A B737-800 crew reported that in level flight at FL380 with the autopilot in VNAV path the aircraft ...

Attributes
ACN	879559
Time
Date	201003
Local Time Of Day	0601-1200
Place
Locale Reference	ZZZZ.Airport
State Reference	FO
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Make Model Name	B737-800
Operating Under FAR Part	Part 121
Flight Phase	Cruise
Flight Plan	IFR
Component
Aircraft Component	Altitude Hold/Capture
Person 1
Function	Pilot Flying First Officer
Person 2
Function	Pilot Not Flying Captain
Qualification	Flight Crew Air Transport Pilot (ATP)
Events
Anomaly	Aircraft Equipment Problem Critical Deviation - Altitude Excursion From Assigned Altitude Deviation - Procedural Clearance

Narrative:

Upon reaching initial cruise altitude; FL380; VNAV and autopilot B were engaged. The aircraft was in VNAV pth for approximately 1 minute when it started an uncommanded descent in VNAV pth. Several seconds later; VNAV disengaged and cws P engaged. I disconnected the autopilot and manually flew the aircraft back to FL380 from FL375. Center called and asked if we had switched our altimeter to 29.92 since there was a 250 ft altitude difference. I explained the problem and stated that we were climbing back to FL380. Our gross weight was approximately 138;000 pounds. The optimum altitude on the act econ crz page was FL380 and maximum was FL400 so we were unable to determine the reason for the uncommanded descent. There were no further problems with the VNAV during the flight.

Original NASA ASRS Text

Title: A B737-800 Crew reported that in level flight at FL380 with the autopilot in VNAV Path the aircraft began an uncommanded descent. The autopilot disconnected and transitioned to CWP P. The crew recovered with a 250 FT altitude loss.

Narrative: Upon reaching initial cruise altitude; FL380; VNAV and autopilot B were engaged. The aircraft was in VNAV PTH for approximately 1 minute when it started an uncommanded descent in VNAV PTH. Several seconds later; VNAV disengaged and CWS P engaged. I disconnected the autopilot and manually flew the aircraft back to FL380 from FL375. Center called and asked if we had switched our altimeter to 29.92 since there was a 250 FT altitude difference. I explained the problem and stated that we were climbing back to FL380. Our gross weight was approximately 138;000 LBS. The optimum altitude on the ACT ECON CRZ page was FL380 and maximum was FL400 so we were unable to determine the reason for the uncommanded descent. There were no further problems with the VNAV during the flight.

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.

A B737-800 crew reported that in level flight at FL380 with the autopilot in VNAV path the aircraft began an uncommanded descent. The autopilot disconnected and transitioned to cwp P. The crew recovered with a 250 ft altitude loss.