A cessna 421C pilot reports that his autoplt allowed the aircraft to fluctuate in altitude.

Attributes
ACN	339490
Time
Date	199606
Day	Thu
Local Time Of Day	1801 To 2400
Place
Locale Reference	atc facility : gij
State Reference	MI
Altitude	msl bound lower : 12800 msl bound upper : 13500
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zau
Operator	general aviation : corporate
Make Model Name	Golden Eagle 421
Operating Under FAR Part	Part 91
Flight Phase	cruise other
Flight Plan	IFR
Person 1
Affiliation	Other
Function	flight crew : single pilot
Qualification	pilot : commercial pilot : instrument
Experience	flight time last 90 days : 700 flight time total : 1300 flight time type : 150
ASRS Report	339490
Events
Anomaly	aircraft equipment problem : less severe altitude deviation : excursion from assigned altitude non adherence : clearance non adherence : far other anomaly other
Independent Detector	other flight crewa
Resolutory Action	flight crew : became reoriented flight crew : overcame equipment problem flight crew : returned to intended course or assigned course
Consequence	Other
Supplementary
Primary Problem	Aircraft
Air Traffic Incident	other

Narrative:

I was level at 13000 ft with autoplt on. I was reviewing charts for the rest of the flight and setting power. The altitude hold disengaged and I gained 500 ft in altitude before I caught it. I went back to 13000 ft and tried to reengage the altitude hold, but it would only work intermittently. The altitude fluctuated 100 ft to 200 ft the rest of the flight.

Original NASA ASRS Text

Title: A CESSNA 421C PLT RPTS THAT HIS AUTOPLT ALLOWED THE ACFT TO FLUCTUATE IN ALT.

Narrative: I WAS LEVEL AT 13000 FT WITH AUTOPLT ON. I WAS REVIEWING CHARTS FOR THE REST OF THE FLT AND SETTING PWR. THE ALT HOLD DISENGAGED AND I GAINED 500 FT IN ALT BEFORE I CAUGHT IT. I WENT BACK TO 13000 FT AND TRIED TO REENGAGE THE ALT HOLD, BUT IT WOULD ONLY WORK INTERMITTENTLY. THE ALT FLUCTUATED 100 FT TO 200 FT THE REST OF THE FLT.

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.