The Individual Aircraft Tracking (IAT) plan is an integral
element of MIL-HDBK-1530. The plan is
constructed by the airframe contractor as part of the Task IV, Force Management
Data package. The plan is implemented
by the Air Force under Task V, Force Management. The objective of IAT is to provide data on each aircraft that
reflects differences in usage from that of the baseline spectra of the FSMP.
The basic concept of the IAT plan is as follows. The FSMP specifies the timing of required
structural inspections and modifications and estimates the costs for repairs
and inspections. These times and
quantities are based on the FSMP crack growth curves as calculated from the relevant baseline (average) spectrum. Since the baseline stress histories that
were used to generate these crack
growth curves are not necessarily representative of the actual experience of
individual aircraft, a method is needed to account for the individual
differences. This is done in the IAT
Program by collecting, processing and accumulating data descriptive of every
flight of each airframe in a fleet.
There is considerable variability in the degree of complexity of the
necessary data systems required for different fleets of aircraft.
From the parameters measured in the IAT program, a crack growth
increment per flight or per flight hour is computed and accumulated for each
aircraft in the fleet. Comparing the
observed crack growth plot with the predicted plot provides a determination of
equivalent flight units for the current usage level. Figure 8.1.6 from Berens, et al.
 shows this comparison. The baseline usage life remaining until damage size af is reached is (t*-t).
The life t* defined a specified
maintenance action time. At any specific time, the total fleet can be
viewed as having a distribution of remaining life as expressed in terms
of the baseline flight hours. Such
information is then used for scheduling the maintenance activity.
Figure 8.1.6. Relating Individual Aircraft Usage to FSM Plan Usage [Berens, et
Establishing the IAT plan involves the following steps:
selection of the aircraft flight condition descriptions or parameters,
2) the development of a method to translate
these parameters into incremental crack growth,
translation of this crack growth into a measure of time which can be projected
to a future date for the scheduling of some maintenance activity, and
definition of a data processing system for maintaining and updating all of the
analyses and record keeping.
There are many approaches to IAT as driven by the use and
structural complexity of the fleet.
Generally, in the past bomber/transport type aircraft have been tracked
using crew reporting forms while attack/fighter/trainer aircraft have used load
and flight parameters to reflect the more variable usage. See Clay, et al.  for a description of
the crack growth tracking methods developed during the 1970’s. Many of these methods are still in use but
the modern micro-processor based data recording systems are permitting the use
of more sophisticated methods. See
Selder & Liu  for an example that calculates damage based on
cycle-by-cycle crack growth analysis at each control point. These processors are also blurring the
distinction between data collection for L/ESS and IAT.