By Jim Ferguson Aviation analyst

Salvage experts working on the helicopter which crashed into the North Sea on Tuesday night, killing at least five people, face a difficult task.

Despite their understandable frustration, the team will first have to wait until the wreckage of the Sikorsky S-76, owned by Bristow Helicopters, is actually found.

They will then begin the recovery operation from about 30 metres of water.

If the crash has claimed the lives of all 11 occupants, survivor evidence is denied to the investigators

Theirs is a complex task made more difficult by the fact that the wreckage probably still contains the bodies of at least some of the six occupants still missing.

They also need to ensure that as little additional damage as possible is caused to the structure.

The helicopter, which crashed while ferrying staff from the Shell/Esso-operated Clipper to the Global Santa Fe Monarch drilling rig, is being hunted by vessels equipped with sonar equipment.

This kit is capable of "homing" on the Dukane beacons carried as standard by all support helicopters operating in the UK.

Currently it is uncertain just how many of these "pingers" were installed in this particular helicopter, but one will be fitted as standard to the cockpit voice/flight data recorder package (the "black box", which is in fact painted bright orange).

Strong currents

If the crash has claimed the lives of all 11 occupants, survivor evidence is denied to the investigators.

However, they will have taken detailed statements from the several offshore workers who saw the crash.

An inability to salvage the remains of the aircraft will leave many unacceptable uncertainties

Much may also be deduced from the fragmentary items of wreckage left floating on the surface post-impact, and the state of the five bodies already recovered.

But apart from the obvious need to recover the remaining occupants, an inability to salvage the remains of the aircraft will leave many unacceptable uncertainties.

The helicopter is at a relatively shallow water depth, but this is subject to strong currents and, twice-daily, to a two to two-and-a-half metre tidal range. Considerable difficulties could also stem from the unstable nature of the sea bed.

Previous crash

The shifting sands off the East Anglian coast have claimed far more than their fair share of mariners and their vessels over the years.

Safe navigation is a product of the kind of extensive local maritime knowledge and experience garnered by lifeboatmen, fishermen and others.

The shifting sands off the East Anglian coast have claimed far more than their fair share of mariners

In August 1981 the crash of a Bristow Wessex helicopter in much the same area brought into sharp focus the difficulties accident investigators faced.

Due to a reported initial unwillingness by senior civil servants to authorise the charter of a suitable salvage ship, the wreck was beyond recovery by the time the operation began.

As a result the reason for this tragedy and its loss of 13 lives remains a mystery to this day.

Well-equipped

Given the kind of sophisticated underwater technology carried on board the diving support vessel Mayo, which will be used for the salvage operation, and the expertise of her specialist crew members, some, at least, of these difficulties should be minimised.

The Mayo carries... some of the world's most experienced divers

The Mayo is a veteran of such delicate tasks as the recovery of the Russian submarine Kursk from the cold waters of the Barents Sea last year.

It carries a wide range of equipment for which the relatively shallow depths of the crash area is almost "over-engineered".

This ranges from various types of remotely-controlled underwater cameras and mini-submarines, through some of the world's most experienced divers, decompression chambers and supporting medical equipment, lifting tackle and cranes.

However, until the helicopter's remains are actually located all this material will be to no avail.

And it may be that the unpredictable characteristics of the seabed offshore East Anglia will once more live down to their evil reputation.