Definition
A secondary aircraft system, instrument, or power source designed to take over a specific function when the primary system fails or becomes unreliable. In instrument flying, common examples include a standby attitude indicator, a secondary altimeter or airspeed source, an emergency battery bus, or an alternate static source.
Plain English
A second system kept ready in case the main one stops working. If the primary fails, the backup keeps that function available so the pilot can continue flying safely.
Context Anchor
Seen in instrument flying discussions, especially when planning for or responding to instrument, electrical, vacuum, or display failures.
Derivation
Backup' comes from the idea of standing behind something to support it if it falls. 'Alternate' comes from the Latin 'alternare,' meaning 'to do by turns' — one system takes over when the other can't. Both words point to the same idea: a ready substitute.
Why Pilots Care
Allows continued safe flight and landing instead of declaring an emergency or losing control when a primary instrument fails.
Intuition Check
Do not assume “backup or alternate” means automatic, identical, or unlimited. It means there is another usable way to perform the function, but it may need to be selected and may have different limits.
Example Sentence 1
When the primary attitude indicator failed, the pilot transitioned to the backup system mounted on the lower panel.
Example Sentence 2
The checklist directs the pilot to verify the backup or alternate system is available before entering instrument conditions.