Small Transport; Low Wing; 2 Turbojet Eng
Flight Crew Flight Instructor; Flight Crew Instrument; Flight Crew Multiengine; Flight Crew Commercial
Flight Crew Last 90 Days 34; Flight Crew Total 1235; Flight Crew Type 19
Situational Awareness
Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Flight Crew Became Reoriented; Flight Crew Returned To Clearance
Human Factors
While on the RNAV Z RWY 9 at TEX; we descended from 13000 ft. and leveled off at 12900 ft. Shortly after this I noticed we began descending again. Confused by this I double checked our position and the approach plate and realized we were descending prior to the final approach fix. I voiced my concerns to the PIC that we should still be at 12900 ft; the PIC immediately agreed and began a climb back to 12900 ft. The correction was made between 12200 ft. and 11900 ft.
Small Transport First Officer reported an altitude deviation occurred on the RNAV Z Approach to Runway 9 at TEX.