Cheyenne pilot has autoplt problem resulting in altitude deviation on descent.

Attributes
ACN	323520
Time
Date	199511
Day	Wed
Local Time Of Day	1201 To 1800
Place
Locale Reference	airport : pbi
State Reference	FL
Altitude	msl bound lower : 5500 msl bound upper : 7000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : pbi
Operator	general aviation : personal
Make Model Name	PA-31T Cheyenne II
Operating Under FAR Part	Part 91
Flight Phase	descent other
Flight Plan	IFR
Person 1
Affiliation	Other
Function	flight crew : single pilot
Qualification	other other : other pilot : private pilot : instrument
Experience	flight time last 90 days : 75 flight time total : 5000 flight time type : 50
ASRS Report	323520
Person 2
Affiliation	government : faa
Function	controller : approach
Qualification	controller : radar
Events
Anomaly	aircraft equipment problem : less severe altitude deviation : overshoot non adherence : clearance
Independent Detector	other controllera other flight crewa
Resolutory Action	none taken : insufficient time other
Consequence	Other
Supplementary
Primary Problem	Aircraft
Air Traffic Incident	Pilot Deviation

Narrative:

I was on IFR flight plan in VFR WX. Flight plan hot, pbi at FL250. Descended in steps. Last was from 11000 ft to 7000 ft. Autoplt with altitude preselect was in use. Had worked perfectly, but failed to lock on hold at 7000 ft. I was looking at chart and didn't notice until 5500 ft. Just as I saw problem and took corrective action, approach control advised to stay at 5000 ft. This autoplt had been so reliable I became too confident in relying on it.

Original NASA ASRS Text

Title: CHEYENNE PLT HAS AUTOPLT PROB RESULTING IN ALTDEV ON DSCNT.

Narrative: I WAS ON IFR FLT PLAN IN VFR WX. FLT PLAN HOT, PBI AT FL250. DSNDED IN STEPS. LAST WAS FROM 11000 FT TO 7000 FT. AUTOPLT WITH ALT PRESELECT WAS IN USE. HAD WORKED PERFECTLY, BUT FAILED TO LOCK ON HOLD AT 7000 FT. I WAS LOOKING AT CHART AND DIDN'T NOTICE UNTIL 5500 FT. JUST AS I SAW PROB AND TOOK CORRECTIVE ACTION, APCH CTL ADVISED TO STAY AT 5000 FT. THIS AUTOPLT HAD BEEN SO RELIABLE I BECAME TOO CONFIDENT IN RELYING ON IT.

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.