By Simon Finch and Gabriel Range
Co-writers, producer and director of The Day Britain Stopped
In the drama The Day Britain Stopped, that was shown on Tuesday 13 May, two planes collide over London following a "missed approach" at Heathrow.
UK air traffic control handled 2m flights in 2002
A far-fetched and unlikely scenario? Not at all, say the programme's producer and director.
You're circling Heathrow and hear the pilot finally tell the crew to prepare for landing.
Maybe you're an anxious passenger, maybe you're a confident flyer. Either way there's always a slight frisson of nerves as your plane makes its final approach to the runway.
You know that flying is incredibly safe but you're probably also aware that statistically, landing and taking-off are when things are most likely to go wrong.
Then you're passing the end of the runway, and your fingers tighten on the arm of the seat in anticipation of the usual jolt as the wheels touchdown.
But your plane doesn't touch the ground and instead of the jolt you hear a sudden howl as the turbines are thrown into overdrive and the plane suddenly tips skyward at an incredible rate.
Your pilot has just carried out what's known as a missed approach or "go-around" - at the last minute, for any one of a whole host of reasons - a squall of wind, poor visibility, or another aircraft on the runway - he has called off the landing.
Anyone who has been through a missed approach will agree that they are at best an unnerving experience. At worst they are terrifying.
But as National Air Traffic Services (Nats) are at pains to point out, missed approaches are a very common procedure - at Heathrow they occur on average about once a day.
They also stress pilots are trained to carry out such procedures and to expect them whenever they approach an airport.
Air space 'becoming saturated'
During our research, we spoke in confidence to several commercial pilots, quizzing them about Heathrow's arrangements, and heard the same message time and again.
"I've done lots - the chances of a pilot messing it up are fairly remote." "It's not that stressful - remember it's practised in the simulator regularly".
So why would we make a documentary set one year on from a fictional mid-air collision in London - a disaster we suggest could occur immediately after a Heathrow missed approach?
Well, we also spoke to other people who determine what happens during a missed approach, the air traffic controllers themselves, and got a much less reassuring message.
In common with so much of our transport infrastructure, Heathrow is running at full capacity and it just takes the smallest event to throw it into chaos.
In 2002, UK air traffic controllers handled 2 million commercial flights, and that figure is expected to more than double within the next 20 years.
Whilst official rhetoric suggests that technology and human resources will be able to to keep up with predicted increases in air traffic volumes, the controllers themselves paint a different picture.
"An air crash is more likely than you would think. The chances of an accident are increasing by the day." "There is no spare capacity within the system anymore. Delays build up very quickly." " Air space is becoming saturated and we get nervous in holding situations."
More and more overload reports are being filed. Morale has plummeted because it is now harder to take leave. Many people are now taking early retirement.
Controllers speaking confidentially painted a 'gloomy picture'
But if the controllers painted a gloomy picture, the safety experts we spoke to were downright scary in their assessments.
One set of findings we came across was contained within a report so confidential that even its co-author is now not entitled to see a copy.
And the author is not to be dismissed lightly, he is a leading authority on air traffic control safety and trains air accident investigators around the world. And his work has identified serious concerns about the missed approach procedures at Heathrow.
When we embarked on the protracted process of requesting this document under the current open government code the Civil Aviation Authority (NATS was part of the CAA at the time of the report's publication) at first denied it was subject to its terms.
Nats seemed remarkably shy when we wrote to them about this report back in early January not replying until three and a half months later.
In their eventual email - swiftly followed by a phone call from the chief executive suggesting they had after all been anxious for a meeting in the meantime - they not only confirmed its existence, they also said it calculated there would be one collision following a missed approach at Heathrow, on average, every 20 years.
This report was published 10 years ago when traffic levels at the airport were lower than today.
But they insist they could not endorse its conclusions. Curiously however, they came up with no reason except the following: "Heathrow has been operating for more than fifty years without a single collision."
Illogical though it is, their argument appears to boil down to this: "We haven't killed anybody so far."
And the statistics do remain firmly on the side of those who wish to fly - we're not trying to dissuade people from taking to the skies.
But, despite official declarations that nothing ever compromises safety, we think our research suggests otherwise.
The Day Britain Stopped was broadcast on BBC Two on Tuesday, 13 May, 2003 at 2100 BST.