Narrative:

This occurred on approach to atl. It was a low altitude alert call from tower while on PAPI glidepath. I was the pm. We were arriving at the airport from the northwest for an ILS 8L. There was weather and moderate chop on the arrival all the way down to breaking out of the clouds at 5700 ft. We were on autopilot and had been given direct to bazar; the fix before the final approach fix schel. Due to cells at bazar we were then given direct to schel. As the first aircraft into atl this morning; approach asked us for a PIREP for the ceiling and cleared us to 3000 ft. Approach then asked if we had the field in sight. Since we had broken out at 5700 ft with 8 miles visibility we did have atl in sight and he cleared us for the visual. The PF adjusted the heading to intercept the localizer outside of schel and approach mode was selected. We were flying the visual and backing up with the localizer.we intercepted the localizer but the autopilot overshot and had to correct back. We had selected 3000 ft with flch and the altitude at schel would have been 2900. The glide slope indicator had come into view and was indicating we were intercepting glide path as expected. As we were approaching schel; the autopilot continued descent through 3000 ft even though the glideslope had not captured and was still showing a 1/2 dot low. I live in atlanta and am very familiar with the approaches and obstacles around atl. As we were well clear of obstacles; we allowed the autopilot to continue the descent while we determined what it was doing. As we crossed schel; we were at 2700 and as the PAPI for 8L was showing all red we agreed to level off there. At that time the glideslope indication disappeared. We continued level until the PAPI showed we were on glidepath. As we were approaching the PAPI glidepath; the glideslope indication returned and showed we were below glideslope then as the PAPI showed we were on glideslope; the glideslope indication also agreed. We continued the visual approach. Tower cleared us to land 8L when I called per approaches instructions at schel. At about 700 AGL; when we had been correctly on PAPI and ILS glidepath; tower called with a low altitude alert and check altimeter 29.85 (we were on 29.87 per ATC). I replied that we showed we were currently in the center of the PAPI glidepath. The landing continued normally. Upon exiting the runway and receiving taxi instructions; I queried tower about the low altitude alert. He said it was an automated response from the system; but that he agreed our approach looked normal.I have to assume the system generated low altitude alert had to have been generated earlier when we were below glidepath. I believe we were receiving a false glideslope indication prior to schel and the correct indication when we were on the PAPI glidepath. I still do not know why the aircraft started a descent early.while we confirmed the correct ILS frequency during the approach brief; I never identified the frequency. I allowed myself to become distracted by the moderate turbulence; giving a PIREP; looking for the airport; and monitoring the localizer overshoot. It is my habit to ensure one of us identifies the frequency and this underscores the reason for that. I have to believe when we were getting the false glideslope; we would not have gotten a good identification. That would have made us extra cautious about that information. We brief the glideslope intercept altitude during the approach brief primarily to ensure that we are not responding to a false glideslope. We should have simply disconnected the autopilot and leveled off at 2900 until crossing schel. As pm it was my responsibility to provide that backup and call that out. It's not the automations fault. Even though we were flying a visual approach; we were backing it up with the ILS. We do this for safety to ensure our visual is on target. Regardless of my familiarity with an airport; good discipline and safety dictate that if the altitudes or indications of the ILS is off we do the cautious thing and stick to the most conservative altitudes or indications until we have more information.

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Original NASA ASRS Text

Title: B767 flight crew reported they received a low altitude alert from ATL Tower while on the Runway 8L PAPI glidepath.

Narrative: This occurred on approach to ATL. It was a Low Altitude Alert call from Tower while on PAPI glidepath. I was the PM. We were arriving at the airport from the northwest for an ILS 8L. There was weather and moderate chop on the arrival all the way down to breaking out of the clouds at 5700 ft. We were on autopilot and had been given direct to BAZAR; the fix before the final approach fix SCHEL. Due to cells at BAZAR we were then given direct to SCHEL. As the first aircraft into ATL this morning; Approach asked us for a PIREP for the ceiling and cleared us to 3000 ft. Approach then asked if we had the field in sight. Since we had broken out at 5700 ft with 8 miles visibility we did have ATL in sight and he cleared us for the visual. The PF adjusted the heading to intercept the localizer outside of SCHEL and approach mode was selected. We were flying the visual and backing up with the localizer.We intercepted the localizer but the autopilot overshot and had to correct back. We had selected 3000 ft with FLCH and the altitude at SCHEL would have been 2900. The glide slope indicator had come into view and was indicating we were intercepting glide path as expected. As we were approaching SCHEL; the autopilot continued descent through 3000 ft even though the glideslope had not captured and was still showing a 1/2 dot low. I live in Atlanta and am very familiar with the approaches and obstacles around ATL. As we were well clear of obstacles; we allowed the autopilot to continue the descent while we determined what it was doing. As we crossed SCHEL; we were at 2700 and as the PAPI for 8L was showing all red we agreed to level off there. At that time the glideslope indication disappeared. We continued level until the PAPI showed we were on glidepath. As we were approaching the PAPI glidepath; the glideslope indication returned and showed we were below glideslope then as the PAPI showed we were on glideslope; the glideslope indication also agreed. We continued the visual approach. Tower cleared us to land 8L when I called per Approaches instructions at SCHEL. At about 700 AGL; when we had been correctly on PAPI and ILS glidepath; Tower called with a low altitude alert and check altimeter 29.85 (we were on 29.87 per ATC). I replied that we showed we were currently in the center of the PAPI glidepath. The landing continued normally. Upon exiting the runway and receiving taxi instructions; I queried Tower about the low altitude alert. He said it was an automated response from the system; but that he agreed our approach looked normal.I have to assume the system generated low altitude alert had to have been generated earlier when we were below glidepath. I believe we were receiving a false glideslope indication prior to SCHEL and the correct indication when we were on the PAPI glidepath. I still do not know why the aircraft started a descent early.While we confirmed the correct ILS frequency during the approach brief; I never identified the frequency. I allowed myself to become distracted by the moderate turbulence; giving a PIREP; looking for the airport; and monitoring the localizer overshoot. It is my habit to ensure one of us identifies the frequency and this underscores the reason for that. I have to believe when we were getting the false glideslope; we would not have gotten a good ID. That would have made us extra cautious about that information. We brief the glideslope intercept altitude during the approach brief primarily to ensure that we are not responding to a false glideslope. We should have simply disconnected the autopilot and leveled off at 2900 until crossing SCHEL. As PM it was my responsibility to provide that backup and call that out. It's not the Automations fault. Even though we were flying a visual approach; we were backing it up with the ILS. We do this for safety to ensure our visual is on target. Regardless of my familiarity with an airport; good discipline and safety dictate that if the altitudes or indications of the ILS is off we do the cautious thing and stick to the most conservative altitudes or indications until we have more information.

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.