Narrative:

On approach to runway 25L at vhhh; we were cleared to 8;000 MSL and were proceeding direct to td VOR; and cleared for the ILS runway 25L approach. We were in a descent to cross the td VOR at 8;000 MSL and had arrived at altitude slightly before the fix with VNAV engaged I began a speed reduction and flap selection to 180 KTS; I called for 4;500 to be placed in the MCP altitude window which was the altitude restriction at lotus the join point on the ILS. At approximately 7;500 MSL the ATC controller advised us there was a possible altitude fluctuation; however VNAV vpi showed us on path; this transmission contributed to us feeling as though we were high; though I believe he merely saw us in a slowing segment. To be sure I would make the restriction I selected V/south and placed the altitude predictor approximately 1 mile prior to lotus. I then selected approach mode and planned to arrive at lotus at 4;500 MSL capture the altitude; and await localizer and glide slope capture. As we approached lotus we received an altitude cap on my FMA and I checked the glide slope; to my surprise it showed us 1 dot high on the glide slope; which surprised me because at lotus at 4;500 MSL we should have been below the glide slope and have been awaiting capture from above. (The profile view indicated glide slope intercept at .9nm inside of lotus). At that point I tried to capture the glide slope by selecting a lower alt and engaging V/south. We captured the glide path and continued down; for about 30 seconds when I received a degraded glide slope signal FMA; followed sometime later by complete loss of the glide slope indication. Since we were in VMC conditions at that point and I thought it would only be a momentary loss of signal by a vehicle or something similar I selected V/south and continued the descent awaiting re-acquisition of the glide slope signal. At that point we received a transmission from ATC asking our current altitude; we informed him it was 3;700 ft and he informed us that that altitude was below glide slope for that distance. Almost simultaneously we reacquired the glide slope and it showed us 1.5 dot low on the glide slope; I immediately disconnected the autopilot and hand flew the aircraft back to glide slope and reengaged the autopilot. We landed without further incident. In reviewing the chain of events that lead up to and contributed to this incident I realized there were several factors. 1. First; I believe there was an element of mental fatigue; as it was the end of an 8 hour flight arriving in vhhh; this I believe hindered our recognition of what was happening. 2. My approach briefing did not emphasize the importance of 3 key elements: a. Glide slope ball note-- glide slope signal unusable beyond 7 degrees left of localizer course. B. The profile view shows that there is actually a level segment at 4;500 ft for .9 NM before glide slope intercept; so any capture or indication supporting a descent prior to 14.1 nm should have been disregarded. C. Taking those factors into account I should have briefed that I would only select localizer until sure of a reliable glide slope signal 3. Another contributing factor was the ATC transmission we received notifying us of 'possible altitude fluctuation' as it served to contribute to the crews belief we were high; supported by a false high glide slope indication; when in fact we were not high. This combined with an over reliance in glide slope indication vs. Approach plate altitude discipline influenced my decision to start a descent to capture the glide slope. D. Lastly; in retrospect with confusion on why we were indicating above glide slope; at a point in the approach where I knew we should have been below the glide slope; followed by loss of glide slope indication I realize that I should have executed an immediate go around to determine the inconsistencies regardless of the VMC weather conditions.

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Original NASA ASRS Text

Title: An air carrier crew described a confusing VHHH Runway 25L ILS approach. The ILS was predictably unreliable at their location; ATC communications were misleading and the crew was fatigued thereby not understanding what the flight instruments were depicting.

Narrative: On approach to Runway 25L at VHHH; we were cleared to 8;000 MSL and were proceeding direct to TD VOR; and cleared for the ILS Runway 25L approach. We were in a descent to cross the TD VOR at 8;000 MSL and had arrived at altitude slightly before the fix with VNAV engaged I began a speed reduction and flap selection to 180 KTS; I called for 4;500 to be placed in the MCP altitude window which was the altitude restriction at LOTUS the join point on the ILS. At approximately 7;500 MSL the ATC Controller advised us there was a possible altitude fluctuation; however VNAV VPI showed us on path; this transmission contributed to us feeling as though we were high; though I believe he merely saw us in a slowing segment. To be sure I would make the restriction I selected V/S and placed the altitude predictor approximately 1 mile prior to LOTUS. I then selected approach mode and planned to arrive at LOTUS at 4;500 MSL capture the altitude; and await localizer and glide slope capture. As we approached LOTUS we received an ALT CAP on my FMA and I checked the glide slope; to my surprise it showed us 1 dot high on the glide slope; which surprised me because at LOTUS at 4;500 MSL we should have been below the glide slope and have been awaiting capture from above. (The profile view indicated glide slope intercept at .9nm inside of LOTUS). At that point I tried to capture the glide slope by selecting a lower alt and engaging V/S. We captured the glide path and continued down; for about 30 seconds when I received a degraded glide slope signal FMA; followed sometime later by complete loss of the glide slope indication. Since we were in VMC conditions at that point and I thought it would only be a momentary loss of signal by a vehicle or something similar I selected V/S and continued the descent awaiting re-acquisition of the glide slope signal. At that point we received a transmission from ATC asking our current altitude; we informed him it was 3;700 FT and he informed us that that altitude was below glide slope for that distance. Almost simultaneously we reacquired the glide slope and it showed us 1.5 dot low on the glide slope; I immediately disconnected the autopilot and hand flew the aircraft back to glide slope and reengaged the autopilot. We landed without further incident. In reviewing the chain of events that lead up to and contributed to this incident I realized there were several factors. 1. First; I believe there was an element of mental fatigue; as it was the end of an 8 hour flight arriving in VHHH; this I believe hindered our recognition of what was happening. 2. My approach briefing did not emphasize the importance of 3 key elements: A. glide slope ball note-- glide slope signal unusable beyond 7 degrees left of LOC course. B. The profile view shows that there is actually a level segment at 4;500 FT for .9 NM before glide slope intercept; so any capture or indication supporting a descent prior to 14.1 nm should have been disregarded. C. Taking those factors into account I should have briefed that I would only select localizer until sure of a reliable glide slope signal 3. Another contributing factor was the ATC transmission we received notifying us of 'possible altitude fluctuation' as it served to contribute to the crews belief we were high; supported by a false high glide slope indication; when in fact we were not high. This combined with an over reliance in glide slope indication vs. approach plate altitude discipline influenced my decision to start a descent to capture the glide slope. D. Lastly; in retrospect with confusion on why we were indicating above glide slope; at a point in the approach where I knew we should have been below the glide slope; followed by loss of glide slope indication I realize that I should have executed an immediate go around to determine the inconsistencies regardless of the VMC weather conditions.

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.