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Chapter 10: Gas Turbine Engine Support Systems

Ignition System

  • Purpose and nature:
    • In a gas turbine engine, ignition is generally simpler than in piston engines because the engine is self-sustaining once running; it does not require a continuous spark or precise timing throughout the cycle.
    • However, a very powerful spark is still required at start, typically achieved with a high-energy ignition system.
  • How the ignition energy is supplied:
    • Low-voltage current is supplied from an aircraft power source (APU or battery) to the ignition unit.
    • This is converted to a high-voltage current to generate a powerful spark.
    • The high-energy spark is stored to allow a rapid energy release when needed.
  • Delivery to the combustion chamber:
    • The high-voltage current is delivered to the ignitor plug in the combustion chamber via high-voltage ignition leads.
    • Ignitor plugs in gas turbine engines are designed to handle much higher energy levels than piston-engine spark plugs.
  • Redundancy:
    • Most engines have two independent ignition systems for reliability in case of a failure.
    • This typically means two ignitor plugs located in different positions within the combustion section.
  • Operation during flight:
    • The ignition system is usually only required for a short period during engine start.
    • Once air is flowing smoothly and the engine is self-sustaining, constant ignition is not required.
  • Conditions that disrupt airflow and increase flameout risk:
    • Sudden changes to the aircraft’s pitch (e.g., turbulence).
    • Ice or water ingestion into the engine.
    • Disruptions in the airflow can extinguish the fuel–air mixture (flameout).
  • Continuous ignition (to reduce flameout risk):
    • Continuous ignition can be activated to prevent flameout during conditions that disrupt airflow.
    • Used during critical flight phases such as take-off and approach, and in icing or severe turbulence conditions.
    • Automatic activation can occur when engine anti-icing is on or when stall warning is activated.
  • Energy output considerations during continuous ignition:
    • Some ignition units may have a lower energy output during continuous ignition to prolong plug life.
    • Some systems may use a separate low-energy plug for continuous operations.
  • Case note (illustrative caution): even with continuous ignition, some conditions can overwhelm the engine, as shown in a case study.

Case Study: 24 May 1988 – Boeing 737 over New Orleans

  • Scenario:
    • A brand-new Boeing 737 during an approach to New Orleans encountered substantial thunderstorm activity.
    • Pilots selected continuous ignition and activated engine anti-icing.
    • While descending through 16,000 ft, both turbofan engines flamed out after ingesting a large amount of water with low RPM.
  • Actions and outcome:
    • Pilots attempted to restart but were unsuccessful.
    • They conducted an emergency landing, selecting a grass area beside a canal for a safe off-field landing.
  • Significance:
    • Illustrates how severe weather and water ingestion can overwhelm ignition and airflow robustness, underscoring the importance of continuous ignition strategies during high-risk phases.

Fuel Control Unit (FCU)

  • Purpose:
    • The FCU regulates the amount of fuel delivered to the combustion section to control engine thrust.
    • Proper fuel delivery is essential across a wide flight envelope, from startup through rapid acceleration or deceleration.
  • What the FCU monitors and responds to:
    • Mass of air flowing through the engine.
    • Changes in air temperature and pressure.
    • Pilot’s thrust setting or autothrottle setting.
  • Startup and dynamic response:
    • During start-up, the FCU carefully delivers fuel to light the engine.
    • During rapid changes in air flow (acceleration/deceleration), the FCU must react quickly to maintain the desired thrust.
  • Location:
    • The FCU sits between the fuel tank and the engine, allowing it to control the fuel that reaches the engine(s).
  • Fuel spray nozzle design implications:
    • The fuel spray nozzle designs are chosen based on engine burning requirements.
    • Some nozzles are designed to be efficient over a wide range of fuel flows (useful for large engines where fuel flow varies significantly).
    • Some designs include a swirl chamber to help disintegrate fuel into tiny droplets for better burning.
  • Relationship to combustion:
    • The FCU helps ensure the fuel enters the combustion chamber in the right condition to promote efficient combustion.
  • Reference in material:
    • The FCU’s role is depicted conceptually in Figure 10.1: The Fuel Control Unit (FCU) ensures sufficient fuel is supplied to the engine.

Lubrication and Cooling

  • General purpose:
    • Like piston engines, lubrication and cooling in gas turbine engines protect moving parts, remove heat, and keep contaminants out.
    • Demands are higher due to higher speeds and temperatures.
  • Recirculatory oil system:
    • Most gas turbine engines use a recirculatory oil system that pressurizes oil to lubricate key components and returns it to an oil tank via a scavenger pump.
  • Two basic oil system configurations:
    • Pressure relief valve system:
    • Oil flow to the bearing chamber is controlled by a relief valve that opens at a preset pressure, allowing excess oil to return to the tank.
    • As engine speed increases, bearing chamber pressure rises; some systems adjust the relief valve setting with speed to maintain constant bearing lubrication.
    • Not suitable for engines with very high bearing chamber pressure (common in turbofan engines).
    • Full-flow system:
    • Oil is delivered directly to oil jets via a small pump; no pressure-relief valve controlling flow to the bearings.
  • Gearbox lubrication (reduction gearbox):
    • Turboprop and turboshaft engines usually have a separate oil system for the reduction gearbox.
    • The reduction gearbox experiences heavy loads and benefits from higher-viscosity oil.
  • Filtration and cooling:
    • The lubrication system includes an oil filter to remove contaminants and an oil cooler to remove heat.
  • Fuel–oil heat exchange (FOHE):
    • In some aircraft, oil cooling is integrated with the fuel heating system via a FOHE.
    • Operation: passes fuel through a metal tube surrounded by hot oil; this heats the fuel and cools the oil, improving fuel temperature and reducing oil temperature.
  • Fuel freezing and the Heathrow 2008 incident (illustrative safety point):
    • A British Airways 777 on a Beijing–London flight encountered engine failures just before landing.
    • It had been exposed to extremely cold cruise temperatures:
    • The fuel had a freezing point of -47^ ext{^ alseC} ext{C}, while ambient was as low as -74^ ext{^ ext{0}}^ ext{C} (converted to proper notation below).
    • Although the fuel did not freeze, small amounts of water in the fuel froze into ice and clogged fuel lines, restricting flow near the end of the flight.
    • The incident underscored the importance of fuel conditioning and heating systems to prevent fuel lines from icing. (Note: Corrected units for clarity: -74^"C is not a standard representation; typical safe notation would be -74^ ext{^ deg} ext{C} and the fuel freezing point given as -47^ ext{^ deg} ext{C}.)
  • FOHE and temperature management rationale:
    • High engine temperatures are required for maximum thrust, so cooling and materials selection are critical.
    • Turbine cooling often relies on directing cooler air through internal passages to protect turbine blades and nozzle guide vanes.

Bleed Air

  • Primary role of bleed air:
    • Bleed air is used to support aircraft systems beyond thrust, such as pressurized cabins and ice protection.
  • How bleed air is obtained and used:
    • Bleed air is taken from the compressor stage of the engine.
    • It is conditioned (temperature adjusted) before entering cabin systems and other aircraft systems.
  • Downsides of bleed air:
    • Using bleed air reduces the amount of air available for thrust; in some cases, more than 20% of air entering the engine may be used for systems other than thrust.
    • Engines typically have excess thrust, so some bleed air extraction can be tolerated, but during performance-limited phases (e.g., short-runway takeoffs) bleed air may be reduced or valves closed to preserve thrust.
  • Practical implications:
    • Bleed air management is a trade-off between cabin/system needs and available engine thrust.

Ice Protection Systems and Pressurized Air

  • Ice protection systems:
    • Any aircraft flying in icing conditions requires an ice protection system.
    • Systems include heated electrical elements and inflatable de-icing boots.
    • Gas turbine engines commonly use a thermal ice protection system that uses warm air bled from the engine to melt or prevent ice accumulation.
    • Ice protection is applied to critical engine surfaces (e.g., parts of the engine like the nose cowl, fan nose cone, and the first set of stator blades).
    • Anti-icing is activated before icing conditions, while de-icing is activated once ice has formed.
  • Pressurized air (cabin pressurization):
    • Pressurized cabin systems rely on bleed air to maintain cabin pressure at altitude.
    • Bleed air is conditioned to appropriate temperature before entering the cabin.

Reverse Thrust

  • Purpose:
    • Reverse thrust helps slow the aircraft after landing, supplementing wheel brakes, especially on slippery runways.
  • How reverse thrust works by engine type:
    • Turboprop: uses propeller blade angle adjustment to generate a backward force.
    • Turbojet/T turbofan: deflects exhaust flow to reduce forward thrust or generate reverse thrust by redirecting airflow at certain engine sections.
  • Methods for turbojet/turbofan reverse thrust (two main approaches):
    • Clamshell reverser: large clamshell plates deflect exhaust gases by rotating to block the normal exit and exposing a side duct for deflected gases; plates require heat-resistant material.
    • Bucket reverser: external buckets rotate to block the end of the normal exhaust, deflecting exhaust gases similarly; also uses heat-resistant materials.
  • Cold-stream reverse thrust in high-bypass engines:
    • These engines use a cold stream reverser that deflects cooler bypass air; the hotter exhaust flow remains largely unaffected and contributes little to thrust reversal.
    • Operation involves translating cowls and a blocker door that redirects bypass air through cascade vanes to produce reverse thrust.
    • Components in cold-stream reversers are typically lighter due to the use of cooler air.
  • Safety features and incidents:
    • Reversers have safety mechanisms to prevent inadvertent deployment in flight.
    • In-flight deployment incidents have occurred due to electrical faults, leading to loss of pitch control or other destabilizing effects.
  • Notable case studies:
    • 26 May 1991 – Boeing 767 (Bangkok–Austria): left engine reverser deployed in climb, causing disrupted airflow over the wing, leading to a stall and breakup; cause linked to an electrical issue in the circuit designed to prevent in-flight deployment.
    • 31 October 1996 – Fokker 100 (Brazil): right engine reverser deployed after takeoff due to an electrical fault; safety system reduced throttle on the affected engine to idle, which the pilots did not know about; resulted in asymmetric thrust and a crash.

Modern Engine Health Monitoring and Summary

  • Modern gas turbine engines rely on integrated support systems to handle harsh operating conditions and ensure continued operation.
  • Engine health monitoring and cockpit instruments:
    • A range of instruments provides telemetry and health information to pilots.
    • This information enables early intervention if something is wrong, helping to prevent more serious failures.
  • Final takeaway:
    • The support systems (ignition, FCU, lubrication/cooling, bleed air, ice protection, and reverse thrust) work together to maintain engine performance, reliability, and safety across all flight regimes and environmental conditions.
  • Next topic hint:
    • The chapter will further explore engine starting and other related systems in the upcoming sections.