How Airports Work

Most of us think AIRPORT, not airport, when we hear the term. We imagine massive sites covering hundreds of acres and filled with hangars, terminals, control towers and parking garages. In reality, many airports don't fit this description. Some are nothing more than a strip of grass, dirt or pavement placed in the middle of a farm or field. These so-called rural airstrips often serve just one or two pilots and don't have any other structures beyond the crude runway itself. Small community airports, on the other hand, may supplement a single airstrip with a few hangars and facilities to train student pilots, although they usually don't have operating control towers.

A regional community airport offers more, including a control tower and an automated weather observation system to provide pilots with real-time weather data. Such a facility usually has multiple Tee hangars -- simple metal enclosures -- and tie-down spots for permanently based or visiting pilots to house or stow their aircraft. A terminal building, with a pilot's lounge, restrooms, vending area and conference rooms, is often available, as is a fuel farm to provide either kerosene-based jet fuel or aviation gasoline (avgas), which is similar to automobile gasoline (mogas) but with some additives to make it perform better in spark-ignited internal combustion engines common in light aircraft. Because of its larger size, a regional community airport can accommodate a wider range of aircraft, from small prop planes to regional jets that seat no more than 20 passengers to larger three-engine jets, such as the Boeing 727, capable of seating up to 189 passengers.

In the U.S., most smaller airports fall into the category of general aviation [source: Airlines for America]. They don't handle military flights or common commercial transport but instead provide facilities and resources for personal flying, business flying, instructional flying and certain commercial flying activities, such as aerial photography and skydiving. Airports that handle passenger planes operated by companies such as Southwest, Delta and United Airlines and cargo planes operated by FedEx, DHL and other similar entities belong to the commercial aviation category. These large facilities are almost always situated near major urban areas, and they can handle national and international flights and support jumbo jets, such as 747s.

The United States boasts one of the world's most extensive aviation systems, with almost 20,000 small, medium and large airports. Nearly 17 percent of these make up the national airport system, which means they are eligible for federal assistance to go toward improvements that increase safety and security or that mitigate environmental impact. The national system includes a network of just over 500 commercial airports, all of which receive public funding and handle at least 2,500 passenger boardings a year [source: Airlines for America]. For the rest of this article, we will focus almost exclusively on these larger, more complex facilities.

Commercial airports require huge amounts of land for runways and other facilities. As a result, few have been built in North America and Europe since the 1950s because metropolitan areas simply don't have any suitable sites. Even airports built on the outskirts of a city in the 1950s now find themselves hemmed in by urban and suburban development. This leaves planners with no choice but to renovate existing structures or expand within long-established property lines.

With that said, some cities have managed to add a commercial airport in the last couple of decades. Denver, for example, began construction of its international airport in 1989 and, after numerous construction delays, opened the facility in February 1995. Since then, it's grown to be the fifth-busiest airport in the United States, with more than 145,000 passengers passing through its gates each day [source: Denver International Airport].

Building a commercial airport on the scale of Denver's requires years of planning and a number of complex decisions. Here are some of the things airport planners must consider:

Once a site is chosen, planners must then design and lay out the airport's major structures. Much of their efforts center on how to best configure runways and terminals for the most efficient flow of traffic on the ground and in the airspace during departures and landings.

At its most basic level, an airport has just two parts -- an airfield and a terminal. The airfield consists of runways and ramps, known as taxiways, that connect to the terminal. When laying out runways, engineers borrow conventions used in navigation and surveying, fields that indicate direction with a compass reading. For example, the 0/360 position on a compass marks north; 90 degrees marks east; 180 degrees marks south; and 270 degrees marks west. Every runway has a different number, expressed in a shorthand format, painted at each end to designate its orientation. A runway facing due west on one end and due east on the other would be marked 27 and 9 respectively. A runway with a northwest/southeast orientation might be marked 31 on one side (short for 310 degrees), 14 on the other (short for 140 degrees).

The U.S. Federal Aviation Administration (FAA), which provides guidelines for airport layout, specifies 20 different ways to arrange runways, although many of these are variations on four basic configurations:

To determine the length of a runway, airport planners consider a number of factors, including airport elevation, temperature, wind velocity, airplane operating weights, takeoff and landing flap settings and runway surface condition (dry or wet). A typical runway at a commercial airport, which must be able to accommodate jumbo jets, is between 10,000 and 12,000 feet (3,048 and 3,658 meters) long. Runways at small community airports, which serve aircraft with 20 or fewer passenger seats, may only be 3,000 to 4,000 feet (914 to 1,219 meters) long [sources: Bennett, Landrum & Brown].

Length isn't the only consideration. The runway surface itself must be much thicker than a highway to handle the extra weight of modern aircraft. Imagine a fully loaded Boeing 777 touching its wheels to the ground. That's 660,000 pounds (299,370 kilograms) coming down hard on the runway surface! Runways have to be specially constructed to take that strain without cracking or, worse, buckling. When the Denver International Airport was built, it took 2.5 million cubic yards (1.9 million cubic meters) of concrete to create five 12,000-foot (3,658-meter) runways, plus taxiways and aprons. First, 6 feet (1.8 meters) of compacted soil was put down; then, a 1-foot-deep (0.3-meter) layer of soil was spread, topped by an 8-inch- (20-centimeter-) thick cement-treated base; that was followed by 17 inches (43 centimeters) of concrete paving [source: Denver International Airport].

After landing, pilots use taxiways to get from a main runway to the terminal area, with its many gates. That's where we're headed next.

At a busy airport like Atlanta's Hartsfield International, 2,500 flights take off and land every day. That means that, each day, perhaps as many as 250,000 people move through the airport and need certain services. Airports provide those services in their concourses and terminals, the heart of any airport. There you'll find the space for airlines to handle ticket sales, passenger check-in, baggage handling and claims.

Terminals come in many sizes and designs. At small airports, a single building holds a common ticketing and waiting area with several exits -- or gates -- leading to aprons, where aircraft park and boarding takes place. At large airports, this basic design can be expanded, which results in a linear or curvilinear terminal, often a very long building with plenty of room to accommodate ticketing and check-in and multiple gates for access to aircraft. In the 1950s, the linear concept evolved into the pier-finger terminal. In this configuration, passengers are processed in the main terminal and then directed down one of several piers, where aircraft await in gates known as finger slots. Concourses, the open areas formed where the main terminal building and the various piers meet, provide spaces for shops, restaurants and lounges.

The terminal complex at Atlanta's Hartsfield airport covers 130 acres -- that's 5.7 million square feet (529,547 square meters) of space available to handle all of the necessary aviation activities! The complex includes the domestic and international terminals and seven concourses. Within these concourses, there are 114 food and beverage vendors (most of these are owned and staffed by private companies), 90 retail and convenience stores (also owned and staffed privately) and 56 staffed service outlets (places where you can get your shoes shined or connect to the Internet) [source: Hartsfield-Jackson].

Hartsfield's concourses also lead to the gates. There are 207 gates in all (167 domestic and 40 international) [source: Hartsfield-Jackson]. The gates are where the airplanes park for passenger boarding and deplaning. Passengers wait in the immediate area of each gate to board the plane. Gates are rented by each airline from the airport authority, and some airlines may rent a whole terminal building in their "hub" airport, in which case the rental fee alone can run into the millions of dollars.

Routine airplane maintenance, such as washing, deicing and refueling, is done by airline personnel while the plane is parked at the gate. In some cases, other maintenance tasks might be performed at the gate, possibly with passengers onboard the plane – it's not uncommon to sit on a plane at the gate while maintenance personnel replace something like a hydraulic brake line on an aircraft.

Commercial airplanes are rarely flown empty, which means they must receive a few important items before they take to the skies. Cargo carriers load their planes with different types of freight -- packages and mail, perishable items, even human remains. Passenger airlines load civilian passengers, their luggage (and sometimes their pets), snacks and drinks. And, of course, both types of aircraft require fuel.

When you think of the size of an airport and the enormous volume of people it serves, you can imagine that providing these logistics services, as they're called in the industry, can be a daunting challenge. Consider baggage handling as an example. You can get the full scoop in How Baggage Handling Works, but we'll cover the essentials here to show the journey your checked bag takes from the ticket counter to the plane.

The first step, which occurs as part of the check-in process, is generating a bar code that includes information about your flight, including layovers and final destination. An airline attendant attaches this code to your bag and then places it on a check-in conveyor, which carries it away. A machine on the other side of the desk, armed with an array of scanners capable of "seeing" the bar code regardless of its angle, reads the encoded data and then routes the bag accordingly. Based on these instructions, your bag follows a continuous chain of conveyors, which both move it along rapidly and change its orientation as necessary. Additional scanners along the way check and double-check the bar code to make sure your bag continues to follow the right path. Eventually, your bag reaches human handlers, who load it onto a trolley and drive it to the plane, where it is loaded into the hold.

At the same time, an airline must keep track of the people flying on their planes. During the check-in process, an agent must scan an e-ticket or manually input passenger data, noting any upgrades (to first class, for example) or special needs (wheelchairs, infants, lap children). For international flights, agents must also enter a passenger's passport information. All of this data, plus a code for the final destination, appears on a boarding pass, which is printed out and given to the passenger.

At the gate, during boarding, the passenger hands the pass to another airline agent, who scans the barcode and confirms that person on the flight manifest. All passengers then pass through a door and onto a boarding bridge, or jet bridge, an enclosed, movable connector that links the gate area to the aircraft. When all passengers have boarded the aircraft, the agent prints a copy of the manifest so a member of the flight crew can check that all passengers successfully boarded. Once all passengers have been accounted for, the airline issues a final manifest, which can be used to notify relatives in the unlikely event of an air disaster.

After takeoff, the flight crew may offer food and beverage service. The food that passengers eat while onboard the airplane is usually provided by private companies contracted by one or more airlines at an airport. The food is prepared in a building that is off the airport grounds, shipped to the airport by truck and loaded onto the plane by the catering company's personnel. For example, LSG Sky Chefs is one of the catering contractors at Denver International Airport. They prepare and load thousands of meals per day for various airlines.

Airport security changed radically after Sept. 11, 2001. Before the terrorist attacks, private companies provided airport screening services. These companies worked under the guidance of the U.S. Federal Aviation Administration, and although their representatives scanned passengers using metal detectives and X-rayed bags, limited federal security requirements existed for cargo and baggage screening. After 9/11, the U.S. government established the Transportation Security Administration to improve and strengthen aviation security. By November 2001, the TSA was rolling out a number of new security measures: armed air marshals, reinforced cockpit doors and no-fly lists identifying people who could pose a threat and designating them for enhanced screening or, as appropriate, prohibiting them from boarding an aircraft.

Today, the TSA screens 100 percent of checked baggage for explosives. Most airports integrate this screening process into their baggage claim solutions. Using combinations of software, conveyance systems and screening technologies -- computer tomography, X-ray machines and explosive trace detection -- airports can now conduct full in-line screening without interrupting or delaying the movement of luggage from the ticket counter to the aircraft.

The TSA also subjects passengers to much more rigorous screening procedures that can involve pat-downs and whole-body scans. The latter involves so-called advanced imaging technology machines, which have been installed at some 200 airports since 2008 [source: TSA]. The machines come in two flavors, based on the type of electromagnetic radiation they use to make a scan. Backscatter machines send low-energy X-rays to bounce off a passenger's body. Millimeter wave (mmv) scanners emit energy more akin to microwaves. Both see through clothing to produce a 3-D image of the person standing in the machine, revealing any threats he or she may be trying to conceal. In either machine, the scanning process is the same. Passengers must remove everything from their pockets, as well as belts, jewelry, lanyards and cell phones. Then they step up a small ramp and, stand in the center of the machine, raise their arms, bent at the elbows, and remain motionless as the device completes a scan. For backscatter machines, the process takes about 30 seconds. For mmv scanners, it takes about 10 seconds.

Airports have also increased their on-site police forces since 9/11. At Los Angeles International Airport, for example, the police crew has grown from 100 sworn officers before the terrorist attacks to 430 today. This makes the airport force almost as large as others performing their duties out on the city streets [source: McCartney]. Some airport police are members of the city or municipality assigned to the airport, while others are from private security companies. Either way, their highest priority is thwarting a threat to a plane or to the airport itself. They question people who are photographing aircraft, conduct random searches of cars to turn up illegal guns and drugs, monitor traffic on the tarmac and around the terminals and investigate reports of theft at security inspection points. Many airports also use bomb-sniffing dogs to check out unattended bags, garbage cans and vehicles.

Airports complement their police forces with separate crews to handle fire and emergency medical services (EMS). An airport may have several fire/EMS stations on the ground because the FAA requires that emergency crews be able to reach the midpoint of a runway within 3 to 5 minutes. The crews are usually employees of the city or municipality and are stationed at the airport. At their disposal are specially designed and equipped fire and rescue vehicles capable of extinguishing jet-fuel fires with thousands of gallons of foam. These 44-ton, six-wheel vehicles can accelerate from 0 to 50 miles per hour in less than 35 seconds and come equipped with a variety of turrets, nozzles and booms to attack a fire efficiently and protect escaping passengers [source: Rosenblum].

An airport can't exist in isolation. It depends on a massive surface-transportation system so that people can get to and from the airport, park and get from place to place within the airport structure itself. While your first thought about an airport is air travel, ground transportation is pretty crucial, too.

The busiest airport in the world is Atlanta's Hartsfield International Airport. You can take an extensive tour of Atlanta's ground transportation services on the airport's website, but here are some highlights of how ground transportation is critical:

In many ways, airports act as transportation hubs for entire regions. A person flies, say, from Washington, D.C., to Atlanta. After he arrives, he takes the SkyTrain from the main terminal to the rental car center. He rents a car, then drives downtown for a business meeting. In a little more than two hours, he has completed a trip that would have taken him 9.5 hours in a car. But think of the people, processes and modes of transportation he has engaged to make his quick trip. It's a highly complex system with hundreds, if not thousands, of moving parts and interconnecting strands. Managing all of this, as we'll see next, is a massive undertaking.

Not surprisingly, airports are huge businesses. How big? Well, Denver's airport cost about $5 billion to build, and operating costs are $160 million per year. But its economic impact on the state is huge. According to the Colorado Department of Transportation, Denver International Airport generates $22.3 billion for the state every year [source: Denver International Airport].

Commercial airports are publicly owned and generally financed through municipal bonds. Airports typically own all of their facilities and make money by leasing them to airlines, air-freight companies, and retail shops and services, as well as by charging for services like fuel and parking and through fees and taxes on airline tickets. The revenues pay off the municipal debt and cover the operating costs. Airports often require other sources of funding as well, such as airport bonds and government grants. But most airports are self-sustaining businesses once they become operational.

About 90 percent of employees at airports work for private companies, such as airlines, contractors and concessions [source: Airlines for America]. Most of the remaining 10 percent work directly for the airport as administrators, terminal- and grounds-maintenance personnel and safety crews. Air traffic controllers are employees of the federal government. Airports have their own departments of finance, personnel, administration and public relations, much like any city or municipality.

Whether or not you find heaven or hell in the complex world of a commercial airport largely depends on your personality and the circumstances of your travel. A business person trying to make an early-morning meeting tomorrow will likely curse the airport today if her flight is delayed or she has to make a mad dash across two miles of terminal to catch a connecting flight. Someone taking a month-long vacation to Hawaii may be much more forgiving about these realities of flying and may even see the airport as a great place to observe people and study human nature. Either way, airports will continue to be all-important nodes in the international transportation system -- at least until someone perfects teleportation.