Flight crew of medium large transport in descent allowed autoplt to level and start climb before cor...

Attributes
ACN	196449
Time
Date	199112
Day	Mon
Local Time Of Day	0601 To 1200
Place
Locale Reference	atc facility : cle
State Reference	OH
Altitude	msl bound lower : 13000 msl bound upper : 18500
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zob
Operator	common carrier : air carrier
Make Model Name	Medium Large Transport, Low Wing, 2 Turbojet Eng
Navigation In Use	Other Other
Flight Phase	descent other
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : atp pilot : instrument pilot : commercial
Experience	flight time last 90 days : 190 flight time total : 6500 flight time type : 1600
ASRS Report	196449
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : commercial pilot : atp pilot : instrument
Events
Anomaly	aircraft equipment problem : less severe altitude deviation : overshoot non adherence : clearance
Independent Detector	other controllera other flight crewa
Resolutory Action	flight crew : returned to intended course or assigned course flight crew : regained aircraft control
Consequence	Other
Supplementary
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation other

Narrative:

We were given a descent to 17000 ft. After the descent was initiated and we were passing through approximately FL190, we were given 13000, which I dialed in. Both the captain and I were looking in our flight bags to set up for our arrival at our destination. Suddenly ATC came back and asked us our altitude. We both looked up and discovered that the autoplt had changed from a descent mode to a climb and was climbing through FL185. The autoplt was disconnected and the descent continued. During this time, the autoplt was operating off of the VNAV portion of the FMS (profile) and we were on an assigned heading. When the autoplt was reengaged, it tried to climb again. We switched the autoplt to 'level change', which took the FMS out of the loop and everything returned to normal. We never discovered why the FMS did this, so we wrote it up upon landing. The transition from descent to climb was so smooth and gradual that neither one of us felt it. Someone should have been monitoring the autoplts performance instead of both of us having our heads in our bags.

Original NASA ASRS Text

Title: FLC OF MLG IN DSCNT ALLOWED AUTOPLT TO LEVEL AND START CLB BEFORE CORRECTING.

Narrative: WE WERE GIVEN A DSCNT TO 17000 FT. AFTER THE DSCNT WAS INITIATED AND WE WERE PASSING THROUGH APPROX FL190, WE WERE GIVEN 13000, WHICH I DIALED IN. BOTH THE CAPT AND I WERE LOOKING IN OUR FLT BAGS TO SET UP FOR OUR ARR AT OUR DEST. SUDDENLY ATC CAME BACK AND ASKED US OUR ALT. WE BOTH LOOKED UP AND DISCOVERED THAT THE AUTOPLT HAD CHANGED FROM A DSCNT MODE TO A CLB AND WAS CLBING THROUGH FL185. THE AUTOPLT WAS DISCONNECTED AND THE DSCNT CONTINUED. DURING THIS TIME, THE AUTOPLT WAS OPERATING OFF OF THE VNAV PORTION OF THE FMS (PROFILE) AND WE WERE ON AN ASSIGNED HDG. WHEN THE AUTOPLT WAS REENGAGED, IT TRIED TO CLB AGAIN. WE SWITCHED THE AUTOPLT TO 'LEVEL CHANGE', WHICH TOOK THE FMS OUT OF THE LOOP AND EVERYTHING RETURNED TO NORMAL. WE NEVER DISCOVERED WHY THE FMS DID THIS, SO WE WROTE IT UP UPON LNDG. THE TRANSITION FROM DSCNT TO CLB WAS SO SMOOTH AND GRADUAL THAT NEITHER ONE OF US FELT IT. SOMEONE SHOULD HAVE BEEN MONITORING THE AUTOPLTS PERFORMANCE INSTEAD OF BOTH OF US HAVING OUR HEADS IN OUR BAGS.

Data retrieved from NASA's ASRS site as of July 2007 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.

Flight crew of medium large transport in descent allowed autoplt to level and start climb before correcting.