Altitude overshot on descent.

Attributes
ACN	170932
Time
Date	199102
Day	Mon
Local Time Of Day	0601 To 1200
Place
Locale Reference	airport : den
State Reference	CO
Altitude	msl bound lower : 10700 msl bound upper : 11000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : den
Operator	common carrier : air carrier
Make Model Name	Medium Large Transport, Low Wing, 2 Turbojet Eng
Navigation In Use	Other
Flight Phase	cruise other descent other
Route In Use	enroute : on vectors
Flight Plan	IFR
Aircraft 2
Operator	common carrier : air carrier
Make Model Name	Any Unknown or Unlisted Aircraft Manufacturer
Flight Phase	cruise other
Route In Use	enroute : on vectors
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : atp pilot : cfi
Experience	flight time last 90 days : 225 flight time total : 13000 flight time type : 2500
ASRS Report	170932
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp pilot : flight engineer
Experience	flight time last 90 days : 250 flight time total : 8500 flight time type : 1000
ASRS Report	170916
Events
Anomaly	altitude deviation : overshoot altitude deviation : excursion from assigned altitude non adherence : required legal separation non adherence : clearance
Independent Detector	aircraft equipment other aircraft equipment : unspecified other controllera
Resolutory Action	flight crew : returned to intended course or assigned course other
Consequence	Other
Miss Distance	horizontal : 0 vertical : 800
Supplementary
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

We were level at 11000' and given a turn by approach control to a heading of 180 degrees. The autoplt was engaged and in the altitude hold mode. I started a turn from 225 to 180 degrees. As we were rolling out of the turn, approach control told us to maintain 11000'. I looked at the altimeter and noticed it was reading 10800' an descending very slowly. I immediately started a climb back to 11000'. At the same time we got a TCAS traffic advisory. I looked at the TCAS and it showed traffic several miles away at our 12 O'clock position and 800' below us. For reasons unknown the autoplt had slipped off of altitude hold. The maximum altitude deviation I showed was 250'. The captain and I discussed the problem and could find no reason for the autoplt to start descending. All warning systems worked and prevented a more serious altitude deviation.

Original NASA ASRS Text

Title: ALT OVERSHOT ON DESCENT.

Narrative: WE WERE LEVEL AT 11000' AND GIVEN A TURN BY APCH CTL TO A HDG OF 180 DEGS. THE AUTOPLT WAS ENGAGED AND IN THE ALT HOLD MODE. I STARTED A TURN FROM 225 TO 180 DEGS. AS WE WERE ROLLING OUT OF THE TURN, APCH CTL TOLD US TO MAINTAIN 11000'. I LOOKED AT THE ALTIMETER AND NOTICED IT WAS READING 10800' AN DSNDING VERY SLOWLY. I IMMEDIATELY STARTED A CLB BACK TO 11000'. AT THE SAME TIME WE GOT A TCAS TFC ADVISORY. I LOOKED AT THE TCAS AND IT SHOWED TFC SEVERAL MILES AWAY AT OUR 12 O'CLOCK POS AND 800' BELOW US. FOR REASONS UNKNOWN THE AUTOPLT HAD SLIPPED OFF OF ALT HOLD. THE MAX ALT DEVIATION I SHOWED WAS 250'. THE CAPT AND I DISCUSSED THE PROB AND COULD FIND NO REASON FOR THE AUTOPLT TO START DSNDING. ALL WARNING SYSTEMS WORKED AND PREVENTED A MORE SERIOUS ALT DEVIATION.

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.