1. A vibration measurement system for measuring vibration to detect rotor imbalance and monitor engine health for an aircraft engine, comprising: an internally mounted accelerometer for detecting imbalance; an external accelerometer for detecting rotor imbalance; and a signal conditioning computer including software for duplicating a signal from the external accelerometer, modifying said signal by means of an algebraic function, and using the modified signal to provide the function of the internally mounted accelerometer if said internally mounted accelerometer fails.