One approach to this is the "Five Why's" approach, wherein you ask "why" five times. eg, Q1: Why did the plan explode? A1: The engine cowling fell into the en

One approach to this is the "Five Why's" approach, wherein you ask "why" five times. eg, Q1: Why did the plan explode? A1: The engine cowling fell into the engine Q2: Why'd that happen? A2: The tire exploded and damaged it. etc etc. Obviously the number 5 is arbitrary and not always applicable, it's just a heuristic to get to something "root-ish" without getting to ridiculous distant things like "the laws of physics prevent two objects from inhabiting the same position in space-time". More generally, defining something as root vs. not is somewhat of a judgement call. Usually you try to find something that will prevent future problems of this sort and call it the root cause. Ideally something that your organization can mitigate with a reasonable time/cost. Note that the actual mitigation is a separate question. If runway debris is the root cause, then one mitigation is reworking the fuel system. Another would be using tougher tires. Perhaps another would be adding a shield between the tires and the aircraft body. Another might be an automated runway monitoring system that detects debris. etc.