Flight Crew Air Transport Pilot (ATP)
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
Initially cleared to turn left off of 170 heading (for CB avoidance) to proceed direct to fix EG366 for straight in RNAV Rwy 10. Then ATC cleared us to descend and maintain 3500. Fix altitude at EG366 shows 4000 on Jepp chart. MSA in all sectors is 2000 so I did not query ATC on the lower altitude assignment. Check tgt Altitude message appeared in the scratchpad of FMS; I modified FMS altitude with Captain (CA) agreement to /3500 at EG366. Upon intercepting the vertical path at 3500 feet and cleared for the approach with minimums of 800 feet set in the MCP the pitch mode did not change to VNAV PTH; CA went to FLCH mode on the MCP at this time; and asked what the next lower altitude was. I replied 2000 but also attempted to get the CA to go back to VNAV and push the MCP alt button once to start a descent to mins. I think he did briefly go back to VNAV but then returned to FLCH and V/S as well. CA disengaged autopilot and started descending when he saw the aircraft not descending in path and aircraft got below path less than 400 feet deviation as I recall. Check Airman in center seat instructed CA to level off to intercept the path; I engaged VNAV again and left Autopilot; rest of approach and landing continued on profile.Suggestions: I should have been more assertive in getting the CA to manage the approach in VNAV. I also believe the CA was feeling additional pressure due to the presence of the Check Airman observing the approach and also the marginal weather conditions of numerous thunderstorms in the area. Since we rarely fly RNAV approaches other than in the sim; we should have discussed what we expect to see and when regarding VNAV on the approach and a backup plan of action should things not occur as planned on the approach profile.
My observation was that the CA was unsure of his exact position on the approach (both in altitude as well as in terms of navigation). This might be the reason why he selected FLCH in order to descend. It might also explain the early configuration (Flaps 10 @ 33 DME)Crew communication is a key in this event. The CA never expressed his thoughts or plans and selected different modes without verbalizing or explanations to the rest of the crew.In order to stay protected in min altitudes; VNAV needs to stay enabled during all phases of the RNAV approach.ALL crew members should be comfortable and assertive enough to voice concerns or questions. Any queries should be addressed immediately until everyone is in agreement or a safe alternate action is agreed on.
B747-400 international first officers reported the pilot flying descended below charted altitude at FAF on RNAV approach to SBEG while using FLCH rather than VNAV.