Definition
A movable reference marker on the vertical speed indicator (VSI) of an electronic flight display that the pilot sets to a chosen rate of climb or descent, typically in feet per minute. When coupled with an autopilot in vertical speed mode, the bug commands the autopilot to fly that rate; when flown by hand, it serves as a visual target on the VSI tape.
Plain English
A small marker the pilot slides along the climb/descent gauge to mark the rate they want to fly at. The autopilot can chase that marker, or the pilot can fly to it manually.
Context Anchor
Seen on glass cockpit vertical speed displays when setting or monitoring a climb or descent rate.
Derivation
The word 'bug' has long been used in aviation and instrumentation for a small movable index or marker set against a scale. It comes from the older mechanical sense of a small attached pointer or tab. Calling it a 'vertical speed bug' simply means the movable marker on the vertical speed scale.
Why Pilots Care
It lets the pilot maintain a chosen vertical speed at a glance without repeated numeric checks, lowering workload during high-task phases of flight.
Intuition Check
Do not read “bug” as an insect or a software problem here. In this context, a bug is a movable target marker on an aircraft display.
Example Sentence 1
Approaching the descent point, she set the vertical speed bug to 500 feet per minute down and engaged the autopilot in vertical speed mode.
Example Sentence 2
On final approach she moved the vertical speed bug to match the 500-foot-per-minute descent rate required by the instrument approach procedure.