On June 1, 2009, Air France Flight 447 descended unexpectedly, hundreds of feet per second, before it slammed its belly into the Atlantic Ocean, shearing the plane apart and killing all 228 passengers and crew members. Over time, accident investigators were able to piece together what went wrong on that fateful night: A combination of severe weather, equipment malfunction and crew confusion caused the aircraft to stall and drop from the sky.
Flight 447 sent a shock wave through the aviation industry. The aircraft -- an Airbus A330 -- was one of the world's most reliable planes, with no recorded fatalities flying commercially until the doomed Air France flight. Then the crash revealed the frightening truth: Heavier-than-air vehicles operate under very narrow tolerances. When everything is five by five, an airplane does what it's supposed to do -- fly -- with almost no apparent effort. In reality, its ability to stay aloft relies on a complex interplay of technologies and forces, all working together in a delicate balance. Upset that balance in any way, and a plane won't be able to get off the ground. Or, if it's already in the air, it will return to the ground, often with disastrous results.
This article will explore the fine line between flying high and falling fast. We'll consider 10 innovations critical to the structure and function of a modern aircraft. Let's begin with the one structure -- wings -- all flying objects possess.
Birds have them. So do bats and butterflies. Daedalus and Icarus donned them to escape Minos, king of Crete. We're talking about wings, of course, or airfoils, which function to give an aircraft lift. Airfoils typically have a slight teardrop shape, with a curved upper surface and a flatter lower surface. As a result, air flowing over a wing creates an area of higher pressure beneath the wing, leading to the upward force that gets a plane off the ground.
Interestingly, some science books invoke Bernoulli's principle to explain the uplifting story of airfoils. According to this logic, air moving over a wing's upper surface must travel farther -- and therefore must travel faster -- to arrive at the trailing edge at the same time as air moving along the wing's lower surface. The difference in speed creates a pressure differential, leading to lift. Other books dismiss this as hogwash, preferring instead to rely on Newton's tried-and-true laws of motion: The wing pushes the air down, so the air pushes the wing up.
Heavier-than-air flight began with gliders -- light aircraft that could fly for long periods without using an engine. Gliders were the flying squirrels of aviation, but pioneers like Wilbur and Orville Wright desired a machine that could emulate falcons, with strong, powered flight. That required a propulsion system to provide thrust. The brothers designed and built the first airplane propellers, as well as dedicated four-cylinder, water-cooled engines to spin them.
Today, propeller design and theory has come a long way. In essence, a propeller functions like a spinning wing, providing lift but in a forward direction. They come in a variety of configurations, from two-blade, fixed-pitch propellers to four- and eight-blade models with variable pitch, but they all do the same thing. As the blades rotate, they deflect air backward, and this air, thanks to Newton's action-reaction law, pushes forward on the blades. That force is known as thrust and works to oppose drag, the force that retards the forward motion of an aircraft.
In 1937, aviation took a giant leap forward when British inventor and engineer Frank Whittle tested the world's first jet engine. It didn't work like the piston-engine prop planes of the day. Instead, Whittle's engine sucked air through forward-facing compressor blades. This air entered a combustion chamber, where it mixed with fuel and burned. A superheated stream of gases then rushed from the tailpipe, pushing the engine and the aircraft forward.
Hans Pabst van Ohain of Germany took Whittle's basic design and powered the first jet-aircraft flight in 1939. Two years later, the British government finally got a plane -- the Gloster E.28/39 -- off the ground using Whittle's innovative engine design. By the end of World War II, Gloster Meteor jets, which were successive models flown by Royal Air Force pilots, were chasing down German V-1 rockets and shooting them from the sky.
Today, turbojet engines are reserved primarily for military planes. Commercial airliners use turbofan engines, which still ingest air through a forward-facing compressor. Instead of burning all of the incoming air, turbofan engines allow some air to flow around the combustion chamber and mix with the jet of superheated gases exiting the tailpipe. As a result, turbofan engines are more efficient and produce far less noise.
Early piston-powered aircraft used the same fuels as your car -- gasoline and diesel. But the development of jet engines necessitated a different kind of fuel. Although a few wacky wingmen advocated the use of peanut butter or whiskey, the aviation industry quickly settled on kerosene as the best fuel for high-powered jets. Kerosene is a component of crude oil, obtained when petroleum is distilled, or separated, into its constituent elements.
If you have a kerosene heater or lamp, then you might be familiar with the straw-colored fuel. Commercial aircraft, however, demand a higher grade of kerosene than fuel used for domestic purposes. Jet fuels must burn cleanly, yet they must have a higher flash point than automobile fuels to reduce the fire risk. Jet fuels must also remain fluid in the cold air of the upper atmosphere. The refining process eliminates all suspended water, which could turn into ice particles and block fuel lines. And the freezing point of the kerosene itself is carefully controlled. Most jet fuels won't freeze until the thermometer reaches minus 58 degrees Fahrenheit (minus 50 degrees Celsius).
Flight Controls (Fly-by-wire)
It's one thing to get an airplane into the air. It's another thing to control it effectively without crashing back to earth. In a simple light airplane, the pilot transmits steering commands via mechanical linkages to control surfaces on the wings, fin and tail. Those surfaces are, respectively, the ailerons, the elevators and the rudder. A pilot uses ailerons to roll from side to side, elevators to pitch upward or downward, and the rudder to yaw port or starboard. Turning and banking, for example, requires simultaneous action on both the ailerons and the rudder, which causes the wing to dip into the turn.
Modern military and commercial airliners have the same control surfaces and take advantage of the same principles, but they do away with mechanical linkages. Early innovations included hydraulic-mechanical flight control systems, but these were vulnerable to battle damage and took up a great deal of room. Today, almost all large aircraft rely on digital fly-by-wire systems, which make adjustments to control surfaces based on an onboard computer's calculations. Such sophisticated technology allows a complex commercial airliner to be flown by just two pilots.
Aluminum and Aluminum Alloys
In 1902, the Wright brothers flew the most sophisticated aircraft of the day -- a one-person glider featuring muslin "skin" stretched over a spruce frame. Over time, wood and fabric gave way to laminated wood monocoque, an aircraft structure in which the plane's skin bears some or all of the stresses. Monocoque fuselages allowed for stronger, more streamlined planes, leading to a number of speed records in the early 1900s. Unfortunately, the wood used in these aircraft required constant maintenance and deteriorated when exposed to the elements.
By the 1930s, almost all aviation designers preferred all-metal construction over laminated wood. Steel was an obvious candidate, but it was too heavy to make a practical airplane. Aluminum, on the other hand, was lightweight, strong and easy to shape into various components. Fuselages bearing brushed aluminum panels, held together by rivets, became a symbol of the modern aviation era. But the material came with its own problems, the most serious being metal fatigue. As a result, manufacturers devised new techniques to detect problem areas in an aircraft's metal parts. Maintenance crews use ultrasound scanning today to detect cracks and stress fractures, even small defects that might not be visible on the surface.
In the early days of aviation, flights were short, and a pilot's main concern was not crashing to the ground after a few exhilarating moments in the air. As the technology improved, however, increasingly longer flights were possible -- first across continents, then across oceans, then around the world. Pilot fatigue became a serious concern on these epic journeys. How could a lone pilot or a small crew stay awake and alert for hours on end, especially during monotonous sessions of high-altitude cruising?
Enter the automatic pilot. Invented by Lawrence Burst Sperry, son of Elmer A. Sperry, the autopilot, or automatic flight control system, linked three gyroscopes to an aircraft's surfaces controlling pitch, roll and yaw. The device made corrections based on the angle of deviation between the flight direction and the original gyroscopic settings. Sperry's revolutionary invention was capable of stabilizing normal cruising flight, but it could also perform unassisted takeoffs and landings.
The automatic flight control system of modern aircraft differs little from the first gyroscopic autopilots. Motion sensors -- gyroscopes and accelerometers -- collect information on aircraft attitude and motion and deliver that data to autopilot computers, which output signals to control surfaces on the wings and tail to maintain a desired course.
Pilots must keep track of a lot of data when they're in the cockpit of an airplane. Airspeed -- the velocity of an aircraft relative to the air mass through which it's flying -- is one of the most important things they monitor. For a specific flight configuration, be it landing or economy cruising, a plane's speed must remain within a fairly narrow range of values. If it flies too slowly, it can suffer an aerodynamic stall, when there is insufficient lift to overcome the downward force of gravity. If it flies too rapidly, it can suffer structural damage, such as the loss of flaps.
On commercial airliners, pitot tubes bear the burden of measuring airspeed. The devices get their name from Henri Pitot, a Frenchman who needed a tool to measure the speed of water flowing in rivers and canals. His solution was a slender tube with two holes -- one in front and one on the side. Pitot oriented his device so that the front hole faced upstream, allowing water to flow through the tube. By measuring pressure differential at the front and side holes, he could calculate the speed of the moving water.
Airplane engineers realized they could accomplish the same thing by mounting pitot tubes on the edge of the wings or jutting up from the fuselage. In that position, the moving airstream flows through the tubes and allows for an accurate measurement of the aircraft's speed.
Air Traffic Control
So far, this list has focused on aircraft structures, but one of the most important aviation innovations -- actually a collection of innovations -- is air traffic control, the system that ensures aircraft can take off from one airport, travel hundreds or thousands of miles and land safely at a destination airport. In the United States, more than 20 air traffic control centers monitor the movement of airplanes across the country. Each center is responsible for a defined geographic area, so that as an airplane flies along its route, it gets handed off from one control center to the next. When the airplane arrives at its destination, control transfers to the airport's traffic tower, which provides all directions to get the plane on the ground.
Surveillance radar plays a key role in air traffic control. Fixed ground stations, located at airports and at control centers, emit short-wavelength radio waves, which travel to airplanes, strike them and bounce back. These signals allow air traffic controllers to monitor aircraft positions and courses within a given volume of airspace. At the same time, most commercial aircraft carry transponders, devices that transmit the aircraft's identity, altitude, course and speed when "interrogated" by radar.
Landing a commercial airliner seems like one of technology's most improbable feats. A plane must descend from 35,000 feet (10,668 meters) to the ground and slow from 650 miles (1,046 kilometers) to 0 miles per hour. Oh, yeah, and it has to place its entire weight -- some 170 tons -- onto just a few wheels and struts that must be strong, yet completely retractable. Is it any wonder that landing gear takes the No. 1 spot on our list?
Up until the late 1980s, the majority of civilian and military aircraft used three basic landing-gear configurations: one wheel per strut, two wheels side by side on a strut or two side-by-side wheels next to two additional side-by-side wheels. As airplanes grew larger and heavier, landing gear systems became more complex, both to reduce stress on the wheel and strut assemblies, but also to decrease forces applied to runway pavement. The landing gear of an Airbus A380 superjumbo airliner, for example, has four undercarriage units -- two with four wheels each and two with six wheels each. Regardless of configuration, strength is far more important than weight, so you'll find steel and titanium, not aluminum, in the metal components of a landing gear.
Without the system that pumps unused air from an aircraft's engines, passengers would be unable to breathe in flight. But how does that system work?
Orville Wright once said: "The airplane stays up because it doesn't have the time to fall." After writing this, I would call that an understatement of epic proportions.
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