! MIL-STD-81OD Downloaded from https://www.everyspec.com 19 July ‘1983 1-3.1 ~. Experiments have shown (reference,1) that the onlY ~aY to reproduce the service failure distribution,is to reproduce the service stress distribution. Stress distribution is a range of stresses, in proper proportion, of level and durations determined by mission PrOfiles. The proportioning is applied to vibration, temperature,.thermal shock and : electrical stress. Procedure I uses combinations of temperature, acouatic’ vibration, mechanical vibration, and store operating patterns to simulate in-service missions. I-3.2 ~.
Military aircraft service use may be characterized by a set of specialized missions, with a duration and relative frequency assigned to each mission type. Each mission type is described by its “mission profile an idealized mission history which specifies altitude, velocity and operating % state as a function of time; and which locates the occurrence of stressful events such as special maneuvers, gunfire and landings. From such mission profiles, corresponding mission environment histories (e.g., vibration level.$, skin temperatures) can be constructed. Data from instrumented flights may be 4 used in this construction. By treating the mission environment profiles -.
probabilistioally, summing the durations of each stress level in each mission, and weighting by the relative frequency of each mission,.a service distribution function for each stress may be obtained. A composite environment profile may then be constructed for each stress of intereat. This composite environmental profile is a sequence of stress levels constructed to sj.mulatethe service environment profiles for the different.missions taken together. Its total : duration should be no longer than a few missions, it must represent realistic flight conditions, and it must reflect the calculated combined service distribution function for the stress.
A composite mission,profile consists of the combination of composite envir”onziental profiles for each environment,‘so coordinated that the mixture of stress levels at any point in time represents 0 the typical service condition being’simulated. Simulation of typical (5th to 95th percentile) values is emphasized. If extreme values were used, similar to qualification test levels, the results would not’correlate with field experience. I-3.3 ~. The first step in developing the composite mission profile ie to determine the types of.aircraft and the types of missions with which the store is to be.employed.
For each,aircraft type,each mission will have its own typical mission operational profile, ueually charted as altitude - and velocity versus time and indicating critical points or periods. A typical airoraft.mission operational profile is shown in fisure 523.0-1. ~ The relative frequency for each type of store-carrying mission must .be established for each aircraft type along with.the Proportion Of.tOtal atOre use expected for each aircraft type. Tables, such as table 523.0-11..are usuallY -v prepared to handle this infOrmatiOfi.
Since both ,vibration-causingacoustic fields and skin’temperature .Of~ store can be related to altitude and aircraft speed,”the pattern of expected altitude/velocity combinations will be needed”for each relevant mission. Table ~ 523.0-111 shows one method organizing such data, by dividing each mission into segments or phases. Similar charts for each mission of each aircraft type are suggested. Additionally, a frequency-weightedmean of mission durations ehould be calculated.’ METHOD 523.0 19 JUIY 1983 523.0-4 ...