Declared emergency result of mountain wave.

Attributes
ACN	255750
Time
Date	199311
Day	Wed
Local Time Of Day	0601 To 1200
Place
Locale Reference	atc facility : ekr
State Reference	CO
Altitude	msl bound lower : 37000 msl bound upper : 41000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zmp
Operator	common carrier : air carrier
Make Model Name	Widebody, Low Wing, 2 Turbojet Eng
Navigation In Use	Other Other
Flight Phase	cruise other
Route In Use	enroute airway : j84
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : flight engineer pilot : atp
Experience	flight time last 90 days : 200 flight time total : 17000 flight time type : 1300
ASRS Report	255750
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : atp
Events
Anomaly	altitude deviation : excursion from assigned altitude inflight encounter other non adherence : clearance
Independent Detector	other flight crewa
Resolutory Action	controller : issued new clearance flight crew : regained aircraft control
Consequence	Other
Supplementary
Primary Problem	Weather
Air Traffic Incident	other

Narrative:

Cruising at FL410, mach .80 on J84 60 west den. Previous conditions off and on light turbulence with light to moderate turbulence reported down through FL290. Winds at 410 about 270/60. Observed acsl's to south and was aware of turbulence PIREP southwest of den. Encountered severe wave action resulting in airspeed fluctuations to in excess of .86 mach (accompanied by overspd warning) followed by rapid deceleration to 200 KIAS. Only light turbulence encountered, began descent without clearance from ATC as stall was believed imminent. ATC was notified of descent and cleared flight to FL370 as it was transitioning FL400. TCASII was available and in use and helped establish clear track for descent (no conflicts were involved). When aircraft accelerated, idle power and full speed brakes would not slow aircraft out of overspd regime at assigned altitude. As aircraft subsequently slowed, maximum cruise power would not accelerate the aircraft and it was feared that downstream turbulence might cause a stall. A descent was initiated to regain safe flying speed. First officer had reported FL410 winds as 270/006. As plane entered acceleration phase of encounter, flight continued to destination cruising at FL370 till well east of den where climb to FL410 was executed. Winds at FL410 east of den were in the 270/60 range again.

Original NASA ASRS Text

Title: DECLARED EMER RESULT OF MOUNTAIN WAVE.

Narrative: CRUISING AT FL410, MACH .80 ON J84 60 W DEN. PREVIOUS CONDITIONS OFF AND ON LIGHT TURB WITH LIGHT TO MODERATE TURB RPTED DOWN THROUGH FL290. WINDS AT 410 ABOUT 270/60. OBSERVED ACSL'S TO S AND WAS AWARE OF TURB PIREP SW OF DEN. ENCOUNTERED SEVERE WAVE ACTION RESULTING IN AIRSPD FLUCTUATIONS TO IN EXCESS OF .86 MACH (ACCOMPANIED BY OVERSPD WARNING) FOLLOWED BY RAPID DECELERATION TO 200 KIAS. ONLY LIGHT TURB ENCOUNTERED, BEGAN DSCNT WITHOUT CLRNC FROM ATC AS STALL WAS BELIEVED IMMINENT. ATC WAS NOTIFIED OF DSCNT AND CLRED FLT TO FL370 AS IT WAS TRANSITIONING FL400. TCASII WAS AVAILABLE AND IN USE AND HELPED ESTABLISH CLR TRACK FOR DSCNT (NO CONFLICTS WERE INVOLVED). WHEN ACFT ACCELERATED, IDLE PWR AND FULL SPD BRAKES WOULD NOT SLOW ACFT OUT OF OVERSPD REGIME AT ASSIGNED ALT. AS ACFT SUBSEQUENTLY SLOWED, MAX CRUISE PWR WOULD NOT ACCELERATE THE ACFT AND IT WAS FEARED THAT DOWNSTREAM TURB MIGHT CAUSE A STALL. A DSCNT WAS INITIATED TO REGAIN SAFE FLYING SPD. FO HAD RPTED FL410 WINDS AS 270/006. AS PLANE ENTERED ACCELERATION PHASE OF ENCOUNTER, FLT CONTINUED TO DEST CRUISING AT FL370 TILL WELL E OF DEN WHERE CLB TO FL410 WAS EXECUTED. WINDS AT FL410 E OF DEN WERE IN THE 270/60 RANGE AGAIN.

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.