The direct maintenance costs (DMC) is the primary economic index that the aircraft manufacture guarantees to the operator. DMC is regarded as a reference baseline for monitoring and controlling the maintenance expenses during the operating phase of aircraft. In this paper, by considering the guaranteed DMC value and the obligated maintenance interval listed in schedule maintenance requirements, we put forward an approach to determine the conductible window of a preventive maintenance task. The data items and process for employing the method are introduced. The economic index of a preventive maintenance task is defined and the principle for obtaining the conductible window is addressed. The conductible window is the interval within which the maintenance expense is regraded as acceptable. To control and optimize the maintenance expense, the aircraft operator can plan to repackage multiple maintenance tasks, if the conductible windows of them overlaps each other.