Definition
A movable marker on the compass card of a Horizontal Situation Indicator (HSI) or heading indicator that the pilot positions to a chosen heading. It serves as a visual reference for the desired heading and, when the autopilot is coupled in heading mode, commands the autopilot to turn to and hold that heading.
Plain English
A small pointer on the heading instrument that the pilot sets to the heading they want to fly. It marks the target heading on the dial, and the autopilot can follow it.
Context Anchor
Seen on an HSI or heading indicator when setting a heading to fly, especially during instrument flying or when using heading mode.
Derivation
The word 'bug' is used here in its mechanical sense — a small, movable marker or pointer set on a dial. The same usage appears in 'altitude bug' and 'airspeed bug.' It comes from the older mechanical convention of a tiny tab attached to an instrument face that the operator could nudge into position.
Why Pilots Care
It allows precise, hands-off heading control, reducing workload during enroute navigation and instrument approaches.
Intuition Check
Do not read “bug” as an insect or a fault. In this context, a bug is a movable marker on the instrument.
Example Sentence 1
After ATC issued a vector, the pilot turned the heading bug to 270° and engaged heading mode on the autopilot.
Example Sentence 2
With the heading select bug aligned to the assigned vector, the aircraft maintained course without further input.