Air Carrier First Officer reported they descended below their cleared altitude during approach to SDF resulting in a CFTT event before climbing back to the appropriate altitude.
Synopsis
Air Carrier First Officer reported they descended below their cleared altitude during approach to SDF resulting in a CFTT event before climbing back to the appropriate altitude.
Narrative
Approach cleared us for and approach; later issue us clearance for the approach and to cross the IAF at a lower altitude than what it's depicted and to expedite the descend. To comply with the clearance I started and expedited descend to comply with the new clearance. I selected the FAF altitude pre-select which was 500 ft. bellow the penultimate fix and during the descent trying to comply with the altitude for the IAF I descended bellow the penultimate fix; but not bellow the FAF. We saw we were descending and immediately stop the descend and regain appropriate altitude. No issues after that and continue with the approach.Cause: Attempting to comply with conflicting instructions at a high area of work. Not identifying future conflicts with the clearance. Improper FMS mode and work saturation.Suggestions: If unable to meet desired outcomes inform ATC immediately. Do not accept clearance that might create a hazard. Better crew communications. Not utilizing the proper flight mode to protect my altitudes and restrictions.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.