Definition
A secondary instrument, system, or power source that provides the same essential information or function as a primary one, allowing safe continuation of flight if the primary fails. In analog instrument flying, backup instruments typically include a separate attitude indicator, airspeed indicator, and altimeter powered by an independent source (such as a battery or alternate vacuum/electrical system).
Plain English
A spare instrument or system that takes over when the main one fails, so the pilot still has the information needed to fly safely.
Context Anchor
Seen in instrument failure discussions, especially when a pilot must use another instrument or source of information after a primary instrument stops working correctly.
Derivation
From the everyday English sense of 'to back up' — to support or stand behind something. In aviation it carries the same idea: a system that stands behind the primary one and takes over if needed.
Why Pilots Care
Maintains accurate flight information and prevents loss of control when a primary instrument stops functioning.
Intuition Check
Backup does not mean unimportant or optional here. It means secondary, available, and intended to be used if the primary item fails or cannot be trusted.
Example Sentence 1
When the primary attitude indicator failed, the pilot transitioned to the backup attitude indicator on the right side of the panel.
Example Sentence 2
Backup airspeed and altimeter instruments allowed continued flight in IMC.