Abstract: There is disclosed an aircraft operation scheduling system, wherein: (a) the aircraft operation scheduling system includes a computing arrangement that computes a measure of contaminant exposure for estimating aircraft contamination, wherein the computing arrangement, when in operation, accesses a database of contamination data over location and time; (b) the computing arrangement receives when in operation schedule information related to a plurality of aircraft flights; (c) the computing arrangement receives aircraft data relating to an aircraft fleet being scheduled, and interrogates a database of contamination data to derive: (i) an estimated historical contamination for the aircraft or each aircraft engine of the aircraft to at least one contaminant; and (ii) an estimated expected contaminant exposure for the aircraft or each aircraft engine; and (d) the computing arrangement, when in operation: (i) identifies at least one aircraft engine with a higher estimated historical contaminant exposure or wit