G4 crew had an autoplt malfunction and a +/- 300 ft altitude deviation as a result.

Attributes
ACN	634378
Time
Date	200410
Day	Fri
Local Time Of Day	0601 To 1200
Place
Locale Reference	navaid : als.vor
State Reference	CO
Altitude	msl bound lower : 42700 msl bound upper : 43300
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zkc.artcc tower : dfw.tower
Operator	general aviation : corporate
Make Model Name	Gulfstream IV
Operating Under FAR Part	Part 91
Navigation In Use	other
Flight Phase	cruise : level
Route In Use	enroute : direct
Flight Plan	IFR
Person 1
Affiliation	company : corporate
Function	flight crew : captain oversight : pic
Qualification	pilot : cfi pilot : atp
Experience	flight time last 90 days : 30 flight time total : 10000 flight time type : 500
ASRS Report	634378
Person 2
Affiliation	company : corporate
Function	flight crew : first officer
Events
Anomaly	aircraft equipment problem : critical altitude deviation : excursion from assigned altitude non adherence : clearance other anomaly
Independent Detector	aircraft equipment other aircraft equipment : altimeters other flight crewa other flight crewb
Resolutory Action	flight crew : overrode automation flight crew : regained aircraft control flight crew : returned to assigned altitude none taken : detected after the fact other
Consequence	other
Supplementary
Problem Areas	Aircraft
Primary Problem	Aircraft

Narrative:

Cruising at FL430 on autoplt, the aircraft descended 250-300 ft still on autoplt and altitude hold engaged. Using the vertical speed function, I climbed to 43000 ft and overshot the altitude by 300 ft. I had to disconnect the autoplt and level off at 43000 ft. It felt as if it was mountain waves, but we were 250 NM east of ALS VOR so I doubt it was mountain waves.

Original NASA ASRS Text

Title: G4 CREW HAD AN AUTOPLT MALFUNCTION AND A +/- 300 FT ALTDEV AS A RESULT.

Narrative: CRUISING AT FL430 ON AUTOPLT, THE ACFT DSNDED 250-300 FT STILL ON AUTOPLT AND ALT HOLD ENGAGED. USING THE VERT SPD FUNCTION, I CLBED TO 43000 FT AND OVERSHOT THE ALT BY 300 FT. I HAD TO DISCONNECT THE AUTOPLT AND LEVEL OFF AT 43000 FT. IT FELT AS IF IT WAS MOUNTAIN WAVES, BUT WE WERE 250 NM E OF ALS VOR SO I DOUBT IT WAS MOUNTAIN WAVES.

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.