Definition
A movable marker on the heading indicator (directional gyro or HSI) that the pilot sets to a selected compass heading as a visual reference. It does not steer the airplane unless coupled to an autopilot; on its own, it simply marks the desired heading on the instrument face.
Plain English
A small pointer the pilot moves around the edge of the heading instrument to mark the direction they want to fly, so it's easy to see at a glance whether the airplane is on that heading.
Context Anchor
During the before-takeoff check, a pilot may set the heading bug to the runway heading or to the first heading expected after takeoff.
Derivation
"Bug" is an old instrument-shop word for a small pointer or marker added to a dial. The same usage appears in other instruments (e.g., airspeed bugs). It helps because the word simply means "a little marker you can move" — nothing more technical than that.
Why Pilots Care
It lets the pilot set and visually track a precise heading without constantly reading numbers or performing mental calculations, reducing workload and heading deviations.
Intuition Check
A heading bug is not an insect and not a computer error. Here, “bug” means a small movable marker on an aircraft instrument.
Example Sentence 1
During the before-takeoff check, the pilot set the heading bug to the runway heading of 270 as a reminder of the initial departure direction.
Example Sentence 2
Once airborne the pilot turns the airplane until the heading bug aligns with the lubber line to maintain the assigned course.