and yaw.

If you're still with me, and before committing this all to memory, you'll be thrilled to know there are idiosyncratic variants of almost everything just described. One plane I flew had spoilers used only after landing, others that assisted with turning, and others still for inflight deceleration. Certain Boeing models are equipped not only with conventional trailing-edge wing flaps, but also ones at the leading edge, as well as slats. The Concorde had no horizontal stabilizers, so it had no elevators. But it did have “elevons.” We'll save those, along with “flaperons,” for another time.

At a wing's tip, the higher pressure beneath the wing meets the lower pressure above it, sending out a turbulent discharge of air. Winglets, as they're affectionately called, help smooth this mixing, decreasing drag and, in turn, improving range and efficiency. Because planes have different aerodynamic fingerprints, winglets aren't always necessary or cost-effective. For instance, the 747-400 and A340 have them, while the 777 does not, even though it too is a long-range widebody. Because fuel economy wasn't always the priority that it is today, and because the advantages of winglets weren't fully understood until fairly recently, older models were designed without them. For these planes—a list that includes the 757 and 767—they are available as an option or retrofit. An airline considers whether the long-term fuel savings is worth the cost of installation, which can run millions per plane. It depends on the flying. In Japan, Boeing has sold a number of 747s, used specifically on high-capacity, short-range domestic routes, with winglets
removed
. Winglets provide minimal efficiency gain on shorter flights, and removing them means the plane is lighter and easier to maintain.

Aesthetics are a personal thing. I find winglets attractive on some jets, like the A340, and awkward on others, like the 767. You see them in different forms. Some are large and jaunty, while others are just a tweak. With a “blended winglet,” the wing tapers gradually with no harsh angles. Planes like the 787 and Airbus A350 use a more integrated style, sometimes referred to as a “raked wingtip.”

They're just coverings—streamlining devices called fairings. While they help prevent the formation of high-speed shock waves, mostly their purpose is a nonessential one: smoothing the airflow around the flap extension mechanisms inside.

There was a case not long ago when a group of passengers became alarmed after noticing that one of these fairings was missing from their aircraft. They refused to fly because—as the media reported the incident—“a piece of the wing was missing.” In reality, the fairing had been removed for repairs after being damaged by a catering truck. Flying without a fairing might entail a slight fuel-burn penalty, but the plane remains perfectly airworthy. (Whether any part is allowed to be missing, and what the penalty might be, is spelled out in the plane's Configuration Deviation List.)

Any airplane can perform more or less any maneuver, theoretically, from loops to barrel rolls to a reverse inverted hammerhead Immelman. (During a demonstration flight in the late 1950s, a Boeing 707 was intentionally rolled upside down.) However, the ability to do so is mostly a function of excess thrust or horsepower, and commercial planes generally lack enough engine strength relative to their weight. In any case, it's not a good idea. Airliner components are not designed for aerobatics and could suffer damage—or worse. Plus, the cleaners would be up all night scrubbing out coffee stains and vomit.

Maybe that makes you wonder, how can
any
plane fly upside down given what I said earlier about a wing being curved on top and flat on the bottom, resulting in a pressure differential that produces lift? If you're flying upside down, wouldn't lift act in the opposite direction, forcing the plane toward the ground? It would, to an extent. But as we've already seen, a wing creates lift in
two
ways, and Bernoulli's pressure differential is the less critical. Simple deflection is a lot more important. All a pilot needs to do is hold the right angle, deflecting enough air molecules, and the negative lift from an upside-down airfoil is easily offset by the kiting effect.

Picture the engine's anatomy as a back-to-back assembly of geared, rotating discs—compressors and turbines. Air is pulled in and directed through the spinning compressors. It's squeezed tightly, mixed with vaporized kerosene, and ignited. The combusted gases then come roaring out the back. Before they're expelled, a series of rotating turbines absorbs some of the energy. The turbines power the compressors and the large fan at the front of the nacelle.

Older engines derived almost all of their thrust directly from the hot exploding gases. On modern ones, that big forward fan does most of the work, and you can think of a jet as a kind of ducted fan, spun by a core of turbines and compressors. The most powerful motors made by Rolls-Royce, General Electric, and Pratt & Whitney generate in excess of 100,000 pounds of thrust. The thrust is tapped to supply the electrical, hydraulic, pressurization, and deicing systems. Hence, you'll often hear jet engines referred to as “powerplants.”

All modern, propeller-driven commercial airliners are powered by turboprops. A turboprop engine is, at heart, a jet. In this case, for better efficiency at lower altitudes and along shorter distances, the compressors and turbines drive a propeller instead of a fan. Loosely put, it's a jet-powered propeller. There are no pistons in a turboprop engine, and the “turbo” shouldn't elicit confusion with turbocharging in the style of an automobile, which is completely different. Turboprops are more reliable than piston engines and offer a more advantageous power-to-weight ratio.

Jets and turboprops run on jet fuel, which is basically refined kerosene—a permutation of the stuff in camping lanterns. It's manufactured in different grades, but the flavor used by airlines is called Jet-A. Televised fireballs notwithstanding, jet fuel is surprisingly stable and less combustible than you'd think, at least until atomization. You can stick a lit match into a puddle and it will not ignite. (Neither Patrick Smith nor the publisher shall be responsible for injuries or damage caused in connection to this statement.)

That's the APU (auxiliary power unit), a small jet engine used to supply electricity and air conditioning when the main engines aren't running, or to supplement them when they are. All modern airliners have an APU, and it is typically located in the rear fuselage under the tail. If you're boarding through the old style airstairs and notice a hissing, jet-like noise similar to the sound of ten thousand hairdryers, that's the APU.

It also provides high-pressure air for starting the main engines. The internal batteries on larger planes aren't powerful enough to initially rotate an engine's compressors. Instead they are spun by air plumbed from the APU. The first commercial jetliner with an APU as standard equipment was the Boeing 727, which debuted in 1964. Prior to that, an external air source, referred to as an “air cart” or “huffer,” would be hooked into the plane's pneumatic ducting. You might see one of these carts today on occasions when a plane is dispatched with its APU inoperative, used to get the first engine going. The running engine then becomes the air source for the remaining engine(s).