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MD11 captain experiences an early descent during an RNAV approach with the autopilot engaged in prof. As the aircraft is climbed back to the correct altitude a brief gear warning is heard. A second attempt at engaging the autopilot results in another inappropriate descent and a low altitude alert from the tower.
| Attributes | |
| ACN | 1184705 |
| Time | |
| Date | 201407 |
| Environment | |
| Flight Conditions | Marginal |
| Aircraft 1 | |
| Make Model Name | MD-11 |
| Operating Under FAR Part | Part 121 |
| Flight Phase | Initial Approach |
| Component | |
| Aircraft Component | Approach Coupler |
| Person 1 | |
| Function | Captain Pilot Flying |
| Qualification | Flight Crew Air Transport Pilot (ATP) |
| Events | |
| Anomaly | Aircraft Equipment Problem Less Severe Deviation - Altitude Excursion From Assigned Altitude Deviation - Procedural Clearance Deviation - Procedural Published Material / Policy Inflight Event / Encounter CFTT / CFIT |
Narrative:
[We] briefed RNAV approach. After cleared; protected; established (we both take pains to verify) set MDA in fcp; all magenta in FMA. Aircraft immediately started to descend ignoring hard altitudes remaining. 7 NM prior to the FAF at flaps 15; got brief gear warning as we were setting V/south climb. After reestablished at 1;900 ft again selected prof and jet descended; this time tower called 'altitude alert.' we have reviewed extensively and I can't determine what happened but getting low in marginal conditions is concerning. My theory is this: when being vectored for the RNAV if you don't leave the tp then the aircraft (at least the older generations) can't keep up or they at least are confused and it initiates an 800 FPM descent while it thinks. If you know what we corked up please advise. In any case; this scenario ought to be part of training. I think joining the RNAV course inside the IAF (but seven plus NM outside FAF) and clearing tp creates a hazardous scenario as the jet descends without warning or alert. RNAV ought to be our primary approach unless weather bad.
Original NASA ASRS Text
Title: MD11 Captain experiences an early descent during an RNAV approach with the autopilot engaged in prof. As the aircraft is climbed back to the correct altitude a brief gear warning is heard. A second attempt at engaging the autopilot results in another inappropriate descent and a low altitude alert from the Tower.
Narrative: [We] briefed RNAV approach. After cleared; protected; established (we both take pains to verify) set MDA in FCP; all magenta in FMA. Aircraft immediately started to descend ignoring hard altitudes remaining. 7 NM prior to the FAF at flaps 15; got brief gear warning as we were setting V/S climb. After reestablished at 1;900 FT again selected prof and jet descended; this time Tower called 'altitude alert.' We have reviewed extensively and I can't determine what happened but getting low in marginal conditions is concerning. My theory is this: when being vectored for the RNAV if you don't leave the TP then the aircraft (at least the older generations) can't keep up or they at least are confused and it initiates an 800 FPM descent while it thinks. If you know what we corked up please advise. In any case; this scenario ought to be part of training. I think joining the RNAV course inside the IAF (but seven plus NM outside FAF) and clearing TP creates a hazardous scenario as the jet descends without warning or alert. RNAV ought to be our primary approach unless weather bad.
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.