There is a simple way to track due dates on these items and that is to incorporate a physical check on these items during A checks or phase checks. Removing items which have less than three months remaining. Items with less than 3 months remaining life may not be fitted to the aircraft. This prevents any lifed item being in service beyond due date. In the case of chemical O2 generators these can be checked as part of the C check and removed or scheduled for removal prior to due date, chemical generators typically have a life of ten to twelve years which generally means they are replaced once by the operator. Replaced generators should have at least 18 months remaining. Chemical generators are marked with DOM.