(2) Slip: Error that is occurred in judging correct balance of external information but reflects incorrect behavior or operation
(3) Lapse: Error which occurs as the result of incorrect or poor timing
Considering the relation between these different types of error and base of behavior shown in Fig.3, behavior based on skill and rule is easily prone to operative errors in which a slip and lapse occurs at the initial stage of the behavior. This is because the process of identification by forming characteristics of external information (checking mechanism) is often omitted. On the other hand, behavior based on knowledge will be easily susceptible to acknowledgement errors in which mistake occurs as a result of judging wrong balance with external information [2].
On the other hand, the following categories of human error modes are also generally used sometimes;
(1) Omission error: Error resulting from the omission of behaviors to be taken (omitting/forgetting)
(2) Commission error: Error resulting from taking different behavior to be taken (failing in)
(3) Imperfection error: Error resulting from taking of imperfect behavior to be taken (missing)
3. Analysis for seeking characteristics of human errors
An analysis was carried out using data of major failures on machinery. in engine room of ships registered with ClassNK reported by ClassNK surveyors during the period 1993 - 1998 as failures which resulted in the ship either being towed by tug or operating at reduced speed.
This major failure data contains 499 examples of main engine failure including failure of turbocharger, boiler, exhaust gas economizer, generator diesel, shafting equipment or the like that occurred during 1993 - 1998. In this study the cause of failure was selected automatically from assembling error, design error, fatigue, insufficient maintenance, fire etc., operation error, vibration as well as poor workmanship taking into account the condition the failure itself and environmental conditions surrounding the failure. For example, "insufficient maintenance" was selected with "omission error" when the failure could be considered as being avoidable if sufficient maintenance and checks had been carried out regularly and properly prior to its occurrence, while "operation error" was selected with "omission error" in cases where the failure could be considered as being avoidable if sufficient checks of machinery by such as lubricating oil pressure, cooling water temperature had been carried out regularly and properly, sometimes "commission error" was selected in a case of unsuitable operation. With respect to the fracture of turbochargers, fracture due to intrusion of a foreign substance or wear and tear was selected with "insufficient maintenance" and "omission error" while the fracture of turbocharger due to other causes was selected with "operation error". Almost of the failures concerned with the combustion chamber were judged to be due to "insufficient maintenance" and "omission error". Failure of the crankshaft around a bearing was selected with "operation error" and "commission error", sometimes "omission error". Propeller blade fractures were selected with "operation error" and "commission error", however it is also supposed to be beyond our control.
Fig.4 shows the ratio of each cause for the total of all 499 major failures reported for 1993 - 1998. It can be seen that insufficient maintenance accounts for 53% of the total, while operational error accounts for 33%, assembling error 4%, fire etc. 3%, fatigue 3%, poor workmanship 2%, and so on. Fig.5 shows that the ratio for each cause with each failed machinery or part of main diesel engine for the same 499 major failures. Cams and main diesel engine camshaft driving system were most generally impacted by insufficient maintenance while combustion chambers and turbochargers of main diesel engine have high percentage of operational error, as well.
Fig. 4 Ratio of each cause of major failure in engine room on 1993 - 1998
