Nick
The pilots in this case must take some of the blame as they had all the information in front of them but failed to notice their gradual descent. During my RAF training, however, we were taught and shown examples of the "chain of circumstances" breaking any link in the chain would have prevented the accident.
In this case, the chain includes:- a/c designers, the operator, the pilots, the FAA and the weather.
The a/c designers for not making a simple AVAD system standard equipment.
The operator for not installing an AVAD system and coupled autopilot if these were only supplied by the manufacturer as optional equipment.
The pilots for not noticing the loss of height or slow rate of descent or for not flying with height hold in if fitted.
The FAA for not insisting that a coupled autopilot and AVAD system be fitted to an offshore helicopter, which flew IFR, and at night.
If the weather had been better or it had been daylight the accident may have been avoided.
If any of those links in the chain had been broken then the accident would have been prevented.
The pilot flying the a/c at the time of the accident is not solely to blame it is all the support services behind the pilot who have to accept a proportion of the blame.
In the UK, we had a similar accident when a S61 enroute to the Scilly Isles in IMC slowly descended until it hit the sea killing some of the passengers. After that event there was a concentrated effort to prevent a further incident and rad alts with height warning bugs/voice calls are now standard on offshore helicopters and most also have a height hold facility.
HF