Free Flight 232: A Story of Disaster and Survival by Laurence Gonzales Page B

Electric made the engines for the DC-10. They were known as Model CF6-6 high-bypass turbofans. If you go out to the airport and look at one, you will see a metal fan more than seven feet in diameter at the front. Behind that number one fan are a stage two fan, compressor wheels, a combustion chamber, and turbine wheels. All of those rotating parts make the airplane go.
    The words ejaculate and jet derive from the Latin verb jacere , meaning to throw. A flying machine must throw a fluid—usually air—in order to move itself in accordance with Newton’s laws, one of which says that any action results in an equal and opposite reaction. A wing produces lift only when it’s moving through the air. To achieve that forward motion, the machine has to push a mass of fluid in the opposite direction. (To an engineer, air is a fluid because it flows.) In the CF6 engines the so-called working fluid is 99 percent air, with a little exhaust mixed in from the fuel that the engine burns.
    Each of the CF6 engines that powered November 1819 Uniform on that July day in 1989 was capable of producing on the order of thirty-nine thousand pounds of thrust . GE makes an engine today that produces more than a hundred thousand pounds of thrust. Either way, such engines produce a great deal of energy, and those who make and use them want to be careful where all that energy goes. For example, the number one fan on the CF6-6 engine has thirty-eight fan blades, each of which is twenty-eight inches long and weighs ten pounds. Those blades are mounted in dovetail slots in the rim of a wheel known as the number one fan disk. Without the blades, the fan disk weighs 370 pounds and has a diameter of thirty-two inches. Spinning at about thirty-five hundred revolutions a minute in cruise flight, the centrifugal force that those blades exert on the fan disk amounts to nearly four million pounds.

    A generic turbofan engine is pictured here. Arrows show the path of the air. Direction of travel of the aircraft is to the left. The fan and compressor blades blow air backward, compressing it before directing it into the combustion chamber, where it is mixed with fuel. The exhaust from the burning fuel turns the turbine blades at the rear, thus creating the power that rotates concentric driveshafts, which in turn spin the fans and compressors at the front. Courtesy Richard Wheeler
    Turbofan engines perform their work based on principles that say, in effect, that temperature, pressure, and volume are all interrelated and interdependent. If you force a gas into a smaller space (reducing volume), you will increase temperature and pressure. If you increase the temperature of a gas in a fixed space, you increase the pressure (but not the volume). Conversely, if you increase the volume of a flowing gas, the pressure goes down and the gas moves faster. A gas turbine engine such as the CF6 does all of those things at various points in its operating sequence. The number one fan on the front pushes a large volume of air backward, about thirteen hundred pounds of it every second. That air takes two paths. The innermost path leads to the compressor wheels. The combustion chamber receives the compressed air, accounting for a small amount of the total. The greatest share of the air flows around the whole working assembly, providing thrust while cooling and quieting the engine.
    The compressor is a series of fans that alternate with sets of stationary fins. The fans are wheels that are fitted with blades. They work much the way propeller blades work, pushing air backward. The stationary fins, called stator vanes, redirect the air to keep it moving in a nearly longitudinal direction. Each set of compressor blades and stator vanes increases the pressure and temperature of the air while reducing its volume. The air goes through four low-pressure stages , then through sixteen high-pressure stages, until the air achieves an incandescent heat. All this takes place before the air has