All flights undertaking commercial aerial activity in the London and London City Control Zones are required to operate under a Non Standard Flight authorisation. Pilots will notify their intention to operate such flights to London Terminal Control. Approval will then be granted subject to flight priority, other aerial activity, the likely amount of disruption caused and controller workload issues. The London City Control Zone is Class D airspace which allows visual separation to be applied between flights, therefore flights into London City would not be stopped. Non Standard Flights are usually low priority and they would therefore be asked to leave the area if they were likely to affect scheduled IFR flights. Higher category flights such as the police and the air ambulance helicopters may well disrupt scheduled IFR flights, depending on where they were operating in relation to Heathrow or London City airports.