Sea King helicopters are usually associated with search and rescue operations. Able to fly low at speeds of up to 143 mph, these distinctive craft have played a key role in locating missing people both on land and at sea.

But the helicopters that collided in the Gulf, killing all seven crew members, were the Royal Navy's Mk7 Airborne Early Warning (AEW) Sea Kings. These fulfil a different role entirely.

The craft were brought into service soon after the Falklands conflict in 1982, when it became clear that early warning surveillance was an essential part of air power at sea.

The Sea King was chosen for the job. Using hi-tech radar equipment, it can detect low-flying enemy aircraft that could be about to target surface ships under conventional ship borne radar cover. It can also be used for surface search using computer-controlled long range radar.

The usual crew is one pilot and two observers.

Sturdy 'workhorse'

Manufactured by AgustaWestland in the UK, the basic Sea King model - on which the AEW variation was based - was developed for the Royal Navy and has been in service since 1977.

There are more than 300 in service with both the RAF and Royal Navy, while the helicopters are also used by the navies and air forces of Australia, Germany, India, Pakistan and other countries.

As well as the AEW model, other versions used by the Royal Navy are the Mk6 Anti-Submarine and the Mk4, which is used to support Royal Marine commandos.

The Mk6 has its own weapons, including depth charges and the Sting Ray lightweight torpedo, and monitoring equipment such as a sonar system and a device which detects radar transmissions from other craft.

The Mk4 is known as the Royal Navy's "workhorse". It carries up to 27 troops over a range of about 400 miles. It can also carry Land Rovers, 105mm guns and air defence missile systems. It too has its own defence systems.