18 ATPL Training Gas Turbine Engines #18 Engine Starting

Summary

This detailed guide covers the essential procedures for starting and troubleshooting gas turbine engines, focusing on key requirements like compressor rotation, fuel supply, and ignition. It explains various starting methods, such as electric and air starter motors, and their duty cycles. The script also delves into common issues like wet starts, hot starts, and hung starts, providing troubleshooting strategies for each. Additionally, it highlights the importance of monitoring critical parameters like EGT and RPM during engine start, offering insights into handling failures and ensuring a smooth engine startup or relight in-flight.

Takeaways

• 😀 The basic requirements to start a gas turbine engine include rotating the compressor turbine assembly, providing fuel in the combustion chambers, and ensuring ignition is supplied to start the air-fuel mixture burning.
• 😀 A motoring over cycle (also called a blowout cycle) may be necessary after a failed start to clear out excess fuel and prevent torching during the next attempt.
• 😀 There are several methods to rotate the high-pressure compressor during engine start, with the most common being electric starter motors, air starter motors, and starter generators.
• 😀 The duty cycle of a starter motor defines the maximum time it can be energized before requiring a cool-down period to prevent damage.
• 😀 The air starter motor is commonly used on large gas turbine engines because it is lightweight and economical, using low-pressure air as its power source from an APU, ground start unit, or another engine.
• 😀 Engine start is monitored by critical instruments such as the exhaust gas temperature (EGT) and high-pressure compressor RPM (N2) to ensure safe progression during the start cycle.
• 😀 During a start cycle, ignition is typically engaged when N1 reaches 24%, and engine light-up should occur within 20 seconds, indicated by a sharp rise in EGT.
• 😀 A hot start occurs when the EGT rapidly exceeds the engine's safe limit, often due to too much fuel or insufficient cooling air. Immediate action, such as shutting off the fuel supply, is needed to prevent damage.
• 😀 A wet start, characterized by failure to light up within the required time, requires a blowout cycle to clear the engine of residual fuel and prevent further damage during the next start attempt.
• 😀 Failure to start could be caused by ignition failure, lack of fuel, or insufficient airflow through the engine, and symptoms include low RPM and higher-than-normal EGT without light-up.
• 😀 Engine rundown time, the time taken for the engine to stop after fuel is shut off, should be monitored. A significantly reduced rundown time may indicate an impending engine malfunction.

Q & A

What are the three basic requirements to start a gas turbine engine?

The three basic requirements are: 1) The compressor turbine assembly must be rotated to get air into the combustion chambers. 2) Fuel must be provided in the combustion chambers. 3) Ignition must be provided to start the air-fuel mixture burning.

What is a motoring over cycle, and when is it necessary?

A motoring over cycle, also known as a blowout cycle, is performed when an engine has failed to start. It involves rotating the engine without ignition or fuel supply to clear any remaining fuel from the combustion chamber and avoid excess fuel ignition in a subsequent attempt.

Why is the electric starter motor no longer widely used in larger gas turbine engines?

The electric starter motor has fallen out of favor in larger and more modern engines due to its weight, which makes it less efficient compared to other starting methods.

What is the role of the air starter motor in starting a large gas turbine engine?

The air starter motor is a popular starting method for large engines. It is lightweight, simple, and economical, utilizing low-pressure air to rotate the high-pressure compressor of the engine.

What is the duty cycle of a starter motor, and why is it important?

The duty cycle is the maximum time the starter motor can be energized before it requires a cooling period. This is essential to prevent overheating and ensure the motor's longevity and reliability during the starting process.

How does a starter generator differ from a traditional starter motor?

A starter generator is a combination of a starter motor and a generator, allowing it to serve a dual purpose. This system has a better weight-to-usefulness ratio and is becoming increasingly popular in smaller engines.

What are the preferred air sources for engine starting on modern aircraft?

The preferred air sources for engine starting are, in order of preference: 1) The aircraft's auxiliary power unit (APU), 2) Ground start units, and 3) Air from a running engine (crossbreed start).

What safety mechanism prevents the air starter motor from breaking due to centrifugal force?

The air starter motor is equipped with a ratchet clutch arrangement that prevents it from being driven by the engine once the engine starts, as well as a flyweight cutout switch to automatically shut off the air supply when the engine reaches self-sustaining speed.

What is the significance of exhaust gas temperature (EGT) and high-pressure compressor RPM (N2) during the engine start process?

EGT and N2 are key indicators of engine start progress. EGT shows the temperature rise associated with ignition and combustion, while N2 reflects the rotational speed of the high-pressure compressor, which must increase to initiate combustion and achieve self-sustaining engine speed.

What is a hot start, and how can it be prevented during engine startup?

A hot start occurs when the EGT rises too rapidly, surpassing the engine's temperature limits, typically due to excess fuel or insufficient air for cooling. To prevent this, it is crucial to monitor fuel and air supply carefully, ensure proper RPM, and immediately shut off the fuel supply if temperatures exceed limits.