Final answer:
EGT should rise (light off) during the start-up sequence of a gas turbine engine when the fuel-air mixture ignites. Monitoring EGT is critical because it indicates the engine is starting correctly and to avoid potential engine damage.
Step-by-step explanation:
EGT should rise (light off) when the fuel-air mixture ignites during the start-up sequence of a gas turbine engine, typically seen in aviation.
EGT, or Exhaust Gas Temperature, is an essential parameter to monitor when starting a gas turbine engine. 'Light off' refers to the point where the fuel-air mixture within the combustion chamber ignites, which signals the start of the engine's operation. Engineers monitor EGT to ensure that it rises within expected limits; if it's too high, it can indicate potential problems. The light off should occur after the starter has begun to accelerate the engine and the fuel flow has been initiated.
Once the fuel-air mixture is deemed correct and the ignition system is activated, the EGT should begin to rise. This is usually visible on the gauge within a few seconds. It is critical because a proper light off ensures that the engine is starting correctly and safely. Over-temping on start can cause engine damage, hence the significant focus on the proper rise of EGT during this phase.