Flight crew of B767-200 encounters mountain wave and is unable to maintain assigned altitude.

Attributes
ACN	380505
Time
Date	199709
Day	Thu
Local Time Of Day	1801 To 2400
Place
Locale Reference	atc facility : tbe
State Reference	CO
Altitude	msl bound lower : 38200 msl bound upper : 40500
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zab
Operator	common carrier : air carrier
Make Model Name	B767-200
Operating Under FAR Part	Part 121
Navigation In Use	Other Other
Flight Phase	cruise other
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp pilot : flight engineer
Experience	flight time last 90 days : 180 flight time total : 14000 flight time type : 600
ASRS Report	380505
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : commercial pilot : instrument
Events
Anomaly	altitude deviation : excursion from assigned altitude inflight encounter : weather
Independent Detector	aircraft equipment other aircraft equipment : unspecified other flight crewa
Resolutory Action	flight crew : returned to intended course or assigned course none taken : unable
Consequence	Other
Supplementary
Primary Problem	Weather
Air Traffic Incident	other

Narrative:

When in the vicinity of tobe VOR, wbound at FL390, in clear smooth air, the aircraft encountered a large strong mountain wave on lee side of rocky mountains. There was no warning. The airspeed increased rapidly above barber pole and aircraft climbed on autoplt to approximately FL405. I disconnected autothrottles and then the autoplt. I reduced thrust and lowered nose to return to assigned altitude. While this was happening ZAB called asking about our altitude. We informed center that we were in a strong mountain wave and attempting to return to assigned altitude. The center controller said that there was no problem. When we flew out of the upwind side of the mountain wave the altitude and airspeed decreased rapidly. Maximum continuous power was applied yet airspeed decreased to clean speed plus 10 KTS and aircraft descended to about FL382. When airspeed started increasing I slowly eased aircraft to assigned altitude and re-engaged autoplt and throttles. I don't know of any way this occurrence could have been prevented. During the recovery phase I controled the aircraft attitude while the copilot controled the thrust setting for me which worked well.

Original NASA ASRS Text

Title: FLC OF B767-200 ENCOUNTERS MOUNTAIN WAVE AND IS UNABLE TO MAINTAIN ASSIGNED ALT.

Narrative: WHEN IN THE VICINITY OF TOBE VOR, WBOUND AT FL390, IN CLR SMOOTH AIR, THE ACFT ENCOUNTERED A LARGE STRONG MOUNTAIN WAVE ON LEE SIDE OF ROCKY MOUNTAINS. THERE WAS NO WARNING. THE AIRSPD INCREASED RAPIDLY ABOVE BARBER POLE AND ACFT CLBED ON AUTOPLT TO APPROX FL405. I DISCONNECTED AUTOTHROTTLES AND THEN THE AUTOPLT. I REDUCED THRUST AND LOWERED NOSE TO RETURN TO ASSIGNED ALT. WHILE THIS WAS HAPPENING ZAB CALLED ASKING ABOUT OUR ALT. WE INFORMED CTR THAT WE WERE IN A STRONG MOUNTAIN WAVE AND ATTEMPTING TO RETURN TO ASSIGNED ALT. THE CTR CTLR SAID THAT THERE WAS NO PROB. WHEN WE FLEW OUT OF THE UPWIND SIDE OF THE MOUNTAIN WAVE THE ALT AND AIRSPD DECREASED RAPIDLY. MAX CONTINUOUS PWR WAS APPLIED YET AIRSPD DECREASED TO CLEAN SPD PLUS 10 KTS AND ACFT DSNDED TO ABOUT FL382. WHEN AIRSPD STARTED INCREASING I SLOWLY EASED ACFT TO ASSIGNED ALT AND RE-ENGAGED AUTOPLT AND THROTTLES. I DON'T KNOW OF ANY WAY THIS OCCURRENCE COULD HAVE BEEN PREVENTED. DURING THE RECOVERY PHASE I CTLED THE ACFT ATTITUDE WHILE THE COPLT CTLED THE THRUST SETTING FOR ME WHICH WORKED WELL.

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.