Altitude deviation. Overshoot in descent. Unable callback.

Attributes
ACN	136863
Time
Date	199002
Day	Thu
Local Time Of Day	1201 To 1800
Place
Locale Reference	atc facility : tpa
State Reference	FL
Altitude	msl bound lower : 11750 msl bound upper : 12000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : tpa
Operator	common carrier : air carrier
Make Model Name	Medium Large Transport, Low Wing, 2 Turbojet Eng
Flight Phase	descent other
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 180 flight time total : 15000 flight time type : 3000
ASRS Report	136863
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : atp
Events
Anomaly	aircraft equipment problem : less severe altitude deviation : overshoot non adherence : clearance
Independent Detector	other flight crewa
Resolutory Action	flight crew : returned to intended course or assigned course
Consequence	Other
Supplementary
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

Upon letdown into tpa for landing, we set 12000' in our air carrier medium large transport mode control panel altitude window. In air carrier aircraft the autoplt descends at a high rate of speed until right at level off altitude. After we set 12000' as our level off altitude, the MCP window jumped to 11000', west/O further input from us. As the aircraft was passing 12000' we noticed the error and I disconnected the autoplt and attempted to level off at 12000'. Because of the high rate of descent of the autoplt we lost about 250' on level off. We immediately climbed back to 12000'. ATC asked our altitude and we said leveling at 12000'. This problem has come up before and our company has put out a memo to this effect.

Original NASA ASRS Text

Title: ALT DEVIATION. OVERSHOOT IN DESCENT. UNABLE CALLBACK.

Narrative: UPON LETDOWN INTO TPA FOR LNDG, WE SET 12000' IN OUR ACR MLG MODE CONTROL PANEL ALT WINDOW. IN ACR ACFT THE AUTOPLT DESCENDS AT A HIGH RATE OF SPD UNTIL RIGHT AT LEVEL OFF ALT. AFTER WE SET 12000' AS OUR LEVEL OFF ALT, THE MCP WINDOW JUMPED TO 11000', W/O FURTHER INPUT FROM US. AS THE ACFT WAS PASSING 12000' WE NOTICED THE ERROR AND I DISCONNECTED THE AUTOPLT AND ATTEMPTED TO LEVEL OFF AT 12000'. BECAUSE OF THE HIGH RATE OF DSCNT OF THE AUTOPLT WE LOST ABOUT 250' ON LEVEL OFF. WE IMMEDIATELY CLBED BACK TO 12000'. ATC ASKED OUR ALT AND WE SAID LEVELING AT 12000'. THIS PROB HAS COME UP BEFORE AND OUR COMPANY HAS PUT OUT A MEMO TO THIS EFFECT.

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.