journal of transportation and statistics - Research and Innovative ...

Result 2 (imperfect repair): The rate of unscheduledlandings decreases after a block repair, with thedecrease greater for the initial repair than for subsequentrepairs.To consider the issue of differential effects for theaircraft model and operator, the additional termswith indicator variables were included. Table 6 presentsthe regression results. The additional sum ofsquares for models and operators indicated in table6 are considered after including other effects. Thatis, the outcome (number of unscheduled landings)was adjusted for the model effect when consideringthe operator effect, and it was adjusted for the operatoreffect when considering the model effect. Inboth cases, the effects are statistically significant(i.e., there is a differential effect for operators andmodels). In the context of the regression equation,the effect of maintenance on unscheduled landingsTABLE 6 Comparison of Models and OperatorsVariable Parameter Estimate T Pβ 1X 1 –1.46 –4.73 0.000β 2X 2 2.84 2.19 0.029β 4X 4 0.18 7.51 0.000β 5X 5 0.014 7.13 0.000γ 11V 1 X 1 2.44 6.04 0.000γ 12V 1 X 2 1.88 1.39 0.166γ 22V 2 X 2 3.52 1.57 0.116λ 1UX 4 –0.14 –3.06 0.002λ 2UX 5 0.004 0.99 0.325ANOVA (OPERATOR)AdditionalSource SS df F POperator 131.12 3 13.125 0.000Error 2,104.73 632Total 4,964.00 641ANOVA (MODEL)AdditionalSource SS df F PModel 40.91 2 6.143 0.002Error 2,104.73 632Total 4,964.00 641was not the same for the operators in the study. Aswell, the effect of the time since maintenance wasnot the same for the models selected.Result 3: The relationship between the rate ofunscheduled landings and the time since the lastblock repair and the number of block repairsdepends on the aircraft model and operator.Using the indicator variables, it is possible towrite out the fitted equation for each (operatorand model) type. The estimates for equationparameters are given in table 7. The equation foroperator O 2 (q = 3) is slightly different from theequation using only O 2 data, owing to the greatervariation in using multiple operators and models.However, it is a good reference for understandingthe changes in the equation with the operator/modelvariations. The biggest operator effect is the differenceof O 3 (q = 2,5) from the others on the estimateˆˆβ 1 . For aircraft models, the estimate of β 4 is mostaffected.TABLE 7 Comparison of EstimatesqV 1 V 2 U1 000 –1.46 2.84 0.18 0.0142 100 0.99 4.70 0.18 0.0143 010 –1.46 6.35 0.18 0.0144 001 –1.46 2.89 0.04 0.0185 101 0.99 4.70 0.04 0.018DISCUSSIONβ 1qThe operating condition of aging aircraft has been ahotly discussed topic for more than a decade. TheFederal Aviation Administration’s position is thatthe operation of older aircraft is an economic decisionand not a safety issue; that is, aircraft can berepaired to a safe operating condition and the costof those repairs is the issue.This study considers the trajectory of the healthstatus of an individual aircraft, with an emphasis onepisodes where flights are interrupted because ofmechanical failures affecting safety. In the context ofa model for mechanical failure, two experimentswere carried out. In the first experiment, a singlemodel and carrier were analyzed for the potentialimpact of aircraft age (accumulated use) and repairon schedule reliability. The study assumes that allthe selected aircraft are equivalent except for age,β 2qβ 4qβ 5q10 JOURNAL OF TRANSPORTATION AND STATISTICS V8, N1 2005