Some components fail suddenly with no warning signs. For these components there can be no preventive
maintenance. For them, scheduled maintenance is only a waste of time and money.
Other components show warning signs of impending failure, but for only a short period of time. For these
components, scheduled maintenance may catch the early stages of degradation and produce some beneficial effects,
such as a slight improvement in operational readiness, but the cost of preventive maintenance is too high. The net effect
of scheduled inspections for this class of components is low payoff in operational readiness and high economic cost. An
optimum maintenance system would consider this class of components to be not suitable for preventive maintenance.
A third group of components show warning signs of impending failure for an extended period of time before
failure. This is the group where preventive maintenance has a high payoff in increased operational readiness and in
reduced maintenance costs. By arranging the components in phases over a cycle, scheduling of preventive
maintenance is easily managed. This is the main reason for phasing maintenance over a period of time. In addition,
duplicate inspections routinely conducted on a daily basis by the flight crew, the crew chief and also by maintenance
personnel were assigned during phase checklist workup for only one inspection in order to save work hours. Therefore a
practical optimum maintenance system was developed in the form of phase maintenance.
In preparing the Phase Maintenance Checklists and the Daily Checklists, the following information was used:
(3) Compilation of distinct component mixes for phase maintenance.
(4) Calculation of optimum time interval between phases.
(5) Calculation of number of phases per cycle.
10. Preventive Maintenance Requirements. Preventive maintenance requirements are summarized in the daily and
phase maintenance checklists. The applicable -23 technical manual contains the specific details and procedures for
accomplishment of these requirements. Special inspection requirements are contained in the applicable -23 technical
manual and phase maintenance checklists. In cases where revision to the -23 technical manual has not been completed
to include new requirements, the new requirements for special inspections are covered in the applicable phase
Section IV. INSTRUCTIONS
Changeover Instructions, Periodic to Phase.
Changeover instructions in this technical bulletin cover Army aircraft. As other aircraft are changed to phase
maintenance, these instructions shall apply.