When airline fuel is calculated before flight, ‘contingency fuel’ has been included either separately or in combination in various forms over the years. The primary calculation has always been the fuel required in expected conditions to fly to the destination and thence to a nominated alternate. The next is the provision, in the words of the Air Navigation Order, of a ‘safe margin of fuel’ for contingencies. For aircraft operated under an Air Operator's Certificate, CAP 360 specifies ‘an allowance for contingencies such as errors in forecast winds and temperatures, navigational errors and ATC restrictions on altitude and route’. Also, ‘45 minutes holding reserve’ is stated to be a normally overriding requirement. The wording of CAP 360 is, perhaps intentionally, not precise but is broadly in line with Annex 6 of the ICAO Chicago Convention which requires:

(i) a reserve ‘to provide for contingencies’ plus, for turbojet aircraft,

(ii) 30 minutes' low level holding, plus

(iii) an amount ‘to provide for the increased consumption on the occurrence of any of the potential contingencies specified…’