Bearing envelope analysis (BEA) is a powerful technique in the detection of faults in bearings. The improper selection of the envelope window frequency and window bandwidth can render the analysis ineffective. This can reduce the ability of a health and usage monitoring system (HUMS) to correctly identify a degraded bearing. This occurred recently: a teardown analysis (TDA) of a utility helicopter oil cooler fan housing found extensive bearing damage. The HUMS did not detect the fault. This paper is an analysis of why the BEA failed to detect the damage bearing. A description of the BEA is given. Using raw data that was collected prior to the TDA, various windows where tested on the BEA, a measure of effectiveness for BEA window selection was developed, and a window is suggested that would have detected the bearing fault is given