What must pilots with a beard ensure when using an oxygen mask?

Pilots with a beard must ensure that the fit of the oxygen mask does not interfere with the seal. This is crucial because an effective seal is necessary for the mask to function properly, particularly in emergency situations where supplemental oxygen is needed. A beard can create gaps between the mask and the face, leading to a decreased effectiveness of the mask in delivering oxygen. Therefore, maintaining a proper fit is essential for the safety of the pilot and the effectiveness of the oxygen supply during flight operations.