Skip to content

Single point failure resulting in a dropped object

What happened

A small part weighing 0.3kg fell 20 meters from an OMC-160 wind speed and direction sensor installed on a vessel’s main mast. Two wind speed and direction sensors were installed on the vessel’s main mast; they consisted of a wind speed sensor and a wind direction sensor. The wind direction sensor was of the wind vane type unit and had a counterweight (length 6 cm/diameter 3 cm; 300 grams) fixed to it. This counterweight was only secured to the wind vane by a single grub screw.  The grub screw came loose causing the counterweight to fall off. There were no injuries, nor any damage to the vessel or equipment.

Before: single point failure – one grub screw
After: new securing method – two grub screws

What went wrong

  • The counterweight was secured with only one screw, and this single point failure was not identified during the previous DROPS surveys;
  • The OMC-series wind sensors are designed for minimal maintenance and as such were not part of the vessel’s planned maintenance system;
  • The original grub screw worked loose because of vessel vibration.

Preventative action

  • The crew secured the counterweight by drilling one more hole on the opposite side of the existing one, and thus used two new screws to secure it. In addition, Loctite was applied to secure these screws;
  • After checking for similar situations on the main mast, the same modification was done to the counterweight on the second direction sensor;
  • Check if there is anything similar in your work area that could fall from height following a single point failure, or, is not included in the vessel planned maintenance system – check all instruments on masts;
  • Check fixings at height very thoroughly. Particularly worthwhile and important when access is difficult.

Members may wish to refer to

Safety Event

Published: 18 January 2023
Download: IMCA SF 03/23

IMCA Safety Flashes
Submit a Report

IMCA Safety Flashes summarise key safety matters and incidents, allowing lessons to be more easily learnt for the benefit of all. The effectiveness of the IMCA Safety Flash system depends on Members sharing information and so avoiding repeat incidents. Please consider adding [email protected] to your internal distribution list for safety alerts or manually submitting information on incidents you consider may be relevant. All information is anonymised or sanitised, as appropriate.

IMCA’s store terms and conditions ( apply to all downloads from IMCA’s website, including this document.

IMCA makes every effort to ensure the accuracy and reliability of the data contained in the documents it publishes, but IMCA shall not be liable for any guidance and/or recommendation and/or statement herein contained. The information contained in this document does not fulfil or replace any individual’s or Member's legal, regulatory or other duties or obligations in respect of their operations. Individuals and Members remain solely responsible for the safe, lawful and proper conduct of their operations.