Narrative:

On arrival with captain flying; we monitored ILS glideslope capture at about 1;500 MSL. Descending through 1;200 MSL and in visual conditions; the aircraft began to deviate from the electronic glideslope. Passing through 900 MSL (and coincident to receipt of a 'glideslope' GPWS alert); the captain disconnected the autopilot; corrected the deviation condition; and hand-flew the remainder of the arrival. Also coincident with receipt of the GPWS caution; the tower issued a low-altitude alert of which we were already aware and monitoring. Aircraft was stable by 700 ft AGL (700 ft MSL as well in this case); within the parameters of our stabilized approach criteria. We could have caught the situation sooner and discontinued a coupled approach prior to excessive deviation. Fatigue was a contributing factor; as was a desire to figure out what was causing the deviation.

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Original NASA ASRS Text

Title: MD-11 First Officer reported the autopilot deviated below glide slope on approach. They got a GLIDESLOPE GPWS alert; as well as a call from the Tower. They disconnected the autopilot and returned to normal glide path for a landing.

Narrative: On arrival with Captain flying; we monitored ILS glideslope capture at about 1;500 MSL. Descending through 1;200 MSL and in visual conditions; the aircraft began to deviate from the electronic glideslope. Passing through 900 MSL (and coincident to receipt of a 'GLIDESLOPE' GPWS alert); the Captain disconnected the autopilot; corrected the deviation condition; and hand-flew the remainder of the arrival. Also coincident with receipt of the GPWS caution; the Tower issued a low-altitude alert of which we were already aware and monitoring. Aircraft was stable by 700 FT AGL (700 FT MSL as well in this case); within the parameters of our stabilized approach criteria. We could have caught the situation sooner and discontinued a coupled approach prior to excessive deviation. Fatigue was a contributing factor; as was a desire to figure out what was causing the deviation.

Data retrieved from NASA's ASRS site as of April 2012 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.