GPS Position Misread
| GPS Position Misread |
NZ Official Report. |
The KOTUKU is a coastal tanker of 171.44m in length with a gross tonnage of 15,215 tonnes. This report was compiled by the Maritime Safety Authority of New Zealand and describes the events on a voyage from Wellington to North Island. It was kindly forwarded to me by a member and has a sorry tale to tell, particularly as the majority of the officers involved had Masters Certificates.
Between 0100 and 1605 0n the 10th November 1997, the vessel was subject to a number of shifts around various berths whilst discharging its cargo in Wellington. The vessel cleared Wellington at 1747 and proceeds northwards.
2100 The master leaves night orders that courses are to be followed as laid down in the voyage plan. Standing Orders are to be observed and he is to be called at any time if the officer of the watch thinks it is necessary.
2200 The course is altered to 0590(T).
11th November
0001 The 2nd officer takes over the watch from the 1st officer. Chart NZ 58 is in use and this is to be followed by charts NZ 57 and NZ 56.
0015 Course is altered to 0370(T).
0200 The GPS readout shows 410 01.7' S 1760 21.5' E. This should be plotted on NZ 57 but the 2nd officer misreads it as 400 01.7' S 1760 21.5' E and plots it onto chart NZ 56. He uses only GPS for fixing thereafter and reproduces a similar mistake every half hour until 0400.
0400 The chief officer relieves the 2nd officer and perpetuates the navigation error throughout his watch by misreading the GPS readout and plotting the erroneous result on the chart every half hour for four hours.
0500 The chief officer alters course to 0310(T).
0645 The master relieves the chief officer. The master notes that the plotted position is approximately abeam of Table Cape. He does not question the position because he has always found the chief officer to be competent. The master takes a GPS position but does not take visual bearings or a radar bearing and range to check the ship's position.
0745 He plots an erroneous position on chart NZ 55 from which he notes in the deck log book that Tuaheni Point Light is bearing 2900(T) at a range of 19 miles. On this information he alters course to 0100(T) and (G).
0800 The 1st officer takes over the watch. He obtains a radar echo of land on the starboard side of the ship which matches Mahia Peninsula. The GPS confirms this and the 1st officer brings it to the attention of the master. The tanker is found to be some 75 miles from where the other navigating officers thought the vessel was situated and is in danger of grounding.
0830 With Cape Kidnappers bearing 2600(T), distant 19 miles, course is altered to 0650(T), so as to clear the Mahia Peninsular.
COMMENT
The master, chief officer and 2nd officer held Certificates of Competency as master. The chief officer, 1st officer and 2nd officer kept sea watches whilst the vessel was in port. Over the 24 hour period prior to the incident, the chief officer had been on duty for 15 hours. The day before he was on duty for 12½ hours.
The 2nd officer kept the 12 - 4 watch. He worked a total of 11 hours on the 10th November. He turned in at 1930 that day and went to the bridge at midnight. He was aware of the passage plan and the Safe Bridge Management Procedures which contained a requirement to check the vessel's position by more than one method.
The master, the chief officer and the 2nd officer were aware of the Safe Ship Management (SSM) procedures that were issued by the owning company. These included the following instructions:
"As a minimum, at least two independent means of position fixing are to be used whenever possible. Individual personnel are to assure themselves that they are adequately rested and are capable both medically and physically to undertake the duties assigned to them".
"Personnel are to report to the master immediately if they consider themselves or any other person unfit for duty".
The master and chief engineer are to monitor duty periods to ensure (that) all persons who are assigned duty as officer in charge of a watch, or as rating forming part of that watch, shall be provided a minimum 10 hours rest in any 24 hour period".
The hours of rest may be divided into no more than two periods, one of which shall be at least 6 six hours in length"
"Notwithstanding this, the minimum period of 10 hours may be reduced to no less than 6 consecutive hours provided that any such reduction shall not extend beyond two days and not less than 70 hours rest are provided in each 7 day period".
It would appear that the hours of rest of the master complied with the company's SSM procedures, with the exception that on the day before the incident, the longest period of continuous rest was 5 hours rather than 6 hours. The 2nd officer's periods of rest also complied with the SSM procedures, with the exception that on the day before the incident, the longest period of continuous rest was 5½ hours rather than 6 hours.
It was the duty of the 2nd officer to see that each chart was readily available for the passage and to check that the courses were laid off on the appropriate charts for each voyage.
Chart NZ 57, onto which the position should have been transferred at 0200 hours was later found to be at the bottom of the set of charts which had been set aside in a drawer for the voyage.
The vessel left Wellington making about 11 knots, rolling heavily at times and taking spray on the main deck from a southerly sea with the wind estimated at 40 knots.
CONTRIBUTING FACTORS
Broken work and sleep patterns occurred over the previous 48 hours and an unexpected shifting of ship in Wellington, combined with bad weather as the vessel left Wellington, made at least one of the officers involved in this incident, very tired.
Mistaken acceptance of the accuracy of GPS fixes and the assumption that, since the previous watchkeeper had always read the GPS properly and plotted the position correctly on the right chart, this would always be the case and would never need checking.
Failure of the watchkeeping officers to check the position by reference to other navigational equipment (as required by the Standing Orders and the basic principles of proper seamanship).
Failure to use chart NZ 57 onto which the position should have been transferred at 0200.
Misreading the GPS digital readout and the resultant error in plotting would normally have been obvious but, on the ship's intended track, the limits of latitude and longitude scales on one chart (NZ 56) differed from its predecessor (NZ 57) by exactly one degree and thus the minutes divisions of both latitude and longitude scales were the same.
CAUSE
The vessel strayed off her intended track because the 2nd officer ignored the degree units of the GPS readout. He plotted an erroneous position on a chart which was not next in line for use. He did not check his position by any other means. He repeated this mistake every half hour for the remaining two hours of his watch. This mistake was replicated every half hour for the next four hours by the chief officer and once, later, by the master who was briefly on the bridge to relieve him.