Turbine Engines:

Principles of OperationTurbine engines, whether turboprops or turbofans, have replaced piston engines on most large aircraft, and they've become increasingly popular on executive aircraft with the development of small, efficient turbofan engines.

How It Works

The turbines in a turbine engine are fan-like structures. Blades arrayed around a shaft catch air flowing through the engine and turn the central engine shaft. Most modern turbine engines have several turbines that compress the incoming air in stages before it reaches the combustion chamber. Additional turbines in the exhaust area of the engine capture some of the energy from the exhaust and keep the turbine shaft rotating.

All turbine engines operate according to the same basic principle. A combustible mixture of fuel and air is drawn into the engine. The air and fuel mix in a combustion chamber, where the mixture is ignited. The hot exhaust gas shoots out the rear of the engine at high speed, pushing the airplane ahead. As the hot air flows through the engine, it turns additional turbines in the exhaust stream, which keep the shaft spinning at a high RPM, usually more than 10,000 revolutions per minute.

Turbofan Engines

A turbofan engine has a large-diameter turbine at the front of the engine that accelerates a large mass of air that flows around the central engine core and out the back. This arrangement makes more efficient use of fuel and is much quieter than the older turbojet technology.

Turboprop Engines

A turboprop engine is a jet engine attached to a propeller. The high-speed turbines generate an enormous amount of power that is transmitted to the propeller through a gear-reduction system. The propeller is really a large fan turned by the turbine. Turboprop engines are much more efficient than pure jet engines at speeds in the 250–350 mph (400–560 km/h) range. At higher speeds, propellers lose their efficiency, and pure jet engines are a better choice.

Turbine Engine Controls

From a pilot's perspective, turbine engines are much easier to operate than piston engines. Aircraft equipped with turbofan engines like the Learjet 45 and Boeing 737-400 have one power control—the thrust levers. Automatic fuel control systems take care of mixing fuel and air in the combustion chamber, and there's no propeller control to worry about.

To increase power, move the thrust levers forward. To reduce power, move them back. Keep in mind, however, that it takes a while for jet engines to develop full power or "spool up." It's very important to anticipate the need for more power.

Throttle controls on the Lear-45, notice the lack of mixture, carburetor heat, and propeller controls!

Watch the Temperatures

By far the most important consideration when operating a turbine engine is temperature control. If you shove the thrust levers forward on takeoff, you can easily overheat the engines. If you don't cause an engine failure, at the least you'll run up a very large bill for inspection and repair of critical engine components. So watch the exhaust gas temperature (EGT) and inlet turbine temperature (ITT) gauges carefully when you add power. Keep the needles out of the red zones.

 
Engine instruments on a 737-300 and a 747-400 Glass Cockpit

Reverse Thrust