What is the purpose of a blow-in door in a turbojet engine?

The purpose of a blow-in door in a turbojet engine is to allow more air into the plenum chamber during ground operations. This is particularly important because during ground operations, the turbojet engine may not receive sufficient airflow due to the low speed and static position. By opening the blow-in door, additional air is directed into the engine, which helps maintain proper airflow and ensures optimal performance. This increased airflow is crucial for preventing compressor stall and maintaining engine efficiency when the aircraft is stationary or operating at low speeds.

In scenarios where the engine is operating on the ground, such as during taxiing or run-up, the airflow through the engine is considerably less than it would be in flight. The blow-in door compensates for this by enhancing the amount of air that enters the engine, allowing it to operate more effectively under these conditions.

This function specifically supports the engine's operational needs during ground operations, rather than impacting pressure release during takeoff, closing mechanisms, or directly enhancing fuel efficiency in flight.