Definition
A secondary system installed in an aircraft to take over a critical function if the primary system fails. Backup systems are designed to maintain safe operation when the main equipment is lost or degraded, and they are typically independent of the primary system in their power source, components, or operating principle so a single failure cannot disable both.
Plain English
A second system that is ready to do the job if the main one stops working.
Context Anchor
Seen in aircraft equipment discussions, preflight checks, emergency procedures, and aircraft manuals.
Derivation
From the everyday phrase 'to back up' -- to support or stand behind something. In aviation it refers to a system standing behind the primary one, ready to step in.
Why Pilots Care
Provides continued operation of essential functions such as power, instruments, or controls, preventing a single failure from ending the flight.
Intuition Check
Backup does not mean optional or unimportant. It means a second way to do a needed job if the normal way is lost.
Example Sentence 1
When the primary attitude indicator failed, the pilot transitioned to the backup system on the standby instrument.
Example Sentence 2
Preflight checks include verifying that the backup vacuum system is available for the attitude indicator.