Narrative:

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.

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Original NASA ASRS Text

Title: 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.

Narrative: 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.

Data retrieved from NASA's ASRS site as of July 2013 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.