B747-400 international first officers reported the pilot flying descended below charted altitude at ...

Narrative:

Initially cleared to turn left off of 170 heading (for circuit breaker 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/south 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.

Original NASA ASRS Text

Title: 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.

Narrative: 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.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.

Attributes
ACN	1212829
Time
Date	201410
Local Time Of Day	0001-0600
Place
Locale Reference	SBEG.Airport
State Reference	FO
Environment
Flight Conditions	Marginal
Light	Daylight
Aircraft 1
Make Model Name	B747-400
Operating Under FAR Part	Part 121
Flight Phase	Initial Approach
Flight Plan	IFR
Person 1
Function	Pilot Not Flying First Officer
Qualification	Flight Crew Air Transport Pilot (ATP)
Person 2
Function	Pilot Not Flying Relief Pilot
Qualification	Flight Crew Air Transport Pilot (ATP)
Events
Anomaly	Deviation - Altitude Excursion From Assigned Altitude Deviation - Procedural Clearance Deviation - Procedural Published Material / Policy

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.