B747-400
Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine
Conflict Airborne Conflict; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter CFTT / CFIT
Flight Crew Took Evasive Action
Human Factors; Procedure
On initial approach to Runway XXL; vectored to position ZZZZZ (IAF) 2;600 ft.; and reducing speed to180 Kts. ATC cleared us for the approach Runway XXL. Pilot Flying descended below the required altitude to 2;420 feet; While the Pilot Monitoring and I (Observer seat) were giving corrective call outs; the Pilot Flying disconnected the autopilot and corrected the altitude and simultaneously ZZZZ Approach Control gave us an advisory call; which was responded appropriately by pilot monitor. Rest of the approach and landing were uneventful.
While being vectored for ILS approach to RWY XXL at ZZZZ; the Pilot Flying momentarily began an early descent through required altitude of 2;600 feet.  I; the pilot not flying; and the jumpseat pilot gave immediate verbal corrections. Aircraft descended to approximately 2;420 feet as the Pilot Flying regained situational awareness and applied corrective action to include resetting the MCP altitude; and then reverting to manual control.  The recovery was smooth and controlled.  As the aircraft was returning to 2;600 feet; ZZZZ Radar Control called to direct climb to 2;600 feet.  I responded to ATC while the Pilot Flying completed the recovery.  Approach and landing were completed uneventfully.   This was the final segment of a 2 leg international flying pairing nearing 17 hours of duty.  All crew members were not at their best state of alertness.  Original arrival setup was for a STAR coupled to an ILS approach.  Flown arrival was very non standard. The aircraft was vectored off arrival path and given clearance to disregard many published mandatory altitudes.  This complicated arrival descent planning and may have led to the Pilot Flying losing some situational awareness.  ATC Traffic on approach ahead was a potential conflict.  This diverted my attention during the approach.Delays in positive corrective action which could have lessened/prevented the altitude deviation were potentially caused by distraction due to traffic call; lessened cognitive performance due to overnight/long duty period; lack of urgency due to VMC weather with runway in view.
Flight crew reported descending below minimum altitude on approach and initiated a climb back up to correct altitude and continued the approach.