DC10-30 experiences mountain wave activity at FL390. Unable to maintain altitude, the aircraft receives vectors for an off course descent and has to start descent early on account of decaying airspeed. Center asked for a call on ground.

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Attributes
ACN	364569
Time
Date	199703
Day	Fri
Local Time Of Day	0601 To 1200
Place
Locale Reference	atc facility : mlp
State Reference	ID
Altitude	msl bound lower : 38700 msl bound upper : 39000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zlc tracon : m54
Operator	common carrier : air carrier
Make Model Name	Widebody, Low Wing, 3 Turbojet Eng
Flight Phase	climbout : intermediate altitude cruise other
Route In Use	enroute airway : zlc enroute other
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : second officer
Qualification	pilot : instrument pilot : flight engineer pilot : atp
Experience	flight time last 90 days : 80 flight time total : 11000 flight time type : 100
ASRS Report	364569
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : flight engineer pilot : instrument pilot : commercial
Experience	flight time last 90 days : 190 flight time total : 9000 flight time type : 1500
ASRS Report	364560
Events
Anomaly	altitude deviation : excursion from assigned altitude inflight encounter : weather inflight encounter other non adherence : clearance other anomaly other anomaly other
Independent Detector	other controllera other flight crewa
Resolutory Action	controller : issued new clearance flight crew : exited adverse environment
Consequence	faa : reviewed incident with flight crew
Supplementary
Primary Problem	Weather
Air Traffic Incident	Pilot Deviation other

Narrative:

Cruising at FL350 in area reported having mountain wave activity between FL260 and FL340. We requested a 'wrong way cruise altitude' of FL370, which was granted. Just after leveling at FL370, center controller asked if we could climb to FL390. After checking our performance manuals and agreeing that we could hold FL390 with 1.3G protection, we climbed to FL390. Just after reaching FL390, we were hit by strong mountain wave activity and turbulence. Aircraft buffeted due to low speed stall and then went into mach buffet. Crew was unable to control airspeed and altitude and immediately requested a lower altitude. Controller had conflicting traffic and either said 'standby' or 'it's not available.' we attempted to hold assigned altitude but lost approximately 300 ft. Captain was getting ready to declare an emergency to descend, when ATC gave us a radar vector and a descent clearance to a lower altitude. We started down immediately. Lower altitudes were also affected by mountain wave activity but we had a wider stall and mach buffet protection. A few mins after recovering at FL370, ATC requested that we call ZLC to explain what happened and our altitude deviation. Even though we were 5000 ft above the forecast mountain wave activity, we still encountered difficulty. Supplemental information from acn 364560: given a 30 degree right turn to facilitate lower. During turn forced to start shallow descent with maximum thrust just to maintain airspeed above stall. Experienced high speed buffet 3 times and low speed buffet once during entire episode that lasted approximately 30 seconds.

Original NASA ASRS Text

Title: DC10-30 EXPERIENCES MOUNTAIN WAVE ACTIVITY AT FL390. UNABLE TO MAINTAIN ALT, THE ACFT RECEIVES VECTORS FOR AN OFF COURSE DSCNT AND HAS TO START DSCNT EARLY ON ACCOUNT OF DECAYING AIRSPD. CTR ASKED FOR A CALL ON GND.

Narrative: CRUISING AT FL350 IN AREA RPTED HAVING MOUNTAIN WAVE ACTIVITY BTWN FL260 AND FL340. WE REQUESTED A 'WRONG WAY CRUISE ALT' OF FL370, WHICH WAS GRANTED. JUST AFTER LEVELING AT FL370, CTR CTLR ASKED IF WE COULD CLB TO FL390. AFTER CHKING OUR PERFORMANCE MANUALS AND AGREEING THAT WE COULD HOLD FL390 WITH 1.3G PROTECTION, WE CLBED TO FL390. JUST AFTER REACHING FL390, WE WERE HIT BY STRONG MOUNTAIN WAVE ACTIVITY AND TURB. ACFT BUFFETED DUE TO LOW SPD STALL AND THEN WENT INTO MACH BUFFET. CREW WAS UNABLE TO CTL AIRSPD AND ALT AND IMMEDIATELY REQUESTED A LOWER ALT. CTLR HAD CONFLICTING TFC AND EITHER SAID 'STANDBY' OR 'IT'S NOT AVAILABLE.' WE ATTEMPTED TO HOLD ASSIGNED ALT BUT LOST APPROX 300 FT. CAPT WAS GETTING READY TO DECLARE AN EMER TO DSND, WHEN ATC GAVE US A RADAR VECTOR AND A DSCNT CLRNC TO A LOWER ALT. WE STARTED DOWN IMMEDIATELY. LOWER ALTS WERE ALSO AFFECTED BY MOUNTAIN WAVE ACTIVITY BUT WE HAD A WIDER STALL AND MACH BUFFET PROTECTION. A FEW MINS AFTER RECOVERING AT FL370, ATC REQUESTED THAT WE CALL ZLC TO EXPLAIN WHAT HAPPENED AND OUR ALTDEV. EVEN THOUGH WE WERE 5000 FT ABOVE THE FORECAST MOUNTAIN WAVE ACTIVITY, WE STILL ENCOUNTERED DIFFICULTY. SUPPLEMENTAL INFO FROM ACN 364560: GIVEN A 30 DEG R TURN TO FACILITATE LOWER. DURING TURN FORCED TO START SHALLOW DSCNT WITH MAX THRUST JUST TO MAINTAIN AIRSPD ABOVE STALL. EXPERIENCED HIGH SPD BUFFET 3 TIMES AND LOW SPD BUFFET ONCE DURING ENTIRE EPISODE THAT LASTED APPROX 30 SECONDS.

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.