A charter captain reported that the autoplt did not hold the captured altitude near slc, ut.

Attributes
ACN	515790
Time
Date	200106
Day	Tue
Local Time Of Day	1201 To 1800
Place
Locale Reference	navaid : ilc.vortac
State Reference	NV
Altitude	msl bound lower : 28000 msl bound upper : 28500
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	tower : vhhh.tower
Operator	common carrier : charter
Make Model Name	Light Transport, Low Wing, 2 Turbojet Eng
Flight Phase	cruise : level
Flight Plan	IFR
Person 1
Affiliation	company : charter
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 75 flight time total : 19700 flight time type : 125
ASRS Report	515790
Events
Anomaly	aircraft equipment problem : less severe altitude deviation : overshoot non adherence : clearance non adherence : far
Resolutory Action	flight crew : returned to assigned altitude
Supplementary
Problem Areas	Flight Crew Human Performance
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

Aircraft cleared to FL280. Autoplt in use with altitude capture selected. The aircraft leveled at FL280. Both pilots' attention was then changed to getting proper charts for the next leg and the aircraft drifted up to 28500 ft before corrective action was taken. Center did not take notice of the deviation. This was a case where the crew did not function as a crew and both were engrossed in duties other than watching the airplane. Reason for the autoplt malfunction is not known.

Original NASA ASRS Text

Title: A CHARTER CAPT RPTED THAT THE AUTOPLT DID NOT HOLD THE CAPTURED ALT NEAR SLC, UT.

Narrative: ACFT CLRED TO FL280. AUTOPLT IN USE WITH ALT CAPTURE SELECTED. THE ACFT LEVELED AT FL280. BOTH PLTS' ATTN WAS THEN CHANGED TO GETTING PROPER CHARTS FOR THE NEXT LEG AND THE ACFT DRIFTED UP TO 28500 FT BEFORE CORRECTIVE ACTION WAS TAKEN. CTR DID NOT TAKE NOTICE OF THE DEV. THIS WAS A CASE WHERE THE CREW DID NOT FUNCTION AS A CREW AND BOTH WERE ENGROSSED IN DUTIES OTHER THAN WATCHING THE AIRPLANE. REASON FOR THE AUTOPLT MALFUNCTION IS NOT KNOWN.

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.