Air accident investigators have urged helicopter manufacturers to amend flight manuals to include details of features that could hamper any crash locator system.
The Air Accidents Investigation Branch (AAIB) made the recommendation following an inquiry into two helicopter ditchings in the North Sea less than six months apart.
In both cases the crews survived but the accidents, caused by gearbox failure, led to new advice on checks for the EC225 Super Puma model.
The AAIB recommends that the European Aviation Safety Agency require manufacturers of all aircraft equipped with a type 15-503 crash position indicator (CPI) system to change their operation manual.
CPI includes a water-activated switch, a beacon and an aircraft identification unit to alert rescuers to the craft's position in the event of a crash.
The CPI system on the Super Puma which crashed about 30 miles (48km) off the coast of Aberdeen on May 10 last year did not release automatically, the AAIB said in its latest report. This is despite the water level in the cabin being above that of the water-activated switch. The AAIB is looking into the function of the helicopter wiring when submerged and the switch's design as possible reasons.
When another Super Puma carrying an oil crew to a rig ditched 86 miles (138km) north-west of Shetland on October 22, the CPI did correctly issue a distress signal but only after a crew member manually selected the "transmit" function, the AAIB said. Had the helicopter not remained upright a beacon would not have been released because the water-activated switch is rendered redundant when the transmit function is selected manually, it warned.
As a result of the findings, the manufacturer of the CPI system has amended its operating manual to underline that it must be reset following a manual selection in order to restore its functions.
The AAIB said the manuals for aircraft equipped with the system should also be amended. The recommendation was also made to the equivalent body in the United States, the Federal Aviation Administration.
Following the second ditching in October last year Super Puma makers Eurocopter advised more thorough and frequent monitoring of gearboxes on the craft.