When is an aircraft's altitude above sea level reported in aviation?

An aircraft's altitude above sea level is reported primarily when using pressure altitude to ensure compliance with air traffic control requirements. This reporting is crucial because air traffic control uses altitude to maintain safe vertical separation between aircraft operating in the same airspace.

Pressure altitude is a standardized reference used to account for variations in atmospheric pressure that can affect actual altitude readings. Reported altitude allows for maintaining safe operations, especially in controlled airspaces where adherence to specific altitude assignments is necessary for collision avoidance and traffic management. Additionally, using pressure altitude allows for the application of standard operating procedures and the use of altitude in navigation and flight planning, contributing to overall safety in aviation operations.

While there are instances when altitude might be noted for fuel calculations or during specific flight phases such as takeoff, these situations do not encapsulate the primary reason for reporting altitude in aviation, which is to facilitate safe air traffic control operations.