B747 Undifferentiated or Other Model
Flight Crew Air Transport Pilot (ATP)
Situational Awareness
Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter CFTT / CFIT
Air Traffic Control Issued Advisory / Alert; Flight Crew Returned To Clearance; Flight Crew Became Reoriented
Human Factors
ATIS calling visual approaches for [Runway] 28C. Pilot flying (PF) briefed the ILS Runway 28C. Upon PF request Tower told us to expect ILS 28R and subsequently PF briefed ILS 28R. The flight was cleared to descend and maintain 4000 feet and at that altitude to intercept LOC 28R. While at 4000 feet on the LOC 28R we were then cleared for the ILS 28R and to report a 5 mile final. While I (Pilot Monitoring) was looking out for traffic the PF set 2200 feet in the altitude window (Glideslope intercept altitude) and pushed Flight Level Change without making me aware of the MCP changes. Tower notified us to check our altitude and gave us an altimeter setting of 30.38 in QNH because he showed us at 2300 feet. I then confirmed to tower our altitude and QNH. Tower said that we appeared low and asked if we had the runway 28R in sight. We confirmed that the runway was in sight and PF initiated and immediate climb towards 4000 feet. Ultimately intercepted glideslope at 3000 feet and landed normally. The PF apologized to the tower controller for his improper altitude and he said in effect 'no problem; just wanted to make sure everything was okay'. Adhering to Company SOP's regarding  MCP changes would have prevented this from happening.
B747 First Officer as Pilot Monitoring reports his Captain descending to the glideslope intercept altitude of 2;200 feet well before the ILS procedure allows descent below 4;000 feet. The reporter was looking for traffic and did not notice the descent. The Tower called the low altitude resulting in a climb to 3;000 feet to intercept the glideslope.