bug

In the cockpit, a bug is a small, manually positionable marker or pointer attached to the face of a flight instrument, most commonly the airspeed indicator, altimeter, or heading indicator. The pilot or flight engineer rotates it by hand before or during flight to mark a target speed, altitude, or heading. It serves as a visual reference point without requiring electronic automation or constant attention to numerical readouts.

The airspeed bug is the most familiar variant. Pilots set it to the target airspeed for the current phase of flight, whether that is climb speed, cruise speed, or approach speed. As the needle moves across the dial, the bug stays fixed, making it immediately obvious whether the aircraft is above or below the desired speed. The altimeter bug marks the target altitude; the heading bug marks the desired compass heading or magnetic track.

Mechanism and limitations

Bugs are mechanical devices, typically a pointer or ring driven by a small knurled knob around the perimeter of the instrument. The marker itself is usually made of phenolic resin or plastic and sits against the instrument face without obscuring the needle itself. Because they are mechanical and require manual adjustment, bugs work regardless of whether the aircraft's electrical or autopilot systems are functioning. This redundancy is valuable in degraded conditions.

The main limitation is that the bug represents only one reference value at a time and requires deliberate pilot action to update. Modern glass cockpits often replace mechanical bugs with digital target indicators that can change automatically or sync with flight management systems. However, mechanical bugs persist in older aircraft, light aircraft, and as backups in transport-category aircraft precisely because they do not depend on computation or power management.

The term "bug" in this context is believed to derive from the insect-like appearance of early marker designs, though formal documentation of the etymology is scarce in aviation literature. It is used consistently across pilot training, maintenance manuals, and approach procedures, making it essential vocabulary for anyone working in flight operations or cockpit systems maintenance.