High on the arrival; the flight crew of a B737-700 abandoned VNAV when cleared to FL260. Although th...

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 altitude/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

Original NASA ASRS Text

Title: 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

Data retrieved from NASA's ASRS site as of April 2012 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.

Attributes
ACN	976914
Time
Date	201110
Local Time Of Day	0601-1200
Place
Locale Reference	ZLA.ARTCC
State Reference	CA
Environment
Flight Conditions	VMC
Aircraft 1
Make Model Name	B737-700
Operating Under FAR Part	Part 121
Flight Phase	Descent
Flight Plan	IFR
Component
Aircraft Component	Altitude Hold/Capture
Person 1
Function	Captain
Experience	Flight Crew Last 90 Days 207
Person 2
Function	First Officer
Experience	Flight Crew Last 90 Days 173
Events
Anomaly	Aircraft Equipment Problem Less Severe Deviation - Altitude Overshoot Deviation - Procedural Clearance

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.