What safety rules govern air shows?

At least 11 people are thought to have died after a Hawker Hunter jet crashed into a road during the Shoreham Airshow. There have been calls for changes to safety rules, but what are the regulations currently in place?

Across the UK, planes are not normally allowed to fly any closer than 500ft (152m) from the closest "person, vessel, vehicle or structure". This rises to 1,000ft (305m) from the "highest fixed obstacle" for built-up areas.

And anyone wanting to take part in aerobatics can't do so any closer than 1,000ft from the highest point in any area, outside of controlled air space.

But air shows need to allow planes to get closer to spectators, so they can experience up close the thrill of displays. This means different rules apply on such occasions.

The Civil Aviation Authority's latest regulations, updated this year, set the minimum distance between the display area - the patch of ground over which the plane is flying - and the sectioned-off crowd area.

If it's flying at less than 100 knots (185kph), the distance between display area and crowd area can be as little as 50m. If the plane is doing aerobatics, the distance is 100m.

Planes that travel at more than 300 knots (556kph), such as the Hawker Hunter jet involved in Saturday's disaster at Shoreham, West Sussex, have to leave 200m, increasing to 230m when performing aerobatics. Between 100 knots and and 300 knots there's a sliding scale.

The distances indicated here do not deal with height, just with the proximity of the display area and crowd area. When it comes to height for planes at air shows there are no fixed minimums.

More experienced pilots are allowed to take the plane lower during manoeuvres than those newer to taking part in displays. Air show directors must write to pilots to inform them of minimum heights applicable for their events and follow this up by telling them again in person or by telephone. But the regulations do not set absolute measurements.

The Hawker Hunter at Shoreham collided with several cars on the A27, just to the north of the airfield, after hitting the ground when the pilot attempted to complete a loop.

Because airports control the airspace surrounding them, this section of the road would have formed part of the designated air show display area. This means planes could fly and perform aerobatics over it, but not above the official fenced-off spectator areas within the airport, where this is banned.

The CAA states that directors of air shows "should consider imposing minimum height restrictions over local sensitive and congested areas".

But, in its advice to pilots, it remarks that the Hunter "has little difficulty sustaining, or regaining speed and/or altitude during a low-level display". The CAA also defines the loop as among the simplest aerobatic manoeuvres.

The CAA says that, as a precaution, it has imposed a temporary ban on flights by Hawker Hunters and that vintage jet aircraft will not be able to perform "high-energy aerobatics" until further notice.

It will also conduct extra risk assessments on all forthcoming civil air displays to establish if additional measures should be introduced.

Already, pilots performing in air shows are urged to fix a "sequence card", laying out the basic details of the routine they have practised, to the instrument panel. They are responsible for their own mental and physical condition, including fatigue, stress and any medication taken, and are expected to carry out a final "mental rehearsal" before flying.

"We are pretty much seen in the UK as having one of the best safety records," says Charles Skiera, executive director of the European Airshow Council. "The UK regulations are held in high regard in Europe and formed the basis for many other European nations' own regulations.

"Air shows are second only to football in terms of popularity as spectator events and it's been 63 years since any spectators have been killed."

There are separate rules for military planes, such as the Red Arrows, that take part in UK air shows. The "normal" separation between the crowd area and display area for planes carrying out aerobatics should be 230m, the Ministry of Defence says. For planes going at more than 300 knots towards the crowd, this should increase to 450m. During aerobatic manoeuvres, they should stay at least 300ft (91m) above ground.

Nato countries have an agreement setting minimum standards when military planes from one country visit another to take place in a display. The UK still imposes its own standards on visitors, as these are deemed to be more rigorous.

"The simple fact is that England is full up," former RAF pilot George Bacon told the BBC's Victoria Derbyshire show. "We must seriously recognise that. Shoreham is by no means the only airfield that operates so close to a major arterial road. Clearly more work needs to be done. Maybe rules will be changed so that certain types of aircraft can only operate in certain locations."

It's been suggested that aerobatic displays should happen only over open water, where there is no danger of hitting spectators or buildings. Bacon is sceptical of this idea. "What happened here was that, for some reason, inexplicable at the moment, the aircraft and the pilot went outside the safety envelope," he says.

In the US, the Federal Aviation Administration says that, for planes with a cruising speed of 245 knots or more, the "optimum" distance between the airspace above spectator and display areas is 1,500ft (457m). The "absolute minimum" is 1,200ft (366m).

Over the past decade there have been several accidents at air shows in the UK. But since the Farnborough Airshow of 1952, in which a De Havilland 110 broke up after passing the sound barrier, showering the crowd with debris, no spectator has been killed by a plane. That disaster, causing 31 deaths, resulted in a full review of safety regulations, and they've often been amended since.

Subscribe to the BBC News Magazine's email newsletter to get articles sent to your inbox.