What is the maximum transient limit for EGT during APU start?

The maximum transient limit for Exhaust Gas Temperature (EGT) during APU start is defined to ensure that the engine can safely handle higher temperatures for a short duration without causing damage. The correct answer indicates that the EGT can reach up to 715°C for a period of 20 seconds during the start sequence. This limit is established based on the design parameters of the APU, which allow it to tolerate these elevated temperatures temporarily to ensure successful operation and ignitions.

The transient limits exist to ensure reliability and safety in operation. Exceeding these limits can lead to excessive wear or failure of the engine components, hence, understanding the specified transient limits is crucial for personnel operating the APU. Keeping the EGT within the maximum specified limit during start-up helps to maintain the operational integrity of the APU and prolong its life, which is vital for mission readiness and overall aircraft performance.

The other choices represent either lower maximum transient limits, which would not align with the established standards for the APU being discussed, or durations that exceed safe operational parameters, potentially resulting in damage.