A300
Flight Crew Air Transport Pilot (ATP)
Aircraft Equipment Problem Critical; 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; Flight Crew Returned To Clearance; Flight Crew Took Evasive Action
Aircraft; Human Factors
The flight was a requalification IOE for the First Officer. The student was making normal progress with no indicated problems toward requaling. Our STAR was discontinued by Approach Control on initial contact; replaced by vectors for an ILS to [Runway] 28R. Initial descent [was] to 6;000 MSL; then to 4;000 then 3;000. Aircraft was capturing 3;000 FT with alt*[capture] exactly at the same time the First Officer selected land to arm the ILS as we were given an intercept at about a seven mile final. I noticed on the FCP and FMAs that ALT HLD and LVL CHG began flashing back from one to the other at a rate of about twice a second for about ten seconds while the airplane leveled at 3;000 FT. I pointed this out to the First Officer and confirmed FCP land mode was armed and FMAs showed good for LOC and GS capture. Flashing FMAs stopped at about LOC* [capture]. As we were cleared to land I had finished the landing check we were twice alerted by GPWS of an obstacle and the terrain display came up automatically on ND. I took control of the airplane and disconnected the autopilot and began a climb that leveled off at about 2;000 FT where I assessed the situation; got the airplane stable and waited for the glideslope to come down to center after which I continued the approach off the autopilot to an uneventful landing. Tower gave us an altitude alert call almost exactly at same time as GPWS obstacle alert. After the capture of 3;000 FT; I am not sure what mode the airplane went into vertically. I'm pretty sure however; it was flying pitch on speed at idle power and responded to the First Officer setting approach speed on the FCP by descending quickly. I figure the airplane descended to about 2;000 FT in about 20 seconds. This put us about 800 FT above the ground at about a five mile final; well below glideslope. There must be a potential for the automation to get confused if LAND is armed exactly at the same time as ALT* [capture] happens. I was aware of abnormal FCP and FMA indications; but dismissed them when they went away. [I] got distracted for about 20 seconds checking in with Tower and finishing the Before Landing Checklist. [We might want to] make crews aware of FMA/FCP indications of an improper sequence of LAND mode arming in conjunction with ALT* [capture]. Make LOC* [capture] and GS*[capture] sterile environment events; not allowing radio communication or checklist activity until confirmed capture of both.
As we were being vectored toward the final approach course for the ILS to Runway 28R; the airplane was in a left hand turn; descending from 4;000 to 3;000. Vector was changed from 010 heading to 330 heading; descend to 3;000; cleared to intercept the ILS 28R. At this point; multiple things were happening to the airplane; descending and leveling off at 3;000; ALT capture; rolling out to intercept the localizer; arming the autopilot to capture the glideslope. I selected LAND on the FCP; armed the second autopilot; and tested the auto land system. The FCP vertical control buttons started flashing between ALT HOLD and LVL CHNG. On the PFD; the FMA's started flashing on what was armed. I got distracted between what the autopilot was indicating; what mode it was in and what it was flying. [While distracted] the airplane entered an unrecognized descent with the autopilot engaged and descended to 2;000; [although we were assigned] 3;000; the glideslope intercept altitude. The Captain recognized the deviation; took control of the airplane; punched the autopilot off and began a climb back to 3;000. The EGPWS system made an 'Obstacle' call out as the Captain; flew the airplane. The ILS glideslope was intercepted and the approach was continued to landing.
An A300 flight crew suffered a CFTT incident when the autoflight system malfunctioned and the aircraft failed to arrest its descent at 3;000 MSL as cleared prior to intercepting the ILS.