Narrative:

The captain was acting as pilot flying. The aircraft had been cleared for the visual approach and cleared to land on runway 24 at ttn. The weather was cavu on this night. The crew had briefed and was flying the RNAV (rnp) Y runway 24 approach at ttn fully managed with the autopilot engaged. The aircraft was on the vertical path and on the lateral path. The aircraft was on managed speed and was configured with flaps 2 and landing gear down. While descending from 2;300 ft to 1;700 ft between caxes and jorut; the aircraft automation was attempting to level off at 1;700 ft but slowly descended to slightly below at 1;620 ft. The captain announced the error and was preparing to take corrective action when the automation began a slow climb back up to 1;700. The control tower announced a 'low altitude alert' but by then the altitude was back to 1;700 ft as required on this segment of the approach. The amount of time the aircraft was between 1;700 ft and 1;620 ft was approximately 15 seconds. The crew noted a tailwind of approximately 20 KTS at the time of the altitude discrepancy and speculated that this may have had some effect on the vertical path during the arc portion of this RNAV approach. Because the flight was cleared for the visual approach and the terrain was clearly visible; at no time did the crew feel that additional corrective action was necessary. However; the crew agreed that if this automation error had occurred in IMC; a different course of action may have been required. The automation is normally quite reliable regarding capturing and holding altitudes on approach. However; after witnessing this error; the crew agreed that taking corrective action more quickly by anticipating this type of error may be appropriate; especially if this occurs in IMC. Aircraft automation detected its error and corrected fairly quickly.

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Original NASA ASRS Text

Title: Air Carrier Captain reports deviating slightly below the segment altitude of 1;700 FT during an RNAV (RNP) Y Runway 24 at TTN with the autopilot engaged in LNAV/VNAV and a clearance for a visual approach. The autopilot is allowed to correct the 80 FT deviation and the approach is continued to landing.

Narrative: The Captain was acting as pilot flying. The aircraft had been cleared for the visual approach and cleared to land on Runway 24 at TTN. The weather was CAVU on this night. The crew had briefed and was flying the RNAV (RNP) Y Runway 24 approach at TTN fully managed with the autopilot engaged. The aircraft was on the vertical path and on the lateral path. The aircraft was on managed speed and was configured with Flaps 2 and landing gear down. While descending from 2;300 FT to 1;700 FT between CAXES and JORUT; the aircraft automation was attempting to level off at 1;700 FT but slowly descended to slightly below at 1;620 FT. The Captain announced the error and was preparing to take corrective action when the automation began a slow climb back up to 1;700. The Control Tower announced a 'low altitude alert' but by then the altitude was back to 1;700 FT as required on this segment of the approach. The amount of time the aircraft was between 1;700 FT and 1;620 FT was approximately 15 seconds. The crew noted a tailwind of approximately 20 KTS at the time of the altitude discrepancy and speculated that this may have had some effect on the vertical path during the arc portion of this RNAV approach. Because the flight was cleared for the visual approach and the terrain was clearly visible; at no time did the crew feel that additional corrective action was necessary. However; the crew agreed that if this automation error had occurred in IMC; a different course of action may have been required. The automation is normally quite reliable regarding capturing and holding altitudes on approach. However; after witnessing this error; the crew agreed that taking corrective action more quickly by anticipating this type of error may be appropriate; especially if this occurs in IMC. Aircraft automation detected its error and corrected fairly quickly.

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.