Shooting down a hijacked passenger plane has, since the attacks of 11 September, been a stark option facing those fighting terrorism in Britain.
The RAF Tornado crews are ready to scramble in a matter of minutes
The BBC's Brian Hanrahan was given unprecedented access to the Quick Reaction Alert crews of RAF Coningsby in Lincolnshire for the BBC's Ten O'Clock News.
At any time in the UK, the crews of four RAF Tornado fighter jets are ready to scramble and shoot down a plane being used as a terrorist weapon.
It is a last resort - and any decision to do so will be taken by the prime minister or a senior cabinet minister.
As crews prepare for Quick Reaction Alert, they check their aircraft so they can take off instantly to challenge and shoot down any plane that poses a terrorist threat to Britain.
"We are fairly close to London and therefore the most obvious choice to launch and protect the capital," said navigator Jules, of 25 Squadron.
The crews live and eat in quarters next to the hangars for 24 hours at a time.
They arrived at RAF Coningsby the day after the 11 September 2001 attacks on New York and the duty crews have been there every minute of every day since.
Once the new watch takes over, they even sleep in their waterproof suits.
It saves seconds if they have to scramble.
I watched as fighter control dealt with a genuine alert - an airliner out of contact.
In an old cold war bunker, sophisticated computers and radar feeds from around the country scan Britain's crowded air lanes.
Thousands of aircraft are flying above UK airspace at any moment
The RAF's master controller wanted more details and warned his fighters to stand by.
When a second plane lost contact, the tension notched up.
Any decision to destroy an airliner would be passed from the control centre to the top of the government.
The most senior of a small group of cabinet ministers can be patched into the command circuit within minutes.
Fortunately, within seconds communication with both planes was established, and everyone stood down.
It all happens frighteningly fast.
"You never know where the threat's going to come from," said Wing Commander Pat McCintic from the control centre at RAF Boulmer.
"It could be any of the aircraft out there and there are thousands flying above the airspace of the United Kingdom every minute of every day."
But if ordered to shot down a plane, "That's exactly what we would do," he added.
"It's that serious."
The pilots are constantly updated over a loudspeaker - they can expect to scramble about once a month.
Encounters with airliners are always tense.
Crews aim to launch a Tornado fighter within 10 minutes
The crews practise regularly to keep up their reaction times.
Scrambling is an adrenalin-charged sprint - unchanged since the Battle of Britain.
And they have exactly the same aim - to get combat aircraft armed and airborne, ready to face an instant threat.
Their target on the drill I watched was to launch a Tornado fighter within 10 minutes. They reckon to beat it regularly.
Their first task is to get alongside a suspect plane, and become the eyes of the decision maker - then, if needed, his finger on the trigger.
They live with everybody else's nightmares.
"Nobody wants to be called upon to do the act for real," said Bradders, a pilot with 11 Fighter Squadron. "You hope the deterrent is adequate enough.
"However, if called upon to do it for real, I probably would have to live with it.
"That's part of my job.
"It probably would be hard to live with it but you would have to live with it."
The fighters are weapons of last resort - fed by intelligence and backed up by a huge counter-terrorism operation on the ground.
But they are the most visible, and are intended to deter any terrorist thinking of staging a 11 September-style attack.