B747-400
Human-Machine Interface
Deviation - Altitude Excursion From Assigned Altitude; Deviation - Altitude Overshoot; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter CFTT / CFIT
Flight Crew Returned To Clearance
Human Factors
The flight conditions were typical for this time of year with considerable weather and turbulence for the last 40 minutes of the flight. As we entered Bagram's airspace we were told that due to traffic we would be held high longer than normal. We approached the airport from the west and were being vectored northbound on the west side of the airport. When finally given descent clearance we were requested to maintain a speed of 300 KTS. This is a higher speed than we normally use when descending into Bagram; as a lower speed allows us to have a greater angle of descent. Once north of the field we were vectored to the east side of the airport for a downwind and base for Runway 03. The Controller was busy with other aircraft in our vicinity so when he turned us southbound for a downwind I asked the First Officer to request direct GINRI and clearance to begin the approach from that point as it would a better option. We were cleared as requested. I then had the First Officer request a speed reduction so that we would not overshoot the DME arc. We slowed to 210 KTS while descending to GINRI. When the aircraft began its turn to join the arc just prior to GINRI (still in the descent to 12;500) the First Officer asked me if I would like him to set a lower altitude in the MCP window. I said no as I wanted the aircraft to capture the altitude so that we could change modes from VNAV SPD to VNAV PTH. At this point Approach Control called out two aircraft in our area and asked us to slow to final approach speed. I was looking for the traffic; while slowing as requested; when the First Officer set the MCP altitude to 7;000 FT anyway thinking that the aircraft would descend on profile. We had not captured our altitude of 12;500 when he did this and the aircraft continued descent in VNAV SPD. When my attention returned inside the aircraft I noticed that we were low for that segment of the arc. There is a mountain ridge between GINRI and DAPUB which requires us to maintain 12;500 FT between these two points. Knowing this I disconnected the autopilot and autothrottles and began a climb to the correct altitude for that segment of the arc. At virtually the same time I did this we received a terrain warning from the GPWS. We reestablished ourselves at the proper altitude on the arc and continued the approach; landing without further incident. Other points to note are that all 4 crewmembers were in the cockpit for the descent and approach. The Captain of the second crew called out altitude as I was beginning the process of correcting it. Approach Control did not query us when we descended below the required altitude. I had weather displayed and the First Officer had terrain displayed.
B747 Captain describes an approach into OAIX where the aircraft is allowed to go below profile due to the First Officer setting a lower altitude in the MCP window before the previous altitude had been captured by the autopilot. This overshoot is quickly detected and corrected by the flying Captain.