TB 55-1500-337-24
c.
When all phase inspections for an aircraft have been completed, the aircraft has completed one cycle and
the sequence of phase inspections repeats. After completing one cycle, the next phase inspection is phase one, cycle
two (i.e., the second cycle).
d.
The phase maintenance inspection system intervals designated are the maximum and shall not be exceeded
except in actual operational emergencies. It is the Commander's responsibility to determine (on an individual aircraft
basis) when inspection intervals may be exceeded. When unusual local conditions (utilization, type of mission personnel,
periods of inactivity, environmental conditions, etc.) dictate, it is the prerogative and responsibility-of the Maintenance
Officer to increase the scope and/or frequency of maintenance or inspection as necessary to insure safe operation (TM
55-1500-328-25).
e.
Because of different maintenance characteristics, different types of aircraft may have a different number of
phases in one cycle and/or a different number of flying hours between phases, as shown below:
No. of Phases
Time Between Phases (Flight Hours)
EH-1H/EH-IX
6
150
UH-1C/H/M/V
6
150
CH-47B/C
4
200
CH-47D
6
100
CH-54A/B
4
100
OH-58A/C
4
300
AH-1G/S (Mod/Prod)
4
150
OV-1/RV-1
2
300
U-21A/G
2
300
RU-21A/B/C/D/H
2
300
After sequence of phase maintenance services, a cycle is completed and the sequence repeats. By the time a cycle is
completed, all parts and systems of the aircraft requiring maintenance will have been inspected and maintained at least
once.
7.
Basic Maintenance Studies.
a.
In 1972-1973, the Army performed studies of UH-1H and CH-47C aircraft component failure and
maintenance characteristics, using a computer program, MAVIS (Model for Analysis of Vehicle Inspection System). The
studies showed that components require less frequent maintenance than prescribed by the intermediate/periodic system.
The Phase Maintenance System is the result of these studies.
b.
To confirm the mathematical evaluation obtained by the computer, a field test of the Phase Maintenance
System was considered to be highly desirable. Therefore, field evaluation programs were set up with the selected active
Army, Reserve, and National Guard Units.
8.
Phase Maintenance Field Evaluation and Optimum Phase Maintenance System.
a.
Field testing and evaluation of the Phase Maintenance System was conducted from August 1974 to
November 1975, and again in 1981 and 1982 to optimize previously obtained data. These field evaluations confirmed
the result of the computer mathematical evaluations. Although both the computer and the field evaluations were made
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