Engine Start and Ignition Systems

Air Sources for Engine Start

• Aircraft Auxiliary Power Unit (APU)

  • Preference: Highest priority due to no operational cost to the airline.

  • Function: Provides low-pressure, high-volume air for engine starting.

• Ground Start Unit (GSU)

  • Preference: Used if APU is unserviceable or unavailable.

  • Cost: Airlines incur a cost for its use, making it less popular.

• Cross-Bleed Start

  • Principle: Air is sourced from an already started engine on the same aircraft.

  • Cost: No direct cost to the airline.

  • Requirements: Requires power to be applied to the supplying engine.

  • Risks & Restrictions: Carries inherent risks on a crowded ramp due to an energized engine; some airport authorities may be reluctant to grant permission for this operation.

Engine Starter Motor Operation

• Air Supply Path: Air from one of the preferred sources is directed through an electrically controlled air start valve to the starter motor's air inlet.

• Function: The supplied air turns the turbine rotor within the starter motor.

• Exhaust: The air is subsequently exhausted to the atmosphere.

• Power Transfer: The turbine rotor drives a reduction gear, which in turn rotates the engine's high-pressure compressor (HPC) shaft.

• Ratchet Clutch Arrangement: This mechanism connects the starter motor to the HPC shaft. It's designed to disengage the starter motor from the engine after engine start, preventing the engine from driving the starter motor.

  • Danger: If the starter motor were driven by the engine post-start, it would rotate at speeds high enough to cause structural breakup due to centrifugal force.

• Flyweight Cutout Switch: An additional safety device that may be included in the air start valve's control circuit.

  • Function: Automatically terminates the engine start cycle by de-energizing the air start valve, cutting off the starting air supply.

  • Activation: Activates when the engine reaches a speed slightly in excess of its self-sustaining speed.

Generic Twin Spool Medium Bypass Turbine Engine Start Sequence

Monitored Instruments During Start

• Exhaust Gas Temperature (EGT): Critical for monitoring combustion and temperature limits.

• High Pressure Compressor Rotational Speed ($N2$): Indicates compressor acceleration and engine speed.

• Fuel Flow (FF): Confirms fuel delivery to the engine.

• Engine Oil Pressure: Ensures proper lubrication.

• Low Pressure Compressor Rotational Speed ($N1$): Indicates the speed of the low-pressure spool.

• System Duct Pressure: Ensures adequate air pressure for the starter motor.

• Start Valve Open Light: Confirms the air start valve is open and air is reaching the starter.

Step-by-Step Sequence (Using Engine Controls & Instruments)

1. Engine Selection: Normally, the number $2$ engine is started first.

2. Ignition Selection: For the first start of the day, "Right Ignition" is selected.

3. Start Position Selection: The number $2$ engine start switch is selected to the "Ground" position.

   • Action: This opens the APU control valve (if APU is selected as source) and the number $2$ engine air start valve.

   • Indication: The "Start Valve Open" light illuminates, confirming air flow to the starter motor.

4. HPC Acceleration: The starter motor turns the high-pressure compressor, indicated by an increase in $N2$ reading.

   • Timing: This is when to start timing the starter motor duty cycle.

5. Oil Pressure: As $N2$ increases, oil pumps build sufficient pressure to extinguish the low oil pressure light.

6. Fuel and Ignition Activation: At $24$, the number $2$ engine start lever is moved to the "Idle" detent.

   • Action: This supplies fuel to the engine and activates the AC ignition system.

   • Safety: The operator should keep a hand on the start lever to immediately close it in case of an engine starting fault.

   • Multi-spool Engines: For twin or triple spool engines, ensure other spools rotate in the correct direction, especially with a tailwind.

7. Light Up: A short time after opening the start lever, the engine should light up.

   • Indication: A sharp increase in EGT and an increased rate of acceleration of $N2$.

8. Automatic Cutoff: At approximately $46$, the start switch automatically moves to the "Off" position, and the "Start Valve Open" light extinguishes.

   • Starter Duty Cycle: The starter motor's duty cycle is now complete.

9. Idle Acceleration: $N2$ continues to increase to the ground idle value, typically between $58$ to $62$.

10. Stabilization & Checks: Once engine RPM stabilizes at ground idle, the fuel control and ignition switch (if appropriate) can be released, and after-start checks performed.

Graphical Representation of Normal Engine Start (RPM vs. EGT)

(Using APU as start air source)

1. Start Selection: Engine start selected, air flows from ducting to starter motor via air start valve.

2. Compressor Acceleration (Starter Only): Compressor accelerates under starter motor's influence, forcing air through combustion chambers. No fuel or ignition yet.

3. Fuel & Ignition Activation: At a specific HPC RPM (e.g., $24$ as a specific example, or $N2$ is also understood as the primary monitoring parameter for activation), the start lever is moved to the idle detent, activating fuel supply and ignition.

4. Engine Light Up: Indicated by a notable increase in EGT, which must occur within a specified time (e.g., $20$ seconds typical).

   • Initial EGT Spike: The first increase in EGT is sharp due to an initial buildup of fuel in the combustion chamber before ignition. Once ignited, this excess fuel temporarily overpowers the cooling effect of compressor air.

   • EGT Stabilization: After the excess fuel burns off and the fuel-air mixture balances, the EGT increase steadies.

5. Continued Acceleration: The produced hot gas imparts impetus to the turbine blades, reducing the load on the starter motor, and the engine continues to accelerate.

6. Second EGT Rise: The accelerating engine drives high-pressure fuel pumps faster, increasing fuel flow. Before the compressor reaches idle RPM, there may be a temporary imbalance of too much fuel and insufficient cooling air, leading to a second steep rise in EGT.

7. Self-Sustaining Speed: The engine accelerates to self-sustaining speed (approximately $36$).

   • Starter Retention: The starter motor remains active beyond self-sustaining speed to assist in further acceleration, helping the engine reach idle RPM without exceeding EGT limits.

8. Automatic Shutdown of Starter/Ignition: Igniters and air supply to the starter motor are automatically canceled by an $N2$ speed switch at around $40$. As acceleration continues, the EGT peaks.

9. Idle EGT & RPM: As airflow and fuel flow balance at their idle RPM values, the EGT drops to a level appropriate for idle RPM.

   • Ground Idle RPM: Approximately $60$ and $25$.

Engine Starting Malfunctions and Procedures

1. Motoring Over / Blowout Cycle

• Purpose: Required after an unsuccessful start attempt to evaporate or blow out any residual fuel from the engine.

  • Prevention: Prevents "torching" (a large jet of flame from the exhaust) caused by excess fuel igniting during a subsequent start attempt.

• Procedure: The starter motor is activated without selecting fuel or the ignition system.

• Duty Cycle: A typical air turbine starter motor has a duty cycle of $2$ minutes "on" followed by a $5$ minute cooling-down period. Always refer to the specific engine manual for exact duty cycles.

2. Airborne Relight (Flameout)

• Circumstances: When an engine flames out during flight.

• Procedure: It may only be necessary to initiate fuel flow and activate the ignition system without operating the starter motor.

  • Reason: The engine should already be "windmilling" (rotating from airflow), eliminating the need for the starter motor.

• Example (Specific Aircraft): If both ignition systems are selected and the appropriate engine selector is moved to the "Flight" position, a relight should occur when the failed engine's start lever is moved to the "Idle" detent.

• Evidence of Success: A rise in EGT and/or $N2$ indicates a successful relight.

3. Hot Start

• Identification: Only identifiable by comparing indications to a normal start.

  • Symptoms: EGT initially rises normally, followed by a rapid, uncommanded rise towards the engine's EGT limit a few seconds after light up.

• Immediate Action: The only chance to prevent exceeding the temperature limit is to immediately switch off the engine's fuel supply. Hesitation can lead to engine damage.

  • Practical Implication: Keep a hand on the fuel supply control during start.

• Consequence of Exceedance: If EGT exceeds the limit by even $1$ degree, the engine is considered unserviceable.

• Common Causes: Primarily an imbalance of too much fuel or insufficient cooling air.

  • Excess Fuel: Throttles not set to idle during pre-flight or being knocked from the idle position.

  • Insufficient Cooling Air: Engine not rotating fast enough due to:

    • Low duct pressure (insufficient starter motor speed).

    • Ice accretion within the compressor annulus causing partial seizure.

    • Aircraft positioned with a tailwind (wind going up the jetpipe instead of into the intake), making the starter's job harder and reducing cooling airflow.

    • Residual heat in the engine after shutdown exacerbating tailwind issues.

4. Wet Start (Failure to Light Up)

• Identification: EGT does not rise, and RPM stabilizes at the maximum speed achievable by the starter motor.

  • Diagnosis: If the engine fails to light up within the required time limit (usually about $20$ seconds).

• Initial Action (after diagnosis): Deselect the ignition supply and shut off the fuel supply.

• Dangers: If the accumulated fuel is ignited (e.g., by a delayed light up or a subsequent start without a blowout):

  • Torching: A very large jet of flame from the exhaust system.

  • Uncontrollable EGT: EGT rise may be uncontrollable and exceed limitations.

• Procedure When Diagnosed: Do not immediately terminate the start cycle if a wet start is diagnosed early. Instead:

  1. Properly shut off the high-pressure fuel supply.

  2. Deselect ignition.

  3. Allow the starter to continue turning the compressor for the remainder of that start cycle.

  • Benefit: This allows the compressor to begin blowing out fuel even before a proper blowout cycle is carried out.

  1. Then perform a blowout cycle before attempting a second start.

• Causes:

  • Lack of Ignition: Though ignition units are generally reliable, they can fail.

    • Clue: A high-energy ignition unit's operation may be heard as a background buzz (asynchronous ticking) on the intercom system if operational (or during relight facility selection).

  • Lack of Fuel: Unlikely if the engine was working perfectly before shutdown. Can be confirmed by checking the fuel flow meter when the start lever is operated.

5. Hung Start

• Identification: EGT is higher than expected for the engine's stabilized RPM, and the RPM is lower than the engine's self-sustaining speed.

  • EGT Limit: The high EGT is usually not greater than the engine's limit.

• Consequence: Maintaining the engine in this state is detrimental and can cause significant harm.

• Action: If a hung start is indicated, the high-pressure fuel supply must be shut off, and the problem investigated.

• Usual Cause: Insufficient airflow through the engine to support efficient combustion.

  • Reasons for Insufficient Airflow:

    • Contaminated Compressor: At high-altitude airfields, a contaminated compressor's inefficiency becomes critical.

    • Insufficient Starter Motive Power: Low duct pressure prevents the starter motor from rotating the compressor fast enough.

    • Insufficient Gas Power: Gases generated in the combustion chambers lack sufficient power to assist the starter in accelerating the engine beyond self-sustaining speed.

    • Outcome: Once the starter motor cycle finishes, engine RPM remains stable below the figure required to accelerate to ground idle speed.

Engine Rundown Time / Spool Down Time

• Definition: The time taken for the engine to stop rotating after the high-pressure fuel supply is shut off.

• Monitoring: After a flight, a mental note should be taken of each engine's rundown time, and these times compared.

• Significance: An engine with an appreciably shorter rundown time than others may indicate an impending malfunction.

Ignition Systems

Continuous Ignition

• Purpose: Energizes the low-energy mode of the igniters ($3-6$ joules) for specific flight conditions.

• Activation:

  • Usually by selecting the appropriate switch on the engine start panel.

  • Some aircraft have an automatic system where the aircraft stall warning system automatically selects continuous ignition upon stall detection.

• Conditions for Use:

  • Takeoff and landing (in case of relight necessity, e.g., very wet runway, ingesting water, preventing flameout).

  • Flying into heavy weather.

• Deactivation: Igniters are automatically deactivated by a speed switch incorporated in the high-pressure compressor RPM indicator system, typically after self-sustaining speed is reached.

High Energy Ignition Units

• Output: Approximately $12$ joules (for starting selection).

• Erosion: High energy output causes rapid erosion of igniter plugs, dramatically shortening their working life.

• Combination Emission Systems: To minimize erosion, some engines utilize a combination system:

  • Low energy ($3-6$ joules) for continuous selection.

  • High energy ($6-12$ joules) for starting selection.

• Principle of Operation: Works by charging a very large capacitor and then discharging it across the face of an igniter plug.

• Safety Features (Due to Lethal Potential):

  • Discharge Resistors: Allow residual charge in the capacitor to leak away to earth once electrical power to the unit is disconnected.

  • Backup Safety Resistors: Act as a safety valve if the igniter plug becomes disconnected. They allow energy in excess of the normal level to flow through them, preventing a potentially disabling explosion that would occur if energy continued to build up in the capacitor.

• Internal Components & Function:

  • Discharge Gap: Located within an evacuated tube (ensures constant spark power regardless of humidity or altitude).

  • Choke: Acts as an inductance to slow down the current flow, which prolongs the duration of the spark.

• Spark Rate: Normal output is between $60$ to $100$ sparks per minute, though sparks are produced randomly.

• Acoustic Clue: High energy outputs can sometimes be heard on the intercom system as a background buzz or an asynchronous ticking noise, indicating igniter unit operation.

Igniter Plug Types

• Older Type:

  • Similar to a piston engine spark plug, but with a much bigger spark gap.

  • Requires approximately $25,000$ volts to jump the gap.

  • Demands very high insulation standards within the unit and cabling.

• Modern Surface Discharge Igniter Plug:

  • The end of the insulator is formed from a semiconductor material.

  • Mechanism: Electrical leakage occurs between the hot electrode and the igniter body, ionizing the semiconductor surface. This provides a relatively low resistance path for the capacitor's stored energy.

  • Discharge: Takes the form of a high-intensity flashover from the hot electrode to the body, requiring only approximately $2,000$ volts.

Miscellaneous

• Thrust Reverser Deployment in Flight: Prevented by the "weight on wheel switch" system. It ensures thrust reversers cannot be deployed unless the aircraft is on the ground.