3% ERA (3% En-Route Alternate)

Location of the 3% En-Route Alternate (3% ERA) aerodrome for the purposes of reducing contingency fuel to 3%.
The 3% ERA aerodrome shall be located within a circle having a radius equal to 20% of the total flight plan distance, the centre of which lies on the planned route at a distance from the destination aerodrome of 25% of the total flight plan distance, or at least 20% of the total flight plan distance plus 50NM, whichever is greater, all distances are to be calculated in still air conditions.

Source: EU-OPS