Just like all combustion engines, jets need to begin spinning to start. Small internal combustion engines, like those in our cars or small piston airplanes, use a small electrical motor. The motor spins the flywheel, which makes the cylinders in the engine move. Once fuel, air, and spark are added in, the engine fires right up. But what about jet engines, how do they start?
A large turbine engine will have an auxiliary power unit (APU) starting system. An APU is a small gas turbine engine that produces bleed air, which is then ducted to the main turbine to begin spinning it. Jets can use other systems instead, but APUs are the most common method used on airliners.
How Does a Jet Engine Work?
Before diving into the details of how starting systems work, it’s helpful to get an idea of how a jet engine functions. Much like the piston engines we are more familiar with, jet engines complete the same four power cycles. But in a jet, the intake, compression, power, and exhaust cycles are completed different.
A jet engine is basically a giant air compressor. It brings a lot of air in from the front, and a series of spinning compressor blades makes that air more and more dense. It also heats up. That air then reaches the combustion chambers, where jet fuel is sprayed into it. Since the air is already hot from compression, the fuel combusts.
After combustion, the hot gasses are forced back through the turbine blades of the engine. These blades spin the main drive shaft, which is connected to the first compressor blades to give them power. Once the fuel is burning in the combustion chamber and air is flowing through the engine, the engine will continue to operate since it is all interconnected.
The resulting exhaust is an explosive jet of hot air and gasses that exits the engine’s back end. The force of that air existing aft forces the plane forward thanks to Newton’s Third Law of Motion–for every action there is an equal and opposite reaction.
Types of Aircraft Engines
There is some confusion since similar words are used to describe all sorts of different engines on planes. In general, there are five main types of engines found on aircraft. The term “jet engine” is used as a shortened form of the turbojet, even though many other types of turbine engines are found in planes.
Many planes still use piston engines to spin propellers. These aren’t jets or turbines, since they use reciprocating pistons to move a driveshaft. The smallest air-cooled examples are used in Cessna, Pipers, and many other small aircraft.
A turboprop is a jet engine connected to a propeller. These turbine engines are used on many regional prop planes like the ATR and smaller private planes like the Pilatus. Since a turbine spins much faster than a propeller can, these engines have a gearbox built-in to reduce the shaft speed.
Straight turbojet engines are seldom used today. These were the first-generation engines found on jets like the Boeing 707 or the Douglas DC-8. They were noisy and inefficient by today’s standards.
Nearly all engines on airliners today are turbofans. Before the first-stage compressor, a set of enormous fan blades brings in extra air. These fans add extra power and insulate the main compressors. They run more efficiently and quieter.
The final type of engine commonly used in aircraft is the turboshaft. These turbines have their power output connected to a drive shaft that is not the same driveshaft that spins the turbines and compressors. These engines are commonly used on helicopters to spin the main rotor.
Types of Aircraft Engine Starting System
No matter what type of turbine is in question, the first step to get it started is to get it spinning. Once it is spinning fast enough to compress the air in the combustion chamber, fuel can be added. Once that happens, the combustion will spin the turbine blades, which will spin the compressor blades, and the engine will keep running on its own.
In the early days of jet aircraft, using a small gasoline piston engine to help start the turbine was common. Today, this has been replaced by a variety of different systems. The designers and engineers determine the system on your airplane, and like everything else, it’s a decision they make based on performance, cost, and reliability.
Like piston engines, it’s possible to start a jet engine using an electric starter. The starter will be connected to the drive shaft via a gearing mechanism. When the motor turns, the compressor blades will begin sucking in air. When the proper pressure is reached, fuel is added and ignited.
Since an electric motor is nearly identical to a power generator, many aircraft combine the functions into one device. Once the engine is running on its own, the generator keeps spinning and produces power for the aircraft’s electrical system.
Hydraulic motors have also been used to start jet engines. This system is used in small turboshaft or turbojet engines, especially those found in helicopters or cruise missiles.
Jet Engine Air Starter
By far, the most common type of jet engine startup is the air-start. The air starter uses some source of already moving high-speed air, forces it over the compressor and turbine blades, which then gets the engine running when fuel is added.
Where does the high-speed air come from? If an airplane has more than one engine, it can get the air from another engine. Some high-pressure air is often ducted from an engine for other uses. This is known as bleed air, and bleed air is often used to help start a turbine engine.
But to use bleed air to start a jet engine, another jet engine needs to be already running. Many planes have an APU, or auxiliary power unit, built into the plane. Airliners often have their APUs mounted in the tail cone.
The APU is really just a tiny jet engine. It’s small enough to be quickly started with an electric starter but big enough to produce enough bleed air to start the main engines and run the aircraft’s electrical and environmental systems.
Ground Carts or Huffer Cart
An alternative to using the APU is a ground-based unit, also called the huffer cart. These ground carts are trailers found around airports parked next to the plane to help it during ground starts. These come in several forms, but the biggest ones are basically mobile APUs. The proper name for them is a GPU, or ground power unit.
If an engine is shut down in-flight for any reason–be it a malfunction, training, or fuel savings–it can be restarted quickly. Since the relative wind to the aircraft is still spinning the fan and compressor blades, chances are introducing fuel back into the system will result in a restart.
If the turbine speed is insufficient, it can be increased by putting the plane into a dive to increase airspeed. Of course, the actual restart procedures vary from aircraft to aircraft, so follow the Pilot’s Operating Handbook’s procedures.