RAF jets have been scrambled twice this month in response to terrorist alerts on passenger airliners flying over Britain, defence sources have revealed. 
 
 Eurofighter Typhoons took off from the quick reaction alert base at Coningsby in Lincolnshire – one of two such bases in the UK – minutes after warnings of suspected attempts to hijack American airliners. 
 
 Procedures were triggered that could ultimately lead to a decision to destroy the aircraft, to avoid mass casualties. That decision would be taken "at the highest levels of government", officials say. Though a decision would be taken by the prime minister, if he could not be contacted or was incapacitated, it would be taken by two other designated members of the cabinet, believed to be the home secretary and the transport secretary. 
 
 Last Monday British air traffic control picked up the words "hostage" and "ransom" from a United Airlines plane bound for Frankfurt in Germany. The plane suddenly dropped height south of Reading in Berkshire and a request for the airliner's auxiliary power unit to be activated – a very unusual occurrence – was heard. 
 
 The Typhoons from Coningsby approached the airliner over Manston airfield in Thanet, east Kent, a matter of minutes – the exact time is classified – after the alert was first sounded, according to officials familiar with the incident. 
 
 In the other incident earlier this month, on 2 March, the crew of an American Airlines plane put out an alert, picked up by the air defence control and reporting centre at Boulmer in Northumberland, warning that somebody was trying to get into the cockpit. 
 
 Pilots of the Typhoons at Coningsby and RAF Tornado F3 fighters at Leuchars in Fife – the UK's other quick reaction alert base – were immediately briefed and ordered to intercept the airliner. The Tornados were soon stood down but the Typhoons flew on until they were close enough to identify the airliner, whose exact position had been unclear. Armed police from Scotland Yard's counterterrorism command were alerted, as was No 10. 
 
 The airliner's crew soon radioed that the incident was not a credible attempt to hijack the plane and the passenger was under control and under guard. 
 
 Both incidents were false alarms. However, they show how emergency procedures, set up after the 9/11 attacks on the US, and refined in 2007, are activated. 
 
 Alerts are picked up by radar stations at Boulmer and Scampton in Lincolnshire. They are then passed to the RAF's national air and space operations centre in High Wycombe. From there, 10 Downing Street is alerted, where officials are ready to warn the prime minister that he may have order an RAF pilot to shoot down a civilian airliner. It is potentially one of the most difficult decisions of a prime minister's tenure, perhaps second only to ordering the commander of a Trident submarine to launch a nuclear missile. 
 
 There is a terrorism alert involving civil airliners in British airspace about once a month – out of some three million which cross it every year – officials say. Under interrogation at Guantánamo Bay and secret prisons, terrorist suspects have claimed al-Qaida planned to attack Canary Wharf in London's docklands and other UK targets. 
 
 Government officials are wary of discussing details of the emergency procedures, saying that the exact time it has taken an RAF jet to reach airliners and what maneouvres the pilot takes up are classified. ||||| By Jonathan Beale 
 
 Defence correspondent, BBC News 
 
 Fighter jets were scrambled to intercept a United Airlines flight RAF Typhoon fighter jets have been scrambled twice this month amid fears of possible terror threats in UK airspace, the BBC has learned. One week ago - on 22 March - the RAF was alerted to a conversation overheard on the airwaves, believed to have come from a plane travelling through UK airspace. It included the words "ransom" and "hostage". Soon afterward a Delta Airlines plane - on its way from the US to Frankfurt - made what is described as an "unusual request" to descend just as it was flying south of Reading. Within minutes, two Typhoon fighter jets were scrambled from RAF Coningsby in Lincolnshire. The fighters circled over Kent as the civilian flight, which had raised concerns, cleared UK airspace. It was, thankfully, a false alarm. It is the latest incident to trigger counter-terrorism measures - first introduced after the attacks on 11 September 2001. Difficult decision On average the alarm is raised every month - perhaps no surprise given that there are three million flights through UK airspace every year. Whenever an alert is issued, Downing Street is immediately informed. One of a small group of senior ministers is kept abreast of developments. In this case, he or she would have been woken up in the middle of the night. Ultimately the minister would make the difficult decision to intervene or, if all else fails, to shoot an aircraft down. Tornado F3 fighters intercepted a Russian Blackjack bomber So far it has not come to that, but the procedures are in place. Typhoon jets were also launched from RAF Coningsby on 2 March, in another incident which has already been reported. They went to "assist" an American Airlines flight travelling from Dallas/Fort Worth to London. It followed reports that a passenger had tried to enter the flight deck. In this case, a female passenger had become distressed during the flight, but the flight crew had managed to calm her down. But the speed of the interception by the RAF Typhoons gives a sense of how seriously these incidents are taken. We are not allowed to report precisely how soon the fighters were at the scene, but again it took a matter of minutes to intercept the American Airlines flight. Cuts 'inevitable' Counter-terrorism operations are not the only thing keeping the RAF busy. Fighter crews have been scrambled more than 10 times in the past year to intercept Russian aircraft approaching UK airspace. Most recently two Tornado F3 fighters were launched from RAF Leuchars in Fife in the early hours of 10 March to intercept a "zombie" - or unknown aircraft. Within minutes the lead Tornado identified and photographed two Russian Tu-160 "Blackjack" strategic bombers. The RAF say such incidents are relatively frequent as the Russians carry out routine patrols over the Arctic and Atlantic oceans, though no Russian planes have ever entered UK airspace. It is perhaps no coincidence the RAF has made public these recent incidents. They are a reminder of the work the air force carries out to defend the homeland at a time when serious questions are being asked about the overall cost of the UK's defence budget. There will be a Strategic Defence Review after the general election that will decide the future shape of the armed forces. Cuts are inevitable. The RAF, like the Royal Navy and the Army are engaged in their own PR battle to ensure they will not end up bearing the brunt of the costs. 
 
 
 
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– Britain's Royal Air Force has scrambled fighter jets twice this month after receiving warnings of possible attempts to hijack American passenger jets, defense sources say. Both incidents proved to be false alarms but they have put fresh focus on Britain's post 9/11 counterterrorism procedures for commercial aircraft, the BBC reports. The Guardian notes that PM Gordon Brown was alerted, and could have faced the decision of whether to shoot an airliner down to avoid a potential attack. In the most recent incident, fighter jets were scrambled after air traffic controllers overheard the words "hostage" and "ransom" in a conversation on a flight from the US, followed by a highly unusual request to carry out a sudden descent to test auxiliary power units. The jets followed the aircraft through UK airspace and into Belgium, where it landed. Defense officials said the matter was resolved but declined to provide further details.