FET means forced expiratory time, which is the time from the beginning of a forced exhalation (the back-extrapolated time zero) maneuver to the end of expiration.
42 CFR § 37.91
Definitions provided in 37.2 will have the same meaning in this subpart. Any term defined in the Federal Mine Safety and Health Act of 1977 (Pub. L. 95-164, as amended) and not defined in 37.2 or this section will have the meaning given it in the Act. As used in this subpart: