Starting, ignition and the APU

A carefully controlled process

Before a turbine engine can produce thrust, it must be rotated fast enough to compress air, ignite fuel safely, and accelerate to self-sustaining speed. Modern transport aircraft use sophisticated starting, ignition and control systems to automate much of this process while protecting the engine from damage. Supporting many of these operations is the auxiliary power unit (APU), a small gas turbine that provides electrical and pneumatic power when the main engines are not running.

A successful engine start is carefully controlled. Too much fuel, insufficient airflow, excessive temperature or prolonged starter operation can quickly damage expensive engine components.

The purpose of engine starting

A turbine engine cannot start itself from rest. The starter provides the initial rotation. Before introducing fuel and ignition, the engine's compressor must already be rotating fast enough to:

• Compress incoming air
• Provide airflow through the combustor
• Cool turbine components
• Support stable combustion

Air turbine starters

Most transport aircraft use an air turbine starter (ATS), a small turbine driven by compressed air from the APU, an external ground-air cart, or another operating engine through a cross-bleed start. The starter is connected through a gearbox to the engine core spool, usually N2 (or N3 on some three-spool engines). As compressed air flows through the starter:

• The starter turbine rotates.
• The gearbox turns the engine core.
• Compressor speed increases.
• Airflow through the engine becomes sufficient for combustion.

A normal engine start

Although details vary by aircraft type, the sequence is similar across most turbine engines:

• Step 1, starter engagement: the starter valve opens, allowing compressed air into the air turbine starter, which begins accelerating the core spool. Crews and technicians monitor N2 (or N3), oil pressure and starter performance.
• Step 2, reaching fuel introduction speed: fuel is not introduced immediately. The engine must first reach a minimum rotational speed specified in the procedure, typically about 20% to 30% N2 depending on engine type, so that airflow through the engine is adequate.
• Step 3, fuel and ignition: at the specified N2, the fuel control switch is moved to RUN, the FADEC commands fuel flow, the igniters begin firing and fuel enters the combustor.
• Step 4, light-off: combustion becomes established, usually shown first by rising EGT. The crew watches for the EGT increase, stable acceleration and normal fuel flow. Most procedures require an EGT rise within a specified time, often around 20 seconds after fuel introduction. Failure to achieve light-off requires immediate action.
• Step 5, acceleration to idle: the engine accelerates on its own, fuel flow increases automatically, oil pressure rises and parameters stabilize. The starter disengages at a predetermined speed, often around 45% to 55% N2, and the engine stabilizes at idle. At this point it is self-sustaining.

Parameters monitored during start

• N2 (or N3): core speed must increase normally. Slow acceleration may indicate weak starter performance, bleed-air problems or internal engine issues.
• EGT: the EGT rise confirms successful ignition. Technicians monitor the time to light-off, the rate of temperature increase and the peak starting temperature. Exceeding limits may damage turbine components.
• Oil pressure: it should rise shortly after engine rotation begins. Low or absent oil pressure may require the start to be aborted.
• Fuel flow: it must be appropriate for the start sequence. Abnormal fuel flow can indicate FADEC or fuel-system issues.

Hot start

Recognizing abnormal starts is one of the most important skills in turbine-engine operation. A hot start occurs when EGT exceeds the allowable start limit, shown by a rapid EGT rise, EGT exceeding the limit and slow acceleration. Fuel must be cut off immediately, because a hot start can severely damage turbine blades, combustor liners and hot-section components. Causes may include:

• Excess fuel
• Insufficient airflow
• Ignition problems
• Fuel-control malfunctions

Hung start

A hung start occurs when the engine lights successfully but stops accelerating below idle speed, shown by stable EGT, a stable but low N2 and failure to reach idle. Fuel must be shut off and troubleshooting performed before another attempt. Causes include:

• Insufficient starter power
• Weak bleed-air source
• Mechanical issues
• Fuel-control abnormalities

No light-off (wet start)

A wet start occurs when fuel is introduced but combustion never begins, shown by fuel flow present, no EGT rise and no acceleration. Unburned fuel may accumulate within the engine, so for safety the engine must be motored to clear residual fuel before another start attempt. Common causes include:

• Failed igniters
• Ignition-system faults
• Fuel-air mixture problems

Tailpipe fire

A tailpipe fire is fuel burning in the exhaust section rather than inside the combustor. Tailpipe fires require immediate action according to the aircraft procedure. It often results from:

• Excess fuel accumulation
• Wet starts
• Interrupted start sequences

Automatic starts and FADEC protection

Many modern engines use FADEC-controlled automatic starts. The FADEC continuously monitors N2 acceleration, fuel flow, ignition, EGT and time limits, and provides hot-start protection, hung-start detection and no-light-off detection. This automation significantly reduces engine damage risk. If an abnormal condition develops, the FADEC may automatically:

• Shut off fuel
• Abort the start
• Record maintenance messages

Starter duty cycles

Starters generate considerable heat, and continuous operation can quickly overheat the starter and associated components. For this reason, every engine has a specified starter duty cycle. Manufacturers commonly specify a maximum crank duration, a maximum number of start attempts and cooling periods between attempts. A typical requirement might allow several short start attempts followed by an extended cooling period. The exact limits vary by engine model.

Starter failures can be expensive and may create additional engine damage, so always follow the approved maintenance data. Exceeding duty-cycle limits can damage:

• Air turbine starters
• Starter gearboxes
• Starter valves
• Pneumatic components

Ignition systems

The ignition system provides the spark required to ignite the fuel-air mixture. Unlike piston engines, turbine engines use ignition primarily during start and during selected abnormal conditions. Continuous ignition is not normally required during cruise.

Transport aircraft use high-energy capacitor-discharge ignition systems made up of exciters, ignition leads and igniter plugs. The exciter stores electrical energy and releases it as a powerful pulse, producing a spark much stronger than that of an automotive ignition system.

Dual igniters

Most transport engines use two igniters, A and B. This improves starting reliability and ignition redundancy. Depending on aircraft procedures, some starts use one igniter and others use both.

Continuous ignition

Continuous ignition may be selected or automatically activated in the conditions below. The purpose is to immediately relight the engine if combustion is briefly disrupted, providing an additional layer of safety in challenging conditions.

• Takeoff
• Landing
• Heavy precipitation
• Icing conditions
• Severe turbulence
• Volcanic ash encounters

Ignition safety

Ignition exciters can store dangerous electrical energy even after power is removed. Never assume a disconnected exciter is immediately safe, because the stored energy can cause serious shock. Maintenance precautions include:

• Disconnecting power
• Following lockout procedures
• Allowing discharge time
• Verifying safe conditions before maintenance

The auxiliary power unit (APU)

The auxiliary power unit (APU) is a small gas turbine engine, usually installed in the tail section. Although much smaller than the main engines, it performs several critical functions:

• Electrical power: powers aircraft systems when the engines are not operating, for cockpit preparation, passenger boarding and aircraft servicing.
• Bleed air: provides compressed air for engine starting, air conditioning packs, pressurization support and pneumatic systems.
• Backup power: in many aircraft, the APU can provide electrical and pneumatic backup during flight, significantly improving aircraft redundancy.

APU starting sequence

The APU itself usually starts electrically. After startup, the aircraft often transfers from ground power to APU power. The sequence includes:

• Battery or external power energizes the starter.
• The APU accelerates.
• Fuel and ignition are introduced.
• The APU reaches self-sustaining speed.
• The generator and bleed-air systems become available.

APU protection systems

Like the main engines, the APU protects itself automatically. Typical shutdown protections include:

• Overspeed: prevents excessive rotor speed.
• Overtemperature: protects hot-section components.
• Low oil pressure: protects bearings and gearboxes.
• Fire detection: protects the aircraft from APU compartment fires.

APU fire protection

Most APUs include fire-detection loops, fire bottles and automatic shutdown functions.

• On the ground: if an APU fire occurs, the APU normally shuts down automatically and the fire bottle often discharges automatically.
• In flight: crew action is generally required. This prevents unnecessary extinguisher discharge from false indications.

Maintenance considerations

Many start-related problems are first identified through FADEC and maintenance computer reports. Technicians commonly inspect:

• Starter performance
• Ignition leads
• Igniter plugs
• Exciter condition
• Starter duty-cycle compliance
• Bleed-air systems
• APU fire protection
• APU oil system
• Start records and fault messages

Key takeaways

• Most transport aircraft use air turbine starters powered by bleed air.
• Fuel is introduced only after the engine reaches the specified N2 or N3 speed.
• A normal start requires light-off, EGT rise, oil-pressure rise and acceleration to idle.
• Hot starts, hung starts and no-light-off starts are the primary abnormal start conditions.
• Wet starts require motoring to clear accumulated fuel.
• FADEC systems often provide automatic start monitoring and abort functions.
• Starter duty-cycle limits prevent starter overheating and damage.
• High-energy ignition systems use capacitor-discharge exciters and dual igniters.
• Exciters can retain dangerous stored energy after shutdown.
• The APU provides electrical power, bleed air, engine-start capability and system backup.
• Modern APUs include automatic protections for overspeed, overtemperature, low oil pressure and fire.

The bottom line

Starting a turbine engine is a carefully managed process that requires proper airflow, fuel scheduling, ignition and temperature control. Systems such as the FADEC, ignition system and air turbine starter work together to bring the engine safely from rest to idle speed while protecting it from damaging conditions. Supporting these operations is the APU, a small but essential gas turbine that provides electrical power, bleed air, engine-start capability and backup functionality throughout the aircraft's operation.