Requirements Evolution is considered one of the most critical issues in developing computer-based systems. Despite the recognised role of requirements in Software Engineering, Requirements Evolution still remains a phenomenon little understood from both quantitative and qualitative perspectives. This paper investigates our ability to understand the Requirements Evolution Process by means of metrics. The empirical investigation of an avionics industrial safety-critical case study assesses our ability to monitor Requirements Evolution by evolutionary trends and the Requirements Maturity Index (RMI). We propose an enrichment of the RMI in order to take into account process aspects and history of changes. The empirical results support the models underlying the proposed metrics. The empirical nature of this work allows to replicate the experiment in other industrial contexts and to benefit of our results.