Starting a jet engine is a carefully controlled sequence that turns a stationary turbofan into a stable source of thrust. The goal is to spin the engine core fast enough for compression, then add fuel and ignition at the right moment. This matters because a mistimed start can overheat the engine, fail to light, or damage expensive components.
Pilots and computers monitor rotor speed, fuel flow, and exhaust gas temperature throughout the start.
Understanding Aviation: Starting a Jet Engine
A turbofan has several rotating sections called spools. Each spool links a compressor at the front to a turbine farther back. During a start, an external energy source must first rotate the high pressure spool.
Many airliners use compressed air from an auxiliary power unit, while some newer aircraft use an electric starter. The rotating compressor draws air inward and squeezes it into a smaller space. This raises its pressure and temperature.
A useful compressor needs a smooth, steady airflow. If the airflow is too weak or disturbed, the combustion chamber cannot support a reliable flame.
Fuel enters through small nozzles that spray it into the combustor as a fine mist. Igniters create sparks, but the sparks do not provide the engine's continuing power. They only begin combustion.
Once burning starts, hot gas expands rearward through turbine blades. The turbine captures some of that energy and uses it to keep the compressor turning. This is the key change during a start.
The engine gradually becomes able to run itself instead of depending on the starter. The fan at the front may turn too, though it is connected to a different spool on many engines.
Engine controls must add fuel at a rate the available airflow can handle. Too much fuel early in the sequence can make the gas temperature rise very quickly. This is called a hot start.
A slow rise in spool speed after ignition can indicate a hung start, where the engine is not gaining enough energy to reach idle. No temperature rise can mean there was no light off. Fuel may then remain inside the engine, creating a wet start.
Procedures normally stop the fuel flow and keep the core turning for a time to clear remaining fuel safely. Modern full authority digital engine controls perform much of this monitoring automatically, but crews still check the indications.
Students can connect this process to energy transfer. Chemical energy in fuel becomes thermal energy in the flame. The hot gas then transfers energy to moving turbine blades.
Those blades provide rotating power for the compressor and fan. The engine cannot simply make unlimited thrust by adding more fuel, because airflow, pressure, temperature, and turbine speed must stay within limits. When learning the sequence, pay attention to cause and effect.
Compressor rotation creates usable airflow. Airflow allows stable burning. Burning drives the turbine.
The turbine sustains rotation. Cockpit displays show these linked changes through spool speed, exhaust temperature, oil pressure, and fuel flow. A normal start is a controlled rise in all of them toward stable idle.
Key Facts
- Starter torque spins the core before fuel is introduced.
- Compressor pressure ratio = compressor exit pressure / compressor inlet pressure.
- Light-off occurs when fuel ignites and exhaust gas temperature begins rising.
- Power = torque x angular speed, or P = tau omega.
- EGT must stay below the engine start limit to prevent hot-section damage.
- At stable idle, compressor speed, fuel flow, and turbine power are balanced.
Vocabulary
- Turbofan
- A jet engine that uses a large fan plus a gas turbine core to produce thrust efficiently.
- Starter
- A device that turns the engine core during start until the turbine can keep the engine running.
- Compressor
- A rotating section that raises the pressure of incoming air before it enters the combustor.
- Light-off
- The moment when the fuel-air mixture ignites in the combustor and the engine begins producing its own hot gas flow.
- Exhaust Gas Temperature
- The temperature of gases leaving the turbine, commonly monitored to protect the engine during start and operation.
Common Mistakes to Avoid
- Adding fuel before enough core speed is reached is wrong because the compressor may not supply enough airflow for safe combustion.
- Thinking the ignition alone spins the engine is wrong because the starter provides the initial rotation while igniters only create sparks.
- Ignoring a rapid EGT rise is wrong because excessive temperature during start can damage turbine blades and combustor parts.
- Assuming idle means no thrust is produced is wrong because a running jet engine still accelerates air and produces some thrust at idle.
Practice Questions
- 1 A starter brings the engine core from 0 rpm to 3600 rpm in 20 s. What is the average angular acceleration in rpm/s?
- 2 During start, EGT rises from 120 degrees C to 540 degrees C in 14 s. What is the average temperature rise rate in degrees C per second?
- 3 Explain why the start sequence requires airflow through the compressor before fuel is sprayed into the combustor.