Altitude deviation. Encountering mountain wave activity, autothrottles rapidly retarded to compensate for overspd. Autoplt changed pitch too slow and reporter captain disconnected it as it was descending. Aircraft lost 500 ft. Flight crew reported altitude deviation to ATC with no conflicts.

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Attributes
ACN	332960
Time
Date	199604
Day	Fri
Local Time Of Day	0601 To 1200
Place
Locale Reference	atc facility : dvv
State Reference	CO
Altitude	msl bound lower : 34500 msl bound upper : 35000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zdv tower : mia
Operator	common carrier : air carrier
Make Model Name	Commercial Fixed Wing
Operating Under FAR Part	Part 121
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 : 200 flight time total : 20000 flight time type : 900
ASRS Report	332960
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : instrument pilot : flight engineer pilot : commercial pilot : cfi pilot : atp
Experience	flight time last 90 days : 200 flight time total : 4500 flight time type : 300
ASRS Report	333399
Events
Anomaly	altitude deviation : excursion from assigned altitude inflight encounter : weather non adherence : clearance
Independent Detector	other flight crewa
Resolutory Action	flight crew : returned to intended course or assigned course
Consequence	Other
Supplementary
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

Just west of denver over the front range at FL350, we experienced strong mountain wave activity. The autothrottles retarded rapidly due to the speed increase and the autoplt pitched the aircraft to maintain the altitude. I disconnected the autothrottles and advanced the throttles toward a nominal setting to prevent the surging that the automatic system was causing. The autoplt was causing another pitch change which I attempted to attenuate by selecting the 'turbulence' mode of the autoplt. I was slow to correct the autoplt induced pitch attitude and we descended 500 ft before I caught up with the aircraft. We advised ATC that we descended from our assigned altitude of FL350 to FL345. ATC acknowledged our report and said that there was no traffic conflict. The episode was smooth and there were no reports of passenger or crew discomfort.

Original NASA ASRS Text

Title: ALTDEV. ENCOUNTERING MOUNTAIN WAVE ACTIVITY, AUTOTHROTTLES RAPIDLY RETARDED TO COMPENSATE FOR OVERSPD. AUTOPLT CHANGED PITCH TOO SLOW AND RPTR CAPT DISCONNECTED IT AS IT WAS DSNDING. ACFT LOST 500 FT. FLC RPTED ALTDEV TO ATC WITH NO CONFLICTS.

Narrative: JUST W OF DENVER OVER THE FRONT RANGE AT FL350, WE EXPERIENCED STRONG MOUNTAIN WAVE ACTIVITY. THE AUTOTHROTTLES RETARDED RAPIDLY DUE TO THE SPD INCREASE AND THE AUTOPLT PITCHED THE ACFT TO MAINTAIN THE ALT. I DISCONNECTED THE AUTOTHROTTLES AND ADVANCED THE THROTTLES TOWARD A NOMINAL SETTING TO PREVENT THE SURGING THAT THE AUTO SYS WAS CAUSING. THE AUTOPLT WAS CAUSING ANOTHER PITCH CHANGE WHICH I ATTEMPTED TO ATTENUATE BY SELECTING THE 'TURB' MODE OF THE AUTOPLT. I WAS SLOW TO CORRECT THE AUTOPLT INDUCED PITCH ATTITUDE AND WE DSNDED 500 FT BEFORE I CAUGHT UP WITH THE ACFT. WE ADVISED ATC THAT WE DSNDED FROM OUR ASSIGNED ALT OF FL350 TO FL345. ATC ACKNOWLEDGED OUR RPT AND SAID THAT THERE WAS NO TFC CONFLICT. THE EPISODE WAS SMOOTH AND THERE WERE NO RPTS OF PAX OR CREW DISCOMFORT.

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.