Abstract

A series of trials have been conducted on a range of offshore helicopter types during the past three years, which have aimed to examine the ability of passengers, during a simulated ditching and capsize, to use the prescribed emergency exits and procedures. As a result of the trials, in which problems of egress were identified under worst case conditions, immediate limitations were advised to Shell Group companies on the cabin capacity of specific helicopter types by Shell Aircraft Limited, pending significant cabin configuration changes which have included enlarged and push-out windows, and changes in seating.

Introduction

Cabin safety in fixed wing aircraft is an area where regulatory authorities have spent considerable time and effort in recent years to enhance standards. However, until recently, the same could not be said for helicopters, particularly those operating in the offshore environment. Whilst the design requirements for medium to large helicopters specify emergency exits required for surface evacuation, they do not address the underwater escape problem. The Westland's study investigated 98 civil helicopter water impacts over a 20 year period; approximately 50% resulted in fatalities. In accidents where the cause of death could be established, 56% of fatalities were the result of drowning. In a similar study of military helicopter water impacts, it was found that 82% of fatalities were due to drowning.

It was this background, and knowledge of recent accidents (in the North Sea area) that led Shell Aircraft Limited in 1992 to initiate a series of trials on a range of offshore helicopter types. These trials aimed to examine the ability of passengers to escape during a simulated ditching and capsize. As a result of the trials, in which problems of egress from an inverted submerged cabin were identified, immediate limitations were recommended to the Shell Group of Companies, on the cabin capacity of specific helicopter types on contract, pending cabin configuration changes which have included enlarged and push-out windows and changes in seating.

The Escape Environment

Captain Brooks highlights the problem of the helicopter ditching, inverting and sinking. With water rushing in through cockpit windows, aircrew and passengers have to overcome inherent bouyancy to make their escape from a flooded compartment through cargo doors, access doors, windows or the windshield. They may even be thrown out through a split in the cabin if the impact is severe. Even if the crew and passengers are uninjured, escape is difficult with loss of vision, disorientation, the requirement to hold breath under water against the gasp reflex and the extreme terror created by the catastrophe. Occupants whose passage is blocked by entanglement with debris, who cannot release their lap straps or who are injured, commonly perish.

Brooks concluded that the crew and passengers of a helicopter flying over water generally have less than one minute of warning of a ditching before they find themselves in the water. The helicopter is unstable in water and will frequently capsize if struck by a breaking wave. While some of the accidents he reviewed were survivable, he concluded that little had been done to reduce contact or acceleration injuries by introducing basic crash-worthiness principles into the helicopter. However if the crew and passengers did survive the initial impact, then the greatest threat to survival was the potential for drowning.

Brooks also pointed out that crew and passenger survival was enhanced if they had a good pre-flight briefing, were sports or professional divers and, most important of all, had practical professional training in under-water escape and the correct crash position to adopt. He also argued that for current in-service helicopters, redesign of parts of the helicopter structure would reduce fatalities, particularly the introduction of crash-worthy seats and four-point restraint for everyone on board. In addition, increasing the number of escape hatches, adding hatching in the deck, lengthening hatches to floor level, shortening the distance to travel from seat to escape hatch, simplifying hatch or window release mechanisms, making each window a push-out window, adding an underwater Braille system that would guide survivors to the escape hatch by touch, and incorporating good under water lighting would all improve the survival rate. He noted also that further opportunities for survival could be achieved by the addition of an under water breathing apparatus and the fitting of externally mounted self-stabilising multi-seat life rafts that jettisoned automatically on ditching.

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