M — Powerplant (Groupe motopropulseur)Chapter 9 · 44 practice questions

Chapter 9: Turbine Engine — Ignition & Starting

Includes 8 animated diagrams — view them live in the interactive theory reader.

Turbine Engine — Ignition & Starting

Overview

This chapter covers the theory, components, operation, and maintenance of turbine engine ignition and starting systems. These systems are critical for reliable engine starts on the ground and in flight, and they incorporate safety features to prevent damage during abnormal start conditions. The chapter also addresses the regulatory framework governing maintenance releases, technical records, and component eligibility under Canadian Aviation Regulations (CARs).

Diagram — Turbine Engine — Ignition & Starting Système d'Allumage et de Démarrage — Moteur à Turbine Légende Alimentation électrique Commande / Signal Sortie haute tension Composant système Batterie / Bus DC 12–24 V CC Interrupteur Allumage / Démarrage Excitateur d'Allumage Convertisseur DC → HT Câble Haute Tension Isolé, blindé 15–20 kV Bougie d'Allumage Électrode centrale Étincelle de surface Chambre de Combustion Allumage du carburant Démarrage moteur Démarreur Moteur électrique Boîte d'Engrenages Entraînement du rotor HP Rotor HP N2 — Mise en rotation Alimentation DC HT Étincelle Allumage Couple mécanique Rotation Note technique : Le système d'allumage haute tension (15–20 kV) produit une étincelle à la bougie pour enflammer le mélange air-carburant dans la chambre de combustion. Le démarreur entraîne le rotor HP via la boîte d'engrenages jusqu'à la vitesse de démarrage (environ 20–25 % N2). Schéma fonctionnel — Système d'allumage et de démarrage (moteur à turbine)

Key Concepts Explained in Detail

High-Energy Ignition System Components

High-energy ignition discharge High Energy Ignition Discharge EXCITER (Ignition Exciter) Capacitor Transformer Vibrator (pulsed flow) DC 12-24 V Output: 20,000 – 30,000 V HIGH-TENSION CABLE Shielded (High-Tension Lead) Critical insulation (prevents short circuits) Shielding braid Pulse SPARK PLUG (Igniter Plug) Fixed gap (non-adjustable) HT Spark very hot COMBUSTION CHAMBER Air-fuel mixture Difficult conditions: high altitude, cold, low fuel pressure CAPACITIVE DISCHARGE IGNITION (CDI) SYSTEM 1. Exciter receives DC from aircraft 2. Vibrator creates a pulse 3. Transformer steps up the voltage 4. Capacitor stores the energy 5. Discharge through plug → spark ⚠ SAFETY — MAINTENANCE Disconnect ignition leads from spark plugs before any maintenance. Ignition switch in OFF position. 1 2 3 Component Combustion zone

A typical high-energy turbine engine ignition system consists of three primary components:

  • Ignition Exciter (Ignition Unit): Converts low-voltage DC aircraft power (typically 12-24V) into a high-voltage, high-energy pulse. The exciter stores electrical energy in a capacitor and discharges it through a transformer to produce a spark. Output voltage is typically in the range of 20,000 to 30,000 volts.
  • Igniter Plug: The spark-producing component located in the combustion chamber. Unlike spark plugs in reciprocating engines, turbine igniter plugs are designed to withstand extreme temperatures and pressures. Most are "set gap" types where the electrode gap cannot be adjusted.
  • High-Tension Lead: A heavily insulated cable that carries the high-voltage pulse from the exciter to the igniter plug. The insulation must be intact to prevent short circuits to ground.

Note: Glow plugs are not used in high-energy turbine ignition systems. They are found in some diesel engines and small reciprocating aircraft engines.

Capacitor Discharge Ignition (CDI) Systems

Capacitor Discharge Ignition (CDI) Circuit Capacitive Discharge Ignition (CDI) Circuit High-Energy Ignition System — Turbine Engine DC Source 12–24 V (aircraft) DC Vibrator Electromechanical Interrupts DC → pulsed current pulsed Transformer Steps up voltage to 20,000–30,000 V HT Rectifier Converts AC to DC Capacitor Stores energy at high voltage Capacitor charging Spark Plug Fixed gap (non-adjustable) Discharge High-energy spark Operation: 1. Vibrator pulses the DC current 2. Transformer steps up the voltage 3. Rectifier converts to DC 4. Capacitor stores the energy 5. Discharge through the plug → spark Sequence: DC → Pulsation → HT → Charge → Discharge AME Training — Transport Canada — M-Powerplant ch9: Turbine Engine Ignition and Starting

Many turbine engines use CDI systems. In these systems:

  • A vibrator is an electromechanical device that interrupts DC current to create a pulsating flow.
  • This pulsating current is transformed and rectified to charge a capacitor.
  • When the capacitor discharges, it produces a high-energy spark through the igniter plug.
  • The vibrator is a mechanical component and is a common source of intermittent faults (e.g., sticking points).

Ignition System Operation During Start

During a normal ground start:

  • The ignition system is activated at the beginning of the start sequence.
  • It produces a continuous spark in the combustion chamber.
  • Once the engine reaches a self-sustaining speed (typically around 50% N2), the ignition system is deactivated.

Continuous ignition is selected only during specific adverse conditions such as heavy rain, snow, or icing, to prevent flameout. It is also used during takeoff and landing in these conditions.

Abnormal Start Conditions

Abnormal starts Abnormal Starts — EGT/N2/Fuel Signatures and Immediate Action Normal Start (Reference) EGT N2 Fuel Ignition active Stable acceleration to idle Hot Start EGT limit EGT OVERSHOOT EGT exceeds max limit shortly after ignition ACTION: Throttle — idle/shut down, fuel cutoff Hung Start N2 stagnates — no acceleration toward idle ACTION: Fuel cutoff, motor purge, investigate Wet Start Fuel injected but NOT ignited (accumulation) No EGT rise — fuel accumulating ACTION: Immediate fuel cutoff, purge, check ignition No Light-Off EGT remains at ambient temperature — no combustion ACTION: Fuel cutoff, check ignition and spark False Start Flame goes out N2 continues (driven by starter) ACTION: Monitor, fuel cutoff if EGT does not recover SUMMARY TABLE — SIGNATURES AND ACTIONS EGT N2 Fuel Immediate Action Hot Start Exceeds limit Stagnates or drops Too rich Reduce fuel, shut down Hung Start High, stable Stagnates (plateau) Insufficient Fuel cutoff, purge Wet Start No rise Increases Accumulates Immediate fuel cutoff No Light-Off Ambient temp Increases Injected Fuel cutoff, check
  • Hot Start: Occurs when too much fuel is introduced relative to airflow, causing an excessively high exhaust gas temperature (EGT) rise. The immediate action is to shut down the engine by cutting off fuel flow.
  • Wet Start: Occurs when fuel is introduced into the combustion chamber but fails to ignite. Unburned fuel accumulates and may ignite later, causing a hot start or tailpipe fire. Common causes include delayed ignition or fuel introduced before the ignition system is armed.
  • Hung Start (Stalled Start): The engine lights off (EGT rises) but fails to accelerate to idle speed. The engine may accelerate to a certain N2 and then stop or decelerate. A failed or slow-responding fuel metering valve is a primary suspect.
  • False Start: EGT rises normally but then suddenly drops back to ambient temperature while N2 continues to increase (due to starter assistance). The flame has been extinguished.

Starter System Components

Turbine Starter System Components Starting System: Pneumatic Starter and Components AIR SOURCE APU / Ground / Cross-bleed STARTER Electric Control valve of the starter air Clutch of the starter air ENGINE N2 rotating drives rotation Cutout switch ~50 % N2 Start lever (fuel shutoff valve) fuel Typical starting sequence 1. Activation Start switch operated by pilot 2. Valve opens Control valve opens → air to starter 3. Engine rotation Starter drives the engine → N2 increases 4. Fuel Lever in start position → shutoff valve open 5. Ignition + injection Ignition system activated Fuel ignited → EGT ↑ 6. Acceleration Engine accelerates to idle speed 7. Starter cutout At ~50 % N2, the cutout switch disengages 8. Ignition off Ignition system turned off after stabilization Pressurized air Fuel Mechanical drive Signal / cutout The pneumatic starter uses air from the APU, a ground cart, or cross-bleed between engines to rotate the engine via a turbine. The electric starter is an alternative: it uses an electric motor to rotate the engine.
Turbine start sequence Turbine Start Sequence From starter to engine self-sufficiency — N2, ignition, fuel, light-off 1. STARTER Valve open / electric motor Air flow 2. N2 ROTATION N2 increasing (15-25%) 3. IGNITION Exciter → spark plug 20-30 kV 4. FUEL Throttle to idle Shutoff valve open 5. LIGHT-OFF Mixture ignited EGT rising 6. ACCELERATION N2 to idle (~50% N2) 7. STARTER CUT-OFF Cut-off switch Clutch disengaged 8. IGNITION OFF Ignition system deactivated 9. IDLE Engine stable Self-sufficient Key parameters to monitor during start N2 (%) Compressor rotation EGT (°C) Gas temperature Fuel pressure Fuel flow Oil Pressure / temp. Abnormal starts — Recognize and respond Wet: fuel without ignition Hot: EGT exceeds limit Hung: N2 stalls below idle False: EGT rises then drops
  • Starter Motor: Can be pneumatic (air turbine) or electric. In pneumatic systems, bleed air from an APU, ground cart, or cross-bleed drives the starter.
  • Starter Control Valve: Regulates the flow of bleed air to the air turbine starter motor.
  • Starter Cutout: Disengages the starter once the engine reaches a self-sustaining speed (typically around 50% N2). This prevents the starter from being overdriven.
  • Start Lever (Fuel Shutoff Lever): The pilot's control that opens or closes the fuel shutoff valve, allowing fuel to flow to the engine.

Troubleshooting Logic

Troubleshooting a "no light-off" (failure to ignite) Decision tree for troubleshooting a no light-off on a turbine engine: checking the spark, then the fuel supply, then rotation. Troubleshooting a "no light-off" (failure to ignite) 1 No ignition observed (No light-off) 2 Check spark Exciter → HT cable → spark plug YES ✓ 3 Check fuel Valve → FCU → injectors NO ✗ Faulty spark: Replace the exciter, HT cable, or spark plug — check continuity and gap Retest No fuel: Check shutoff valve, FCU, injectors YES ✓ 4 Check rotation Starter → clutch No rotation: Check starter, valve, clutch Retest Normal start after correction Follow the start sequence in the manual YES / functional NO / faulty

When troubleshooting a "no start" condition, follow a logical sequence:

  1. Confirm ignition system is functioning (spark present)
  2. Check fuel delivery (fuel control unit, fuel nozzles, shutoff valve)
  3. Verify engine rotation (starter engagement, mechanical connection)

Common failure scenarios:

Important Procedures and Regulations

Maintenance Release Requirements (CARs 571.10 & Standard 571)

A maintenance release is required after:

  • Replacing an igniter plug
  • Performing any maintenance on ignition system components

Who can sign: Only a person who:

  • Holds an appropriate AME license (with the M rating)
  • Is authorized by the AMO (Approved Maintenance Organization)

Requirements before signing:

  • Work must be completed and component installed correctly
  • Visual inspection (gap, torque, lead connection) must be performed
  • Continuity check of leads is standard practice
  • Maintenance record entry must be completed per Standard 571.03

Note: A ground run is not always required. The release can be based on the work performed.

Technical Record Requirements (Standard 571.03)

Mandatory information for a maintenance record entry includes:

  • Brief description of the work performed
  • Product identification (part number/serial number)
  • Date of maintenance
  • Employee identification

Must also record: Any defect found prior to re-assembly (e.g., chafed ignition lead found during inspection).

Not required: Diagnosis of the reason for failure (though good practice).

Component Eligibility (Standard 571.07)

Before installing any new part, the AME must ensure it:

  • Conforms to the type design
  • Is in a safe condition

If a part number has been superseded, the AME must verify eligibility using manufacturer's data (service bulletins, parts catalogs) to ensure the superseded part is an acceptable alternative.

Repair Classification (Standard 571.06)

  • Major Repair: Deviates from type design and has other than negligible effect on airworthiness.
  • Minor Repair: Performed exactly per manufacturer's instructions (approved data); does not deviate from type design.

Manufacturer Data Authority (Standard 571.02)

Where aircraft manufacturer recommendations are incompatible with engine, propeller, or appliance manufacturer recommendations, the aircraft manufacturer's recommendations shall be used.

Safety Precautions

When performing maintenance on high-energy ignition systems:

  1. Disconnect ignition leads from igniter plugs to prevent accidental discharge
  2. Ensure ignition switch is in the OFF position
  3. The exciter capacitor can hold a lethal charge for a long time
  4. Follow safety procedures outlined in aircraft and engine maintenance manuals

Igniter Plug Maintenance

  • Gap: Most igniter plugs are "set gap" type and cannot be adjusted. If out of limits, replace the plug.
  • Torque: Use a torque wrench calibrated in the same units as the specification. A foot-pound wrench should not be used for inch-pound applications due to accuracy concerns.
  • Inspection: An eroded, rounded tip requires replacement. The ignition lead should also be inspected.
  • Loose bushing: Requires replacement of the bushing and inspection of the liner for cracks or deformation.
  • Fouling: Carbon deposits from wet starts can bridge the electrode gap.

Test Equipment Requirements

If required test equipment is not available:

  • Do not perform the task in a non-compliant manner
  • Follow the AMO's Maintenance Control System procedures
  • May use alternate methods approved by the manufacturer or delegate

Common Relationships Between Concepts

Ignition System and Fuel System Interdependence

Ignition-fuel interdependence Ignition-Fuel Interdependence Simultaneous conditions required for successful turbine engine start Combustion chamber 1. Sufficient rotation N2 > 20-25% via starter 2. Atomized fuel Injectors — fine spray 3. High-energy spark CDI exciter — 20,000-30,000 V 4. Airflow Compressor — continuous air flow All 4 conditions must be simultaneous — otherwise: abnormal start Wet start or hot start possible Suspended start or false start CDI exciter + Spark plug Fuel shutoff valve + Injectors Pneumatic or electric starter Engine compressor Legend Fuel Spark Air Rotation 1 2 3 4 ∩ simultaneous

The ignition system and fuel system work together during start. A "no light-off" condition can be caused by either system:

  • Ignition system: No spark (faulty exciter, lead, or plug)
  • Fuel system: No fuel (faulty fuel control, blocked nozzles, shutoff valve)

Starter System and Engine Acceleration

The starter provides initial rotation (typically to 15-25% N2) for light-off. After light-off, the engine must accelerate on its own power. The starter cutout disengages the starter at approximately 50% N2.

Abnormal Starts and Their Causes

Dual Ignition Systems

Dual ignition systems have two independent channels. Failure of only one channel points to a component unique to that channel (exciter, igniter plug, or high-tension lead). A common power supply or switch would affect both channels.

Maintenance Release and Technical Records

The maintenance release certifies that work is complete and satisfactory. The technical record entry must be completed before the release can be signed. The release is typically part of or directly associated with the maintenance record.

Diagram

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