To whom should the current altimeter setting be issued?

The correct approach is to issue the current altimeter setting to all en route aircraft operating below Flight Level (FL) 180. This ensures that these aircraft maintain accurate altitude readings and separation from other air traffic, as altimeter settings can significantly influence the safety and efficiency of operations in controlled airspace.

Providing the altimeter setting to these aircraft allows pilots to adjust their altimeters accurately, which is crucial for maintaining safe vertical separation. This practice helps in preventing altitude deviations that could lead to conflicts with other aircraft, particularly in congested airspace where multiple aircraft are operating at similar altitudes.

In contrast, aircraft operating above FL 410 and those that only request the setting at any time may not need immediate updates or may be in phases of flight where altitude precision isn't as critical. Likewise, only arriving aircraft with filed flight plans may not cover the entirety of en route traffic that requires current altimeter information at lower altitudes.