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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 The likelihood that Malaysia Airlines Flight MH17 was downed by a missile has raised questions over whether the plane should have been flying over war-torn eastern Ukraine – and could have profound consequences for air travel. 
 
 The doomed Boeing 777 was following an air traffic routing between Europe and Asia that is issued by dozens of jets a day and was in a section of airspace deemed safe by international aviation authorities. 
 
 Within hours of Thursday’s disaster, all airspace around eastern Ukraine was shut down. But some airlines had already been avoiding the area amid concerns over the deteriorating security. Two low-flying Ukrainian aircraft– a fighter jet and a military transport plane - were shot down by pro-separatist rebels earlier this week. 
 
 "They chose the most direct and economic flight route possible, which keeps their fuel costs down and is something we expect as customers" 
 
 Aviation regulators– including the FAA – had issued a series of notices to pilots in recent weeks prohibiting airspace very close to the crash site. 
 
 “This was a very commonly used route and passenger jets fly at high altitudes over many of the world’s hotspots all the time,” said Norman Shanks, professor of aviation security at Britain’s Coventry University. “They chose the most direct and economic flight route possible, which keeps their fuel costs down and is something we expect as customers. They were no different from any other international airline.” 
 
 MH17 was flying at 33,000 feet when disaster struck – well above the trajectory of missiles commonly used by militias in ground conflict, and high enough that its routing was approved by the airline’s flight planners, air traffic controllers and ultimately the pilots. 
 
 However, with pro-Russia separatists apparently in possession of surface-to-air missile, airlines might have to be more vigilant about avoiding trouble spots. 
 
 “This incident is unprecedented really,” Greg Waldron, Asia managing editor Flight Global told the BBC. “A lot of the weapons used by separatists and other guerrilla groups simply don’t have the range to get [to 33,000 feet], they don’t have the accuracy to hit something like an airliner.” 
 
 Why was MH17 over a conflict zone? 
 
 Flight plans are drawn up by airlines and pilots and submitted to air traffic controllers for approval. Ultimately, pilots fly the agreed routing – unless they receive or request adjustments from air traffic controllers while en route. 
 
 MH17’s exact flight plan – a routing defined by a series of waypoints and air corridors - has not been confirmed but appears to have included airway L980, a busy section of airspace that acted as an airborne freeway between northern Europe and southern Asia. 
 
 British Airways, Air France, Lufthansa and KLM are among the airlines to have used exactly the same routing over eastern Ukraine in recent days, according to website FlightRadar24. 
 
 “Fifteen out of 16 airlines in the Association of Asia Pacific Airlines fly this route over Ukraine,” Malaysia’s transport minister Liow Tiong Lai said Friday. “European airlines also use the same route, and traverse the same airspace.” 
 
 Airlines can choose to avoid these areas. Australia's Qantas stopped flying over Ukraine several months ago and shifted its London-Dubai route 400 miles to the south. Korean Air said it had rerouted cargo and passenger flights in early March amid the worsening situation over the Crimean peninsula. 
 
 On April 23, the FAA issued a Notice to Airman (NOTAM) prohibiting U.S. carriers from flying over the Crimean region and portions adjacent to the Black Sea and the Sea of Azov. It didn’t cover the airspace where MH17 crashed. “This action was taken due to the unilateral and illegal action by Russia to assert control over Crimean airspace, including international airspace administered by Ukraine, without agreement by the International Civil Aviation Organization (ICAO),” the FAA said in a statement. Similar notices were issued by other countries. 
 
 Another NOTAM was issued on Tuesday prohibiting flights in an expanded area close to where MH17 crashed – but crucially it only applied to operations between 26,000 feet and 32,000 feet. 
 
 The pilots of MH17 filed a flight plan asking to fly at 35,000 feet throughout Ukrainian airspace, the airline said in a statement Friday. However, upon entering Ukrainian airspace, MH17 was instructed by Ukrainian air traffic controllers to fly at 33,000ft. 
 
 “MH17's flight plan was approved by Eurocontrol, who are solely responsible for determining civil aircraft flight paths over European airspace,” the Malaysia Airlines statement said. "Eurocontrol is the air navigation service provider for Europe and is governed under ICAO rules.” 
 
 Tony Tyler, director general of commercial air industry trade organization IATA, said: “No airline will risk the safety of their passengers, crew and aircraft for the sake of fuel savings. Airlines depend on governments and air traffic control authorities to advise which air space is available for flight, and they plan within those limits.” 
 
 He added: “It is very similar to driving a car. If the road is open, you assume that it is safe. If it's closed you find an alternate route. Civil aircraft are not military targets. Governments agreed that in the Chicago Convention. And what happened with MH 17 is a tragedy for 298 souls that should not have happened in any airspace." 
 
 How could air travel change? 
 
 Airlines have rushed to assure passengers they are rerouting flights between Europe and Asia to avoid Ukraine entirely. In part, they have no choice because Ukraine quickly closed all airspace over its eastern zone after news of MH17 broke. 
 
 Longer routings mean longer flights for passengers and higher fuel bills for airlines. Carriers may even be forced to reconsider many international routes. Airline stocks such as Lufthansa dipped in early trading in Europe Friday, Reuters reported, as major travel companies rerouted flights to avoid Ukraine. 
 
 Thomas Routh, an aviation attorney in Chicago, said it was up to airlines and pilots to decide whether a flight will be safe for crew and passengers. "There are airlines flying through Afghanistan's airspace every day," Routh said. 
 
 Some of the other places that the FAA also currently warns pilots to avoid parts of include Iran, Yemen, the Sinai peninsula and North Korea. Gulf-based carrier Emirates stopped flying over parts of Syria as a civil war there expanded. 
 
 Last month, a gunman in Pakistan fired on a jetliner that was landing in Peshawar, part of the country's volatile northwest region, killing a passenger and wounding two other people. Emirates suspended flights to Peshawar, and other carriers canceled some flights while they reviewed airport security. 
 
 Greg Raiff, an aviation consultant in New Hampshire, said that if airlines must avoid flying over all the world's hot spots, flight times would be extended, requiring extra fuel and pilots. That might make some routes uneconomical, forcing airlines to abandon them. 
 
 No commercial airliner is fitted with missile defense technology because of the prohibitive cost relative to the risk. A 2008 Department for Homeland Security report outlined a feasibility study in which missile jamming technology was briefly fitted to three American Airlines Boeing 767s and to MD-10 cargo planes operated by FedEx. 
 
 The Associated Press contributed to this report.
– 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.