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Section 5.4.1. Service Life Description and Mission Profiles
The load sequence developer works from the service life
requirement summary and the mission profiles as given by the aircraft
procurement documents. The service life
data contains the total flight hours, expected calendar year life, number of
missions to be flown, identification of mission types, and number of touch and
go and full stop landings. The mission
profile description provides the time variation of the airspeed, altitude, and
gross weight such as illustrated in Figure 5.4.1. Each mission is divided into segments, as
shown, which can be easily characterized by the type and frequency of the
various load sources.
Figure 5.4.1. Mission Profile and Mission Segments
The load spectrum for each mission segment is characterized by
a table of occurrences of a load parameter.
The commonly used parameter is the normal load factor at the aircraft
center-of-gravity, nz. Such a table can be presented as an
exceedance plot, which shows the number of occurrences that exceed specified
values during a specified time period.
MIL-A-8866 presents tabular exceedance data for six classes of
aircraft, broken out by mission segment.
The number of identified segments varies from three to seven. These tables give the number of exceedances
per 1,000 mission hours. The total
number of exceedances is on the order of 105 - 5 x 105. Figure 5.4.2
shows a plot of the composite maneuver spectrum for the six classes of
aircraft. This composite was made by
summing the exceedances of the mission segments for each class of aircraft.
Figure 5.4.2. Maneuver Spectra According to MIL-A-8866
These three basic pieces of information, the service life
summary, the mission profiles, and the load factor spectra are converted into
the loads history and the stress history at critical locations on the
aircraft. This procedure is briefly
described in the next section.