B727 first officer used the autoplt to descend and level off at 11000 ft. He then selected altitude hold verified by captain. Shortly thereafter, the aircraft began descending. At approximately 10700 ft the crew disconnected the autoplt and returned to cleared altitude.

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Attributes
ACN	415320
Time
Date	199809
Day	Tue
Local Time Of Day	0601 To 1200
Place
Locale Reference	atc facility : pwm
State Reference	ME
Altitude	msl bound lower : 10700 msl bound upper : 11000
Environment
Flight Conditions	VMC
Light	Dawn
Aircraft 1
Controlling Facilities	artcc : zbw tower : elp
Operator	common carrier : air carrier
Make Model Name	B727 Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Flight Phase	cruise other descent other
Route In Use	enroute : on vectors
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 25 flight time total : 4000 flight time type : 700
ASRS Report	415320
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : instrument pilot : commercial
Events
Anomaly	aircraft equipment problem : less severe altitude deviation : overshoot non adherence : clearance non adherence : far
Independent Detector	other flight crewa
Resolutory Action	flight crew : returned to intended course or assigned course
Consequence	Other
Supplementary
Primary Problem	Aircraft
Air Traffic Incident	Pilot Deviation

Narrative:

ZBW was vectoring us into pwm (portland airport) and cleared us to descend to 11000 ft. The first officer was flying the aircraft using the autoplt. The aircraft (on the autoplt) had leveled at 11000 ft and all autoplt progress displays indicated level (ie, green altitude hold light was on) and aircraft was not descending. I (the captain) was looking outside scanning for the airport as was the first officer and so. I glanced then at the aircraft instruments and noticed that the aircraft had started descending and we were approximately 300 ft low (10700 ft). The autoplt was disconnected and we immediately climbed to 11000 ft. ZBW asked us to verify level at 11000 ft which we did. Note: the aircraft autoplt had been previously written up as not capturing altitude and the autoplt control box had been changed. Also note that the captain, first officer, and so had had little sleep in the previous 24 hours due to 24 hour reserve schedule.

Original NASA ASRS Text

Title: B727 FO USED THE AUTOPLT TO DSND AND LEVEL OFF AT 11000 FT. HE THEN SELECTED ALT HOLD VERIFIED BY CAPT. SHORTLY THEREAFTER, THE ACFT BEGAN DSNDING. AT APPROX 10700 FT THE CREW DISCONNECTED THE AUTOPLT AND RETURNED TO CLRED ALT.

Narrative: ZBW WAS VECTORING US INTO PWM (PORTLAND ARPT) AND CLRED US TO DSND TO 11000 FT. THE FO WAS FLYING THE ACFT USING THE AUTOPLT. THE ACFT (ON THE AUTOPLT) HAD LEVELED AT 11000 FT AND ALL AUTOPLT PROGRESS DISPLAYS INDICATED LEVEL (IE, GREEN ALT HOLD LIGHT WAS ON) AND ACFT WAS NOT DSNDING. I (THE CAPT) WAS LOOKING OUTSIDE SCANNING FOR THE ARPT AS WAS THE FO AND SO. I GLANCED THEN AT THE ACFT INSTS AND NOTICED THAT THE ACFT HAD STARTED DSNDING AND WE WERE APPROX 300 FT LOW (10700 FT). THE AUTOPLT WAS DISCONNECTED AND WE IMMEDIATELY CLBED TO 11000 FT. ZBW ASKED US TO VERIFY LEVEL AT 11000 FT WHICH WE DID. NOTE: THE ACFT AUTOPLT HAD BEEN PREVIOUSLY WRITTEN UP AS NOT CAPTURING ALT AND THE AUTOPLT CTL BOX HAD BEEN CHANGED. ALSO NOTE THAT THE CAPT, FO, AND SO HAD HAD LITTLE SLEEP IN THE PREVIOUS 24 HRS DUE TO 24 HR RESERVE SCHEDULE.

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.