Downloaded from https://www.everyspec.com ADS-79D-HDBK A.5.2 Managing service life of Safe-Life Structural components. The service life of structural CSIs on Army aircraft systems is normally managed by a safe life process that is based on a calculation of a fatigue damage fraction. The inputs for establishing the safe lives include usage, flight loads, and fatigue strength with damage fraction calculation based on Miner‘s linear cumulative damage hypothesis.
4 Although there is no identified safety factor used to ensure the reliability of CSI reaching their retirement time without a structural failure, reliability goals are reached by a combination of conservative assumptions employed in developing the usage spectrum and flight loads in conjunction with statistical reductions included in the fatigue strength working curve. Incorporation of the FLM system allows greater certainty of aircraft usage and flight loads severity.
Due to this increased certainty, the analysis of FLM data and correlation with component fatigue capability has great potential of achieving FLM goals of reducing burdensome maintenance tasks, increasing aircraft availability, improving flight safety, and reducing sustainment costs. The following should be considered when implementing FLM in order to maximize benefits. a. Usage: FLM regime recognition monitoring system will track the maneuvers and aircraft gross weight configuration (examples: CG, gross weight, external store.). To properly account for fatigue damage for a flight or mission, fatigue damage should be established for each damaging regime.
In addition, maneuver to maneuver damage including GAG should be evaluated and included in total flight damage calculation. In the event the regime recognition monitoring system is not operational, the fatigue damage should be accounted for by applying the worst case assumed fatigue damage determined from the most current design usage spectrum at a minimum. b. Loads: Maneuver damage assigned to each regime should be based on top of scatter loads (i.e. loads that produce the highest fatigue damage for the regime). Likewise, maximum/minimum loads for maneuver-to-maneuver including GAG cycles should be based on top of scatter loads.
For systems that measure both usage and loads, the reliability of the strength curve and damage sum methodology or reliability of the strength curve or damage sum methodology should provide the reliability guidance of section A.6. c. Fatigue Strength: Fatigue damage should be calculated using the mean minus 3 sigma (μ - 3σ) probability strength with a 95% confidence level or the working S-N curves in the approved fatigue substantiation reports. d. Damage Sum: Component retirement when fatigue damages sum to less than 1 should be considered to ensure that the reliability threshold (i.e.
0.999999 (six nines) component reliability or 0.01 failures per 100,000 flight hour‘s system hazard) is met. A.5.3 Component remediation. There are myriad reasons why structural components are removed from service before reaching their respective component retirement time (i.e. fatigue life). In fact, the majority of Army components are removed due to damage (examples: nicks, corrosion, wear) prior to reaching a retirement life. Remediation is the concept of identifying and mitigating the root causes for part replacement in order to obtain more useful life from structural components (including airframe parts and dynamic components).
The safe life process for service life management bases fatigue strength on ―as manufactured‖ components. Damage, repair, and overhaul limits are established to maintain component strength as controlled by drawing tolerance limits. 4 Collins, J. A. Failure of Materials in Mechanical Design: Analysis, Prediction, Prevention . Wiley & Sons: New York, 1981. 37