What is the minimum altitude at which a ground proximity warning system must be operational?

The minimum altitude at which a ground proximity warning system (GPWS) must be operational is 1,000 feet AGL (Above Ground Level). This requirement is specified to ensure that GPWS can effectively provide alerts to pilots during critical phases of flight, particularly during approach and landing, when the risk of controlled flight into terrain (CFIT) incidents is highest. By having the system operational at this altitude, pilots receive timely warnings that can help prevent accidents caused by unintentional descents toward the ground or obstacles.

Operational effectiveness at this altitude is essential in helping pilots maintain situational awareness and make necessary altitude adjustments as they approach the runway. The other altitude options are higher than the mandated minimum, which would not adequately address the need for timely warnings during critical phases of flight, thereby potentially increasing the risk of an accident.