As part of its Safety Flashes, IMCA analyzes a failure of the winch remote control and emergency stop, during recovery of a rescue boat to the davit onboard. Investigation revealed that the davit winch was being repeatedly switched on and off for very short time intervals, causing a burn out.
The incident
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The incident occurred on completion of rescue boat testing in good weather. The personnel in the small boat were disembarked at sea level and used a ladder to access the main deck.
The AB started recovery of the boat using a remote control. When close to the final stowage position, the remote control failed and the limit switch did not activate as designed.
The main power supply switch was then operated to isolate the power supply to the winch.
Probable causes
Testing revealed that the control circuitry for davit recovery was fully operational. However, the recovery was not automatically stopped by the limit switch.
On inspection and disassembly of the contactor, it was found stuck, with contacts in the closed position.
The system had undergone annual inspection by an approved independent inspector three weeks previously with the following notes:
- Remote control checked all OK.
- Davit fall wire sheave inspected.
- All electronics checked and found OK.
The most probable cause of failure was frequent intermittent use of the winch for fine adjustment of boat level to make it easier for crew to get into and out of the fast rescue craft.
The davit winch was being repeatedly switched on and off for very short time intervals, which can cause the contacts to burn out.
Lessons learned
- Ensure correct retrieval procedure is practiced by all winch operators – limit switches should not be relied on to stop the recovery of craft;
- Confirm correct installation of components including limit/proximity switches for davit systems.
- Where limit/proximity switches have noted systems faults, inspection should be completed before further use;
- Ensure planned maintenance system covers regular function testing of emergency stop switches;
- Limit/proximity switches found to be at fault should be replaced. Replacement switches are not high cost items.
