Large transport has altitude hold disconnect and aircraft climb 300 ft.

Attributes
ACN	211137
Time
Date	199205
Day	Tue
Local Time Of Day	1201 To 1800
Place
Locale Reference	atc facility : mke
State Reference	WI
Altitude	msl bound lower : 33000 msl bound upper : 33300
Environment
Flight Conditions	VMC
Light	Night
Aircraft 1
Controlling Facilities	artcc : zau
Operator	common carrier : air carrier
Make Model Name	Large Transport, Low Wing, 3 Turbojet Eng
Flight Phase	cruise 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 : 5000 flight time type : 2000
ASRS Report	211137
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : instrument pilot : commercial pilot : atp
Events
Anomaly	aircraft equipment problem : less severe altitude deviation : excursion from assigned altitude non adherence : clearance
Independent Detector	other controllera
Resolutory Action	flight crew : returned to intended course or assigned course
Consequence	Other
Supplementary
Primary Problem	Aircraft
Air Traffic Incident	Pilot Deviation

Narrative:

Takeoff, climb, and level-off all normal. Shortly after level- off, ARTCC questioned our altitude, saying we were 300 ft high. Our altimeters showed 200 ft high and that the altitude hold function of the autoplt had disengaged. The aircraft was returned to FL330 and altitude hold re-engaged. My suggestion would be to tie the red autoplt disengaged light into the altitude hold switch to warn of inadvertent disconnects.

Original NASA ASRS Text

Title: LGT HAS ALT HOLD DISCONNECT AND ACFT CLB 300 FT.

Narrative: TKOF, CLB, AND LEVEL-OFF ALL NORMAL. SHORTLY AFTER LEVEL- OFF, ARTCC QUESTIONED OUR ALT, SAYING WE WERE 300 FT HIGH. OUR ALTIMETERS SHOWED 200 FT HIGH AND THAT THE ALT HOLD FUNCTION OF THE AUTOPLT HAD DISENGAGED. THE ACFT WAS RETURNED TO FL330 AND ALT HOLD RE-ENGAGED. MY SUGGESTION WOULD BE TO TIE THE RED AUTOPLT DISENGAGED LIGHT INTO THE ALT HOLD SWITCH TO WARN OF INADVERTENT DISCONNECTS.

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.