Definition
An aerodrome to which an aircraft may proceed when it becomes either impossible or inadvisable to proceed to or to land at the aerodrome of intended landing. Alternate aerodromes include the following: takeoff alternate (an alternate aerodrome at which an aircraft can land should this become necessary shortly after takeoff and it is not possible to use the aerodrome of departure); en-route alternate (an aerodrome at which an aircraft would be able to land after experiencing an abnormal or emergency condition while en route); and destination alternate (an alternate aerodrome to which an aircraft may proceed should it become either impossible or inadvisable to land at the aerodrome of intended landing). Note: The aerodrome from which a flight departs may also be an en-route or destination alternate for that flight.
Plain English
A backup airport you can fly to if you can't or shouldn't land where you originally planned. There are different kinds depending on when in the flight the problem occurs: just after takeoff, somewhere along the way, or at the destination.
Context Anchor
Seen in flight planning, weather planning, and diversion discussions, especially in international aviation wording. In many United States training contexts, pilots more often hear “alternate airport.”
Derivation
From Latin alter, meaning 'the other,' and aerodrome from Greek aero (air) and dromos (course or running track). An alternate aerodrome is literally 'the other airfield' — the other place you can go.
Why Pilots Care
Regulatory requirements for IFR flights often mandate an alternate aerodrome to guarantee a safe landing option and adequate fuel reserves.
Analogy
It is like choosing a backup hotel before a long trip. You hope to use your planned stop, but if that stop becomes unavailable, you already know where you can go instead.
Intuition Check
“Alternate” does not mean a vague or last-minute idea here. It means a specific suitable place the aircraft can use if the planned landing place will not work.
Example Sentence 1
Because the forecast at the destination showed a chance of low ceilings, the pilot filed an alternate aerodrome with better expected weather.
Example Sentence 2
With the runway closed at the destination, the crew diverted to the filed alternate aerodrome.