MD

Gas Turbine Engine Management

Engine Controls

  • Gas turbine engines differ from piston engines in how thrust is produced and how engine settings are expressed.

  • Key objective: manage starting procedures and monitor common problems to keep the engine within safe operating limits.

  • General scope: cover common aspects of gas turbine management, not every engine-specific detail.

  • Engine Pressure Ratio (EPR)

    • EPR provides an accurate measure of thrust output and is often the primary basis for thrust settings.
    • Definition: EPR = \frac{P{\text{exhaust}}}{P{\text{intake}}} where pressure probes are located in the intake and exhaust sections.
    • Typical example values: EPR \approx 1.5, 1.85, 2.10 (varies by engine and operating conditions).

  • Engine RPM and spool architecture

    • Gas turbine engines indicate the rotational speed of the spools (compressor/turbine assemblies connected by a shaft).
    • If an engine does not provide EPR, RPM is the primary thrust reference.
    • Spools in multi-spool engines are denoted as N1, N2, (and N3 in three-spool engines).
    • Two-spool engine:
    • N_1 refers to the fan/low-pressure compressor and their corresponding turbine (low-pressure spool).
    • N_2 refers to the high-pressure compressor and its turbine (high-pressure spool).
    • Three-spool engine: N3 is the high-pressure spool, with N2 as the intermediate spool and N_1 as the fan/low-pressure spool.
    • Spool speeds are usually expressed as a percentage of the maximum normal operating RPM, not the absolute RPM.
    • Example: if the maximum normal RPM is N{ ext{max}} = 30{,}000\,\text{rpm} and the N1 spool is at 15{,}000\,\text{rpm}, then N_1 = 50\%.

  • Engine Torque

    • In turboprop engines, thrust is largely produced by the spinning propeller, so a torque indication (torquemeter) is provided.
    • Torque is a useful indicator because it is proportional to the horsepower produced by the engine.

  • Engine Health and monitoring instruments

    • Common engine health instruments include oil pressure and oil temperature (similar to piston engines).
    • Exhaust Gas Temperature (EGT) is a critical indicator for gas turbine engines.
    • EGT is monitored to ensure it stays within safe limits, particularly during start.
    • EGT readings may be labeled as:
    • Turbine Inlet Temperature (TIT)
    • Turbine Entry Temperature (TET)
    • Turbine Outlet Temperature (TOT)
    • EGT is a primary means of detecting abnormal heat in the turbine/exhaust area.
    • Vibration indicators provide health information; small vibrations can indicate developing problems, while larger vibrations may indicate a failing engine.

  • Case study: importance of vibration indications (pre-incident context)

    • Example: 8 January 1989, a Boeing 737 departed Heathrow for Belfast.
    • During climb to 28,000 ft, a fan blade on the left engine detached, causing significant vibration, smoke and sparks from the engine.
    • The crew misidentified the problem as being with the right engine and shut down the right engine.
    • Reasons for misidentification included cockpit knowledge from a previous aircraft version where cockpit air was supplied from a single engine; newer version supplied air from both engines.
    • The aircraft had engine vibration instruments, which would have shown very high vibrations on the left engine, but these readings were not checked before shutting down the ‘good’ engine.
    • The incident demonstrates how unreliable instrumentation and misinterpretation can have catastrophic consequences.

Starting and Starting Systems

  • After an engine has started, it can become self-sustaining; ignition may not be required once the mixture is ignited.

  • Starting requires rotating the engine core to ensure sufficient airflow into the combustion chamber.

  • In multi-spool engines, the high-pressure spool is typically connected to the starting system.

  • Starter methods (commonly used arrangements):

    • On small aircraft: electric motor starter directly turns the engine core.
    • On larger aircraft: air starters are common; include a starter motor with a turbine turned by directed airstream; the system is connected to the engine core.

  • Air sources for starting air starters:

    • Auxiliary Power Unit (APU): a small gas turbine in the tail section that can supply air to the starter.
    • Ground Start Unit (huffer cart): supplies air when APU is unavailable.
    • Cross-bleed start: air from an already running engine can be directed to start another engine (see Figure 11.4 reference).

  • Start sequence (typical two-spool engine)

    • When the starter engages, it drives the high-pressure compressor and pushes air through the engine.
    • Fuel is injected into the combustion chamber only when sufficient airflow is present; ignition occurs when airflow is adequate.
    • Initial fuel introduction occurs around approximately 10\%-20\%\,N_2 (exact timing varies by engine).
    • A short time later, a spark is introduced to ignite the fuel-air mixture.
    • The starter remains engaged until the turbine and compressor speeds are high enough to sustain operation on their own.
    • With low airflow, engine temperature rises markedly and is observed as a sharp rise in exhaust gas temperature (EGT) as shown in Figure 11.5.
    • Once the turbines drive the compressors fast enough, the starter and igniter are disengaged.
    • The EGT peaks when the fuel-air ratio is balanced and then returns to normal levels.

  • Two main issues during start to monitor

    • Hot Start
    • Definition: start with engine temperature rising too hot during start (EGT continues to rise beyond safe limits).
    • Causes: insufficient airflow (weak starter, inadequate APU air, strong tailwind assisting start), or other air delivery problems.
    • Action: typically shut down the engine immediately to prevent damage.
    • Hung Start
    • Definition: engine fails to accelerate to self-sustaining speeds after ignition (no self-sustaining speed).
    • Causes: weak starter or other failure preventing self-sustaining operation.
    • Action: engine shut down; post-start procedures may be required if fuel remains in the chamber.

  • Post-start procedures for a failed start

    • Blowout cycle: engage the starter but do not energize the igniters to ensure unburnt fuel is blown out of the combustion chamber via airflow.
    • Tailpipe fire risk is mitigated by the blowout cycle when a previous start attempt leaves unburnt fuel.

Engine Issues during operation

  • Compressor Stall (and related compressor surge)

    • Axial-flow compressors consist of a series of small stationary and rotating blades (aerofoil-like).
    • Stall occurs when the angle-of-attack (AoA) of compressor blades becomes excessive, disrupting airflow.
    • In a closely spaced multi-stage compressor, a stall on one stage can propagate to subsequent stages.
    • Common conditions for stall:
    • Air is flowing into the engine slowly but the compressor RPM is high (high RPM, low airflow).
    • Air is flowing quickly but the compressor RPM is low (low RPM, high airflow).
    • Severity ranges from mild rumbling (engine can clear itself) to explosive noises and flames due to pressure buildup and potential reverse flow.
    • Recovery typically involves reducing throttle to restore stable airflow; recovery behavior depends on engine type.
    • Note: The term “surge” is often used to describe a breakdown of airflow over a few stages, whereas “stall” generally refers to a breakdown over the whole compressor; both reduce thrust significantly.

  • Flameout

    • Definition: engine flame extinguishes (loss of flame) due to various factors.
    • Potential causes: incorrect fuel-air mixture (too rich or too lean), insufficient airflow (e.g., high altitude or unusual attitudes), icing, or running out of fuel.
    • Case study: 24 June 1982, a Boeing 747 over Indonesia experienced St Elmo’s fire on the windscreen; engine failure followed by multiple engine flameouts after passing through a volcanic ash cloud that clogged engine interiors.
    • Consequences: difficulty restarting engines; ash also damaged the cockpit windscreen; aircraft eventually restarted engines after descending and diverting.

In-flight Relight and Restart Envelopes

  • Inflight relight envelopes specify the conditions (altitude and airspeed) under which an engine restart is possible.
  • If the aircraft is flying fast enough, the starter system may not be required (e.g., air-start via air starter).
  • Case study: CRJ200 incident (14 Oct 2004, Missouri)
    • Scenario: two pilots attempted to climb the empty 50-seat CRJ200 to 41,000 ft to explore performance limits.
    • Issue: at altitude, the aircraft became too slow, causing a stall and flameout of both engines.
    • Initial restart attempts used a windmill start (no starter, relying on airflow) requiring the aircraft to maintain at least 300\ \text{knots} to provide sufficient airflow to rotate the core.
    • After failing to achieve necessary airspeed, they attempted to restart using APU air.
    • Core heating caused thermal shock, effectively locking the core.
    • They attempted to glide to a nearby airport but crashed; this case underscores the importance of adhering to relight envelope conditions and procedural cautions.

Case Studies and Lessons on Engine Management

  • Misinterpretation of engine health indicators and reliance on unreliable older instrumentation can be dangerous.
  • Real-world case studies emphasize the importance of cross-checking multiple indicators (including vibration, EGT, and other engine parameters) before deciding which engine to shutdown during abnormal events.
  • The evolving design philosophy of engines continues to aim for higher efficiency and lower environmental impact, which includes significant changes in engine architectures and power systems.

Practical and Ethical Implications for Engine Management

  • Respect for engine limits and adherence to published relight envelopes and engine procedures are critical for flight safety.
  • Incorrect engine identification, misinterpretation of instrumentation, or attempting performance beyond envelope limits can lead to catastrophic outcomes.
  • Proper training on modern indicators (EGT, vibration, RPM, EPR) and understanding their significance is essential for pilots.

Connections to Foundational Principles and Real-World Relevance

  • EPR as a practical thrust indicator ties directly to thrust management and engine performance in varying flight conditions.
  • RPM and multi-spool architecture reflect core mechanical constraints of gas turbine design and the relationship between airflow, compression, and turbine power.
  • Monitoring EGT ties to thermodynamic efficiency and turbine health, reflecting the importance of maintaining safe turbine temperatures to avoid material damage.
  • The case studies illustrate the critical need for accurate instrument interpretation, appropriate response to abnormal conditions, and the consequences of not following standardized procedures.
  • The impending shift toward environmental considerations drives ongoing research into more efficient engines and alternative propulsion concepts.

Summary of Key Formulas, Definitions, and Notable Values

  • EPR definition and relation to thrust: EPR = \frac{P{\text{exhaust}}}{P{\text{intake}}}

  • Spool designations (two-spool): N1 (low-pressure/fan side), N2 (high-pressure), (three-spool adds N_3 as the high-pressure outer spool).

  • Example of RPM percentage: If N{\text{max}} = 30{,}000\,\text{rpm} and N1 = 15{,}000\,\text{rpm}, then N1 = 50\%, i.e. N1 = 0.5 \times N_{\text{max}}.

  • Altitude example for windmill/in-flight restart considerations: fast enough airspeed required for windmill starts is typically around 300\ \text{knots} (as per the CRJ200 case).

  • Altitude reference in a key case: 41{,}000\,\text{ft} (CRJ200 incident) for high-altitude testing and relight considerations.

  • Case-study concepts to remember:

    • 1989 Heathrow incident emphasizes the dangers of misidentifying faulty engines and ignoring vibration indicators.
    • 1982 Indonesian ash-cloud flameouts illustrate how environmental hazards (volcanic ash) can cause multiple engine failures and ignition issues.
    • 2004 Missouri CRJ200 incident demonstrates the importance of relight envelopes and the risks of attempting windmill starts at too low airspeed or altitude, plus risks associated with thermal shock when restarting with partial airflow.

Final remarks

  • Gas turbine engine management requires a comprehensive understanding of thrust indicators (EPR), spool speeds (N1, N2, N3), engine health indicators (EGT, vibration), and reliable procedures for starting, stopping, and relight.
  • Real-world incidents underscore the critical importance of proper monitoring, adherence to procedures, and understanding the limitations of engine systems to ensure flight safety and reliability.
  • The field continues to evolve toward more efficient and environmentally friendly propulsion technologies, but the fundamental principles of engine management and safety remain central to piloting and aeronautical engineering.