At all times, safe navigation requires effective command, control, communication and management. It demands that that the situation, the level of bridge manning, and the operational status of navigation systems, the ship's engines and auxiliaries are all taken into account.
It is people that control ships, and it is therefore people, together with management and teamwork, who are the key to reliable performance. People entrusted with the control of ships must be competent to carry out their duties.
People also make mistakes and so it is necessary to ensure that monitoring and checking prevent chains of error from developing. Mistakes cannot be predicted and, once a mistake has been detected, it is human nature to seek to fit circumstances to the original premise, thus compounding a simple error of judgment.
Extracted from : Bridge Procedures Guide, 4th Ed.