A DC9-30 in cruise at FL330 deviated from the assigned altitude by 300 ft while the autoplt was enga...

Attributes
ACN	432422
Time
Date	199903
Day	Fri
Local Time Of Day	0601 To 1200
Place
Locale Reference	navaid : imb.vortac
Altitude	msl bound lower : 33000 msl bound upper : 33300
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zse.artcc
Operator	common carrier : air carrier
Make Model Name	DC-9 30
Operating Under FAR Part	Part 121
Navigation In Use	other vortac
Flight Phase	cruise : level
Route In Use	enroute airway : j15.airway
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : flight engineer pilot : atp pilot : cfi pilot : multi engine
Experience	flight time last 90 days : 64 flight time total : 7000 flight time type : 402
ASRS Report	432422
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Events
Anomaly	aircraft equipment problem : less severe altitude deviation : excursion from assigned altitude other anomaly other
Independent Detector	atc equipment other atc equipment : altitude reporting
Resolutory Action	controller : issued alert flight crew : returned to assigned airspace
Consequence	other
Supplementary
Problem Areas	Aircraft Flight Crew Human Performance
Primary Problem	Aircraft

Narrative:

Crossing kimberly VOR and leveling off FL330, placed autoplt climb/descend wheel in altitude hold detent. Aircraft appeared to level. Captain talking to me regarding route. Shortly center called and said we were route. Shortly center called and said we were showing 300 ft high. Immediately thanked center and moved autoplt out of altitude hold to descend position and continued to climb/hold altitude. Knob had no effect. Turned off autoplt and descended to FL330, and re-engaged altitude hold/autoplt. Aircraft continued to oscillate +/-100 ft above/below selected altitude. Captain said this was due to mountain waves/pressure changes. Monitored autoplt closely and manually overrode oscillations.

Original NASA ASRS Text

Title: A DC9-30 IN CRUISE AT FL330 DEVIATED FROM THE ASSIGNED ALT BY 300 FT WHILE THE AUTOPLT WAS ENGAGED IN ALT HOLD.

Narrative: XING KIMBERLY VOR AND LEVELING OFF FL330, PLACED AUTOPLT CLB/DSND WHEEL IN ALT HOLD DETENT. ACFT APPEARED TO LEVEL. CAPT TALKING TO ME REGARDING RTE. SHORTLY CTR CALLED AND SAID WE WERE RTE. SHORTLY CTR CALLED AND SAID WE WERE SHOWING 300 FT HIGH. IMMEDIATELY THANKED CTR AND MOVED AUTOPLT OUT OF ALT HOLD TO DSND POS AND CONTINUED TO CLB/HOLD ALT. KNOB HAD NO EFFECT. TURNED OFF AUTOPLT AND DSNDED TO FL330, AND RE-ENGAGED ALT HOLD/AUTOPLT. ACFT CONTINUED TO OSCILLATE +/-100 FT ABOVE/BELOW SELECTED ALT. CAPT SAID THIS WAS DUE TO MOUNTAIN WAVES/PRESSURE CHANGES. MONITORED AUTOPLT CLOSELY AND MANUALLY OVERRODE OSCILLATIONS.

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.

A DC9-30 in cruise at FL330 deviated from the assigned altitude by 300 ft while the autoplt was engaged in altitude hold.