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There have always been uncertainties in supply chains. If we knew what products were going to fail or what parts customers were going to order there would be no need to forecast demand. If suppliers always delivered perfect orders on time, there would be no need to buffer their variability. If both were true there would be no need to carry safety stock as the whole supply chain becomes deterministic, without probabilistic uncertainties. If this were the case, we would have no need for material planning systems.
None of those are realistic attributes of real supply chains! Demand, especially in certain verticals such as commercial aviation, has extreme variability of already low demand volumes. Forecasting is clearly required, but not sufficient. While increasing in precision with specialized algorithms, connected assets, predictive maintenance etc., forecasting can help us gauge the variability of demand. Much more comprehensive planning systems are required to account for it though. Lead time variability introduces uncertainty from the supply side of the equation.