The UK Royal Air Force (RAF) scrambled its fighter jets twice in March, after receiving terrorist alerts from transatlantic passenger jets.
On 2 March, after a passenger attempted to get onto the flight deck of the American Airlines plane, the RAF scrambled Tornado F3s from RAF Leuchars in Fife and Typhoons from RAF Coningsby in Lincolnshire to shadow an American Airlines plane on its way to Heathrow from Dallas.
In a similar terror alert on 22 March, the RAF scrambled Typhoons from RAF Coningsby to shadow an American flight across UK airspace into Belgium, where it landed.
During the incident, UK air-traffic controllers picked up phrases including the words "ransom" and "hostage" from the flight.
In such situations scrambled RAF fighters can shoot down the airliners if they are deemed to be an immediate threat.
The decision on the scrambled jets' course of action ispart of a chain of command that is active 24 hours, at the highest level of government, according to the Independent.
On 10 March, the RAF had to scramble fighter jets off the Hebrides to counter a probing Russian Blackjack aircraft, which is capable of carrying a nuclear payload.