MD-11
Flight Crew Air Transport Pilot (ATP)
Human-Machine Interface; Training / Qualification
Aircraft Equipment Problem Less Severe; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter CFTT / CFIT
Air Traffic Control Issued Advisory / Alert; Aircraft Automation Overrode Flight Crew; Flight Crew Became Reoriented; Flight Crew Returned To Clearance
Aircraft; Human Factors
[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.
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.