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How Planes Work
Turn your eyes to the sky and it's likely you'll see more than a few vapor trails—the wispy white lines that jet planes scribble on the great blue canvas stretched above our heads. At the dawn of the 20th century, the very idea of powered flight seemed, to many, like a ludicrous dream. How things have changed! At any given moment, there are something like 5,000 flights zipping through the sky over the United States alone; we're so used to the idea of flight that we barely even notice all the planes screaming above us, hauling hundreds of people at a time to their homes and holidays. Most modern planes are powered by jet engines (more correctly, as we'll see in a moment, gas turbines). What exactly are these magic machines. Let's all take a closer look at how they work.

Jet Engines
A jet engine is a type of reaction engine discharging a fast-moving jet that generates thrust by jet propulsion. This broad definition includes airbreathing jet engines (turbojets, turbofans, ramjets, and pulse jets). In general, jet engines are combustion engines. The term "jet engine" is commonly used only for airbreathing jet engines. These typically feature a rotating air compressor powered by a turbine, with the leftover power providing thrust through the propelling nozzle – this process is known as the Brayton thermodynamic cycle. Jet aircraft use such engines for long-distance travel. Early jet aircraft used turbojet engines that were relatively inefficient for subsonic flight. Most modern subsonic jet aircraft use more complex high-bypass turbofan engines. They give higher speed and greater fuel efficiency than piston and propeller aero engines over long distances. A few air-breathing engines made for high speed applications (ramjets and scramjets) use the ram effect of the vehicle's speed instead of a mechanical compressor.

Altitide
When an aircraft takes off, lift is created by the flow of air around the wings. If the air is thin, more speed is required to obtain enough lift for takeoff; therefore, the ground run is longer. An aircraft that requires 745 feet of ground run at sea level requires more than double that at a pressure altitude of 8,000 feet. [Figure 12-9]. It is also true that at higher altitudes, due to the decreased density of the air, aircraft engines and propellers are less efficient. This leads to reduced rates of climb and a greater ground run for obstacle clearance.Flying at high altitudes is an experience which the majority of near-sea level general aviation pilots will not experience just as they receive their license. Taking to the skies in the higher part of Earth’s atmosphere calls for special training, and the FAA requires supplemental oxygen of pilots flying above certain altitudes for more than half an hour. High density altitude is a concern at higher elevations on days when the air temperature and humidity are also high. It compounds the problems of flying at much higher altitudes, making the aircraft seem as if it were flying at an even higher part of the atmosphere. Climbing to higher altitudes can avoid the traffic of lower altitudes and translate to less drag, as well as less turbulence. That means decreased consumption of fuel, and, depending on the jet stream, can also mean the availability of tailwinds. It also enables pilots to soar over thunderheads or other inclement weather. However, flying at these levels comes with a different set of risks.

Headwind
A tailwind is a wind that blows in the direction of travel of an object, while a headwind blows against the direction of travel. A tailwind increases the object's speed and reduces the time required to reach its destination, while a headwind has the opposite effect. In aeronautics, a headwind is favorable in takeoffs and landings because an airfoil moving into a headwind is capable of generating greater lift than the same airfoil moving through tranquil air, or with a tailwind, at equal ground speed. As a result, aviators and air traffic controllers commonly choose to take off or land in the direction of a runway that will provide a headwind.

Air Pressure
Air pressure is the reason airplanes are able to produce lift. Due to the shape of an airplane wing, air on top of the wings moves faster than air on the bottom of the wings. Air pressure is a difficult concept. How can something invisible have mass and weight? Air has mass because it is made up of a mixture of gases that have mass. Add up the weight of all these gases that compose dry air (oxygen, nitrogen, carbon dioxide, hydrogen, and others) and you get the weight of dry air. So what's the connection between molecules and air pressure? If the number of air molecules above an area increases, there are more molecules to exert pressure on that area and its total atmospheric pressure increases. This is what we call high pressure. Likewise, if there are less air molecules above an area, the atmospheric pressure decreases. This is known as low pressure

Air Density
According to the FAA’s Pilot’s Handbook of Aeronautical Knowledge, density altitude is defined as pressure altitude corrected for variations from standard temperature. When conditions are standard, pressure altitude and density altitude are the same. But when nonstandard conditions are present, including high altitude, high humidity, and high temperatures, air density decreases and density altitude increases. Why is knowing density altitude important? Because high density altitude has a detrimental impact on aircraft performance. It reduces lift and impairs propeller efficiency, reducing thrust as a result. High density altitude can also decrease the engine’s power output. If it’s not accounted for, increased density altitude can cause major problems during takeoff and landing. As the FAA puts it, “hot, high, and humid weather conditions can cause a routine takeoff or landing to become an accident in less time than it takes to tell about it.”