The general principle is that every higher skill level should be replaceable by a skill level immediately below it, maybe not as high, but high enough for the system to continue functioning so that the ship may stay in action.
Umthetho jikelele yeyokuba ubuchule obukhulu obungumlinganiselo funeka kuthathwe indawo noko zikwiqela elinobuchule nje obungu mgangatho ophantsi ngokukhawuleza, babungaphezulu ukuba iqhubekeke ekusebenziseni ukwenzela inqanawa zihlale zilungele ukusebenza.