Not sure of how things affect Vulcans, but Shackletons had varying lives based on the work they were doing, and it had to be put into a Fatigue Index formula. Circuits and bumps seemed a quick way to use up FI, with MOTU use, Maritime and AEW work being less stressful in that order. Just to make it more confusing, various components have different lives, including three parts of the spar boom.