Definition
A movable reference marker on the radio altimeter display that the pilot sets to a specific height above the ground, typically the Decision Height (DH) for a precision instrument approach. When the aircraft descends through the set height, the radio altimeter triggers an aural and/or visual alert.
Plain English
A pointer on the radio altimeter that the pilot moves to the height where they need to be alerted. When the airplane gets down to that height above the ground, the instrument warns them.
Context Anchor
Seen when using a radio altimeter during low-altitude instrument procedures, especially approaches where the pilot wants a clear height reminder near the runway.
Derivation
The term 'bug' comes from older instrument design, where a small movable marker on the dial face looked like a tiny insect on the gauge. Pilots have used 'bug' for decades to mean any settable reference pointer on an instrument (heading bug, airspeed bug, altitude bug). A radio altimeter bug is simply the bug that lives on the radio altimeter.
Why Pilots Care
It gives an immediate visual cue for critical decision heights, helping prevent controlled flight into terrain.
Analogy
It is like putting a reminder mark on a ruler at the exact measurement you care about, so you do not have to search for it while things are moving quickly.
Intuition Check
“Bug” does not mean an insect or a mechanical problem here. It means a settable marker on the instrument.
Example Sentence 1
During the approach briefing, the captain set the radio altimeter bug to 200 feet for the ILS Decision Height.
Example Sentence 2
As the needle reached the radio altimeter bug, the crew initiated the missed approach.