B777-300
Flight Crew Air Transport Pilot (ATP)
Deviation - Altitude Overshoot; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Flight Crew Became Reoriented; Flight Crew Returned To Clearance
Aircraft; Human Factors
First Officer [was] pilot flying. Vectors for EGLL Runway 27R. Cleared to descend to 6;000 FT. FLCH mode used with right autopilot engaged. Transition altitude 6;500 FT. Captain reset altimeter to 1006 Hp leaving 7;400 FT. First Officer did not reset altimeter to QNH until just prior to 6;000 FT. Airplane descended below 6;000 to approximately 5;700 FT with autopilot engaged. Once discovered by Captain; First Officer disengaged autopilot and climbed back to 6;000 FT. EGLL Director did not acknowledge or indicate observation of deviation. Ensure all altimeters are reset to QNH in descent 1;000 FT prior to transition altitude if clearance is below transition.
We were on downwind turning base to Runway 27R at EGLL. We were cleared from FL080 to 6;000 FT with an altimeter of 1006. The transition level was 6;500. I was flying with the autopilot on and selected FLCH to descend to 6;000 FT. We were in the turn to base when I saw BAROSET DISAGREE on the EICAS screen. The Captain had already selected 1006 on his altimeter so I pushed mine from STD to 1006. This was around 6;400 FTto 6;300 FT. We got a caution alert and I saw ALTITUDE ALERT on the EICAS SCREEN. I looked at the altimeter and it showed us 300 FT low (5;700 FT) so I immediately disengaged the autopilot and pulled the nose back up. The altitude 6;000 FT was in the altitude capture window and we were in FLCH. I couldn't understand why it didn't capture it. My guess is that I selected 1006 on the altimeter while it was transitioning to the capture mode and it disrupted the normal automation. ATC did not mention the excursion.I should have selected 1006 right after FLCH going from FL080 to 6;000 FT. The low altimeter setting made the Transition Altitude 6;500 FT that day so I needed to hit it faster. I was monitoring a descent; speed change; my time to the runway; and general visual field orientation and my cross check missed the lack of altitude capture. The B777 is so reliable that I was over confident in letting it back me up. This is a side note and had happened to me once before several months earlier. I was descending to 6;000 FT where 6;000 was the transition level. I switched the altimeter from standard to local just prior to 6;000 FT and it didn't capture the altitude that time either but I was watching it and leveled off at 6;000 FT. We discussed it then and guessed that it might have been because I selected it while it was capturing the altitude. It might be worth asking Boeing if this is a possibility.
B777 flight crew reports descending below their assigned altitude due to the altimeter setting being changed by the flying First Officer from QNE to QNH just as the aircraft is leveling at 6;000 FT. The aircraft levels off 300 FT low on QNH and is immediately returned to 6;000 FT.