Gulfstream IV / G350 / G450
Flight Crew Air Transport Pilot (ATP)
Fatigue; Situational Awareness
Deviation - Altitude Excursion From Assigned Altitude; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Flight Crew Became Reoriented; Flight Crew Returned To Clearance
Human Factors
We were cleared direct to Toboga Island VOR (TBG) and cleared for the ILS 03R MPTO (Panama City; Panama). Prior to crossing TBG we had set 1700 ft (the final approach fix altitude) in the altitude selector and started a VNAV descent with the FMS guiding the aircraft laterally and vertically with the autopilot connected. Upon being cleared for the approach; the pilot flying armed the approach button on the guidance panel. The aircraft overflew TBG and started to turn to track the 041 radial towards the ILS intercept. Because the approach had been armed; the nav systems started to pick up the localizer frequency and switched over to track the localizer inbound. This caused the automation to kick out the VNAV descent and the LNAV/FMS guidance and turn sharper to intercept the ILS. The pilot not flying noticed we were turning off course and pointed it out to the pilot flying. The pilot flying immediately disconnected the autopilot and began to correct back to the intermediate course segment. Concurrently; both flying and non-flying pilots noticed that the VNAV had also kicked off because the FMS guidance had switched to localizer guidance; and the aircraft was descending through 1800 ft. The pilot flying stopped the descent at 1700 ft and immediately climbed back to 2200 ft; returning the aircraft to the appropriate TBG 041 radial outbound at 2200 ft. From this course and altitude; the LOC course 033 was properly intercepted; and appropriate descent to 1700 ft was made to intercept the glideslope once crossing the TBG/041/8.4. The crew continued the approach without further incident to an uneventful landing.  The problem arose when the pilot flying armed the approach too early. The approach should have been armed when the FAF was the 'TO' waypoint. This would have allowed the FMS to guide the aircraft in LNAV/VNAV mode to track the proper feeder route and intercept the localizer inbound at the proper time. This was the end of a nearly 7 hour leg that was operated during a circadian low. Although we had 3 pilots on board so that we could rest and stay alert; the operations during the circadian low were still a factor despite our best efforts to combat the effects. Once the deviation was noticed; proper corrective actions were taken promptly.
[Report narrative contained no additional information.]
G450 flight crew reported lateral and vertical deviations occurred when approach mode was selected too early in the approach. Crew cited fatigue as a factor.