High on the arrival; the flight crew of a B737-700 abandoned VNAV when cleared to FL260. Although they received appropriate FMA advisories the aircraft failed to level at their cleared altitude. The First Officer disconnected the autopilot and returned manually to the assigned altitude.
Synopsis
High on the arrival; the flight crew of a B737-700 abandoned VNAV when cleared to FL260. Although they received appropriate FMA advisories the aircraft failed to level at their cleared altitude. The First Officer disconnected the autopilot and returned manually to the assigned altitude.
Narrative
We were high on the profile for the TANDY Arrival into SNA and being cleared in 1000 to 2000 foot increments because of conflicting crossing traffic below. When we were cleared to FL260 the First Officer had the B autopilot in CMD using CWS-P to descend as the aircraft was high on the profile. Prior to FL260 the aircraft seemed to be transitioning for a level off at FL260. With CMD selected on the autopilot; we had a green CMD; an amber CWS-P; and ALT/ACQ all illuminated on the FMA. Despite the indications the aircraft flew through FL260 at about a 600-700 FPM rate of descent. The First Officer was quick to disconnect the autopilot and return to FL260; but the aircraft descended about 220 FT below FL260. ATC did inquire at what altitude we were as we were returning to FL260. We explained that our autopilot did not level off as we expected and we had returned to FL260. No further deviations occurred. We notified Maintenance through Dispatch; made a logbook entry; and proceeded to our final destination on schedule under MEL provisions
Second reporter narrative
I disengaged the autothrottles; reduced the thrust to idle; deselected VNAV; and manually pitched for descent with the A/FD in CMD+LNAV. The Captain and I both made the 1000 FT callout and I prepared to manually advance thrust levers for the aut-pilot level off. Approaching FL260; the FMA pitch indication changed from CWS-P to ALT ACQ; but the autopilot failed to level off. By the time I disengaged the autopilot and corrected the pitch attitude to return to FL260; we were approximately 300 FT below FL260.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.