Section 8.1.2. Loads/Environment Spectra Survey (L/ESS)
As noted earlier, the initial FSMP is based on the design load
spectrum with its corresponding stress sequences at the critical
locations. Experience has shown that
the actual usage spectrum usually differs significantly
from that anticipated at the design stage of development. Accordingly, ASIP calls for a data
collection and analysis program to ascertain the baseline usage spectrum of the
fleet. The results of L/ESS provide the
data for checking design load assumptions and for updating the FSMP through new
crack growth curves of updated damage tolerance analyses. L/ESS does not directly impact decision
making in the development of the FSMP.
The L/ESS objectives are met through the collection of time
histories of sufficient parameters to characterize
the magnitude, frequency, and order of the stresses being encountered at the
monitored structural locations.
MIL-HDBK-1530 recommends that 100 percent of operational aircraft be
instrumented to measure relevant load parameters but that the data from 10 to
20 percent of the fleet be used to capture
valid operational loads spectra. The
assumption is made that the monitored flights are representative of all
flights in a known stratification of usage such as type of mission or mission
segment. The collected data are
compared to the design spectrum and analyses are updated as needed. The L/ESS process continues through the life
of the fleet so that data are available when change in usage dictates the need
to update the damage tolerance analyses.
The L/ESS influences the FSMP through the damage tolerance
analyses and analyses that feed the crack growth curves of the IAT. When sufficient data have been collected
from the L/ESS to begin to define a spectrum, it can be compared with the
design data that were used to generate the IAT analyses. Variations in the usage parameter
distributions can be determined. Various comparisons can be made depending on
the parameters being measured and analyzed in the tracking function. It is noted that the IAT parameters
typically comprise a subset of the L/ESS parameters.
Exceptions occur when strains are used as IAT parameters but not used in the
L/ESS. Commonly, comparisons are
made on the basis of differences in the load factor spectra. If the L/ESS is representative of the force
usage, then the comparisons should be within sampling variation. If the spectra are significantly different,
the L/ESS methods should be examined and possibly modified or the IAT methods
should be examined and modified.