In a split second, a perfect spring day turned into a scene from hell.

A jet carrying 92 passengers smashed into a light aircraft in the skies over Sussex, exploding into a fireball and sending flaming wreckage crashing to the ground.

Many were killed and hundreds were injured as parts of the fuselage fell on homes, schools, factories and a hotel.

Thankfully, yesterday's terrifying scenes were simulated, part of a mock 999 disaster to test the emergency services' response to a major incident like an air crash, a terrorist attack or last week's train crash in North Korea.

But "victims" and emergency workers played their roles for real, simulating the carnage and the chaos that would be caused by such a catastrophic incident.

The six-hour exercise, codenamed Operation Woodchat, was the biggest emergency planning try-out in Sussex for ten years and took 14 months to plan.

More than 1,000 police, fire, ambulance, medical and military personnel were involved, along with the Coastguard, the Environment Agency and local authorities. All scrambled emergency teams to deal with the incident which was spread over fives sites.

The first emergency calls were received just after 10am, saying a Boeing 737 with 92 passengers and six crew had been involved in a mid-air collision with a Cessna 414 with four people on board near the approach to Shoreham Airport.

Crews were told part of the fuselage had crashed on to a fictitious hotel and a factory.

Debris had also rained down on Shoreham and Portslade, hitting two imagined schools and a chemical warehouse, causing highly-toxic substances to leak into the harbour.

Other notional incidents were dealt with at Fishersgate and at Isfield, near Lewes.

For the purposes of the exercise, the old cement works on the A283 near Upper Beeding was turned into a disaster site.

By the time the first emergency crews arrived, all that was left of the five-star hotel was a mound of rubble, scattered with bodies.

Next door was the remains of a factory in which people were trapped. Fire crews from East and West Sussex used listening devices to locate bodies and heavy-duty equipment used in earthquakes to cut through concrete.

As the crews worked to release those trapped, medical staff began assessing casualties.

Inflatable tents and a temporary mortuary were erected. The injured were colour-coded for levels of treatment and emergency doctors from Sussex and Surrey Immediate Medical Care Team (SIMCAS) treated the most seriously injured before they were ferried to hospital.

Above them, the Coastguard set up a winch on the cliff face to rescue people stranded on a flat roof.

SIMCAS spokesman John Moule said: "When you arrive at the scene of a major incident like this, it is chaos. It is how quickly you can organise the chaos that counts.

"From our point of view, if someone with life-threatening injuries is going to survive a major incident, they need to be away from the scene within an hour. That is where the triage system of identifying the severity of someone's injuries comes in.

"The first doctor on the scene will not treat people. He is there to assess the need. In the same way, the first ambulance crew that comes doesn't treat anyone, it assess.

"That system works and has come from very bitter experience with incidents like IRA bombings."

Mike Hetherington, assistant divisional officer for West Sussex Fire and Rescue Service, was in charge of retrieving bodies and casualties from the collapsed hotel.

He said: "In this case, there were 70 people in the hotel and only 20 walked away, leaving us with 50 casualties on the ground and an unknown number of people on the plane.

"While searching the site, the building was still alight and there was the ever-present danger from aviation fuel.

"Although an exercise like this does require some imagination - the rubble didn't look much like it had ever been a five star hotel - it does help us work together."

A 12-strong search and rescue team shored up the party-collapsed "factory" while it dug out trapped survivors.

Dave Dickson, urban search and rescue co-ordinator said: "From an outsider's point of view it may not be realistic but when you are involved, it is as realistic as you can get. There is a real concrete wall to get through, there are real people trapped in a void beneath a floor slab.

"It has been a worthwhile exercise and there will be a number of learning points to come out of it."

One of the "victims" lying face down in the casualty tent was Leanne Pead, 15, who had escaped the hotel impact but was impaled on a metal pipe.

Her real-life younger sister Amy, 11, had already been rushed to hospital with a fractured skull.

Leanne said: "I was found among the rubble of the hotel. My injuries are life-threatening but I will survive. Amy and I are members of the Casualties Union so we spend quite a few weekends going out and re-enacting disasters like this."

At Shoreham Harbour, East Sussex Fire and Rescue Service dealt with the aftermath of debris crashing into a chemical factory that had polluted the harbour. Firefighters set up mass decontamination tents.

Exercise director Steve Voice, of Sussex Police Operations Department, said Operation Woodchat was crucial to test resources.

He said: "This has been a tremendously successful exercise and there will be a considerable number of learning points we can draw from it.

"What it has shown is that when they are put to the test, the emergency services and all the other agencies in Sussex, can and do work together effectively."

He said: "We hope our activities did not cause too much disruption or alarm to people enjoying the fine spring weekend but we need to test our procedures so people can be assured we are in good shape if we need to respond to a major emergency."