An altimeter setting must be issued to an en route aircraft operating below FL180 at least how many times in your area of jurisdiction?

An altimeter setting must be issued to an en route aircraft operating below FL180 at least once during its flight to ensure that the aircraft is operating with the most accurate altitude information. The altimeter setting is crucial for maintaining correct altitude above sea level, which is vital for safety and separation from other aircraft.

While it may be beneficial for aircraft to receive updated altimeter settings as they progress through different areas where atmospheric pressure may vary, regulatory requirements specify that one issuance is the minimum necessary for en route aircraft. This reduces the workload on air traffic controllers while still providing essential information to pilots.

Although there are scenarios where additional updates might be beneficial (for example, if there is a significant change in weather affecting pressure), the requirement is set at once to ensure that essential safety standards are met without overwhelming communication.