Final answer:
During the engine start sequence of an aircraft, the pack flow control valves are typically closed to ensure sufficient air pressure for engine ignition. This is a part of pre-flight checks to maintain operational efficiency and passenger comfort. Once the engines have started, these valves are reopened to resume normal cabin pressure and temperature control.
Step-by-step explanation:
The engine start sequence in an aircraft has a specific effect on the pack flow control valves, which are part of the aircraft's environmental control system (ECS). The packs are responsible for managing cabin temperature and pressurization, typically by controlling the flow of air bled from the engines. When starting the engines, especially in large commercial jets, the ECS may need to temporarily close these valves to ensure there is sufficient air pressure and volume available for the engine start procedure. This ensures the engines have enough pneumatic power to start properly without causing a drop in cabin pressure that could be uncomfortable for passengers or affect safety.
Once the engines are started, the pack flow control valves will open again, resuming normal operation of the ECS and thereby regulating the cabin's temperature and pressure again. The interaction between engine start sequences and the pack valves is crucial for maintaining both operational efficiency and passenger comfort. If these valves were to remain open during an engine start, it could lead to a longer start time for the engines or a failure to start at all due to insufficient air pressure.
This is why, during the pre-flight checks and preparations, pilots and maintenance crews pay close attention to the ECS and its components, including the pack flow control valves. By ensuring these systems are functioning correctly before takeoff, potential issues can be avoided once the aircraft is airborne. It is also an example of how various aircraft systems are interdependent, and how the change in status of one system, such as during an engine start, can influence the performance of another.