Definition
A movable marker on the heading indicator (or HSI) that the pilot manually sets to a chosen compass heading, used as a visual reference for the heading they intend to fly or as the heading reference an autopilot will track in heading mode.
Plain English
A small pointer on the heading instrument that the pilot turns to point at the heading they want to fly, so they can see at a glance whether the airplane is pointing the right way.
Context Anchor
Seen when setting a runway heading, an assigned heading from air traffic control, or a planned departure direction on the cockpit heading display.
Derivation
‘Bug’ here is borrowed from instrument and engineering use, where a small movable index or marker placed on a dial is called a bug. It signals ‘a little thing stuck on the dial to mark a value’ — useful because the pilot doesn’t have to remember the target heading; the bug holds it for them.
Why Pilots Care
It gives an immediate visual reference for the target heading, reducing the need to read numbers while flying.
Intuition Check
Do not read bug as an insect or a fault in the system. In this context, a bug is a movable marker the pilot sets on the heading display.
Example Sentence 1
Before takeoff on Runway 27, the pilot set the heading bug to 270° as a reminder of the runway heading.
Example Sentence 2
With the heading bug aligned to the desired course, the autopilot tracked the new heading smoothly.