The force on a fan blade at takeoff is equivalent to a load of nearly 100 tons. That’s like hanging a Boeing 757 on each blade.
It’s been 45 years since American Airlines ushered in the jet age with the inaugural transcontinental flight from Los Angeles to New York, featuring the brand new Boeing “707 Jet Flagship.” Since then, the turbojet engines that power our fleet have grown progressively more powerful, quieter, more reliable, and more fuel efficient. But did you ever wonder how a jet engine works? Because I know American Way
readers are a curious bunch, I have decided to make this month’s column a quick session of Jet Engines 101.
The jet engines powering our aircraft are a lot like a car’s engine. In both, air is compressed and mixed with fuel. The mixture is ignited and the rapid expansion of the hot gas that results produces power. But while combustion in a car’s engine is intermittent, and the expanding gas produces shaft power through a piston and crank, combustion in a jet engine is continuous, and the power it produces comes from the expanding gas being forced through the rear of the engine. Just as we know that for every action there is an equal and opposite reaction, the flow of expanding gas out the back of a jet engine creates a reaction of equivalent power. This power, or thrust, is transmitted through the engine to the aircraft you’re sitting in right now, propelling it through the air.
A jet engine’s operation can be summed up in four words: suck, squeeze, bang
, and blow
. First, an enormous amount of air is sucked into the engine’s fan and compressor stages. A Rolls Royce Trent 800 engine, the kind that powers American’s 777 aircraft, takes in more than a ton of air per second at a speed of roughly 350 miles per hour. The air is then squeezed through a number of compressor stages to the combustion chamber, where it is mixed with fuel to create the bang and the expansion that forces air into the turbine section of the engine. The reaction of the expanded gas being forced through the turbines drives the front part, or compressor section, of the engine and then blows out the exhaust nozzle at the back of the engine, producing thrust. By the time the air exits through the back, it has been accelerated to a speed of about 1,000 miles per hour. On the most modern turbofan engines, it is the large fan blades that produce most of the takeoff thrust generated by the engine during takeoff, not the hot exhaust gases that propelled the old jet engines. The newer engines also have a high bypass ratio, which means more air passes by the hot turbine section of the engine than passes through the inside core. This bypass air muffles a lot of the engine noise, which is one of the reasons today’s turbofan engines are much quieter than those of 10 years ago.
At takeoff, a Trent engine generates about 90,000 horsepower. That’s roughly the amount of energy generated by 350 F-150 trucks. In terms of fuel efficiency, a Boeing 777 aircraft, powered by 2 Trent engines, and carrying 300 people, gets about 120 passenger miles to the gallon. You would achieve the same level of efficiency if your car, carrying 4 people, got 30 miles to the gallon.
As you would expect, the components inside a jet engine are subject to extreme forces that place enormous demands on the alloys used to build them. The force on a fan blade at takeoff is equivalent to a load of nearly 100 tons. That’s like hanging a Boeing 757 on each blade. The strength of these alloys has contributed to the remarkable reliability of today’s jet engines. Today, a Trent 800 engine typically flies around 13,000 hours between major overhauls and long enough to fly 7 million miles, or around the world 250 times.
During the past 45 years, turbojets have changed our lives. They have stretched the boundaries of possibility and made the world a smaller place. I know I speak for everyone at American Airlines when I say we can’t wait to see the wonders the next 45 years of the Age of Turbojets will bring. We’re going to be there every step of the way, and certainly hope you will come along for the ride. Thanks for flying with us.GERARD J. ARPEY
President & CEO