When transitioning from QNH to QNE, what must pilots do?

When transitioning from QNH to QNE, pilots must set their altimeter to the ISA standard pressure value of 1013.5 hPa. This action is crucial as it allows the aircraft's altitude readings to be based on a standard reference that applies uniformly throughout the entire flight, regardless of local atmospheric pressure variations. By utilizing the standard pressure setting, the altimeter will indicate the aircraft's altitude above sea level, which is essential for ensuring safe navigation and altitude separation from other aircraft during flight.

This practice is particularly important when flying at high altitudes or when transitioning to flight levels, which are defined in standard pressure rather than local pressure settings. By adhering to the standard atmospheric conditions of 1013.5 hPa, pilots can maintain proper situational awareness and ensure compliance with air traffic control instructions.