B737 flight crew encounters turbulence and mountain wave during cruise flight at FL410; elects to de...

Attributes
ACN	688942
Time
Date	200602
Local Time Of Day	1201 To 1800
Place
Locale Reference	navaid : gll.vortac
State Reference	CO
Altitude	msl single value : 41000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zdv.artcc
Operator	common carrier : air carrier
Make Model Name	B737-700
Operating Under FAR Part	Part 121
Flight Phase	cruise : level
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Experience	flight time last 90 days : 240 flight time total : 26000 flight time type : 21000
ASRS Report	688942
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Events
Anomaly	altitude deviation : overshoot inflight encounter : turbulence inflight encounter other other anomaly
Independent Detector	other flight crewa other flight crewb
Resolutory Action	controller : issued advisory flight crew : exited adverse environment flight crew : returned to assigned altitude
Supplementary
Problem Areas	Flight Crew Human Performance Environmental Factor
Primary Problem	Ambiguous

Narrative:

Cruising at FL410; with continuous light chop; seatbelt sign was on. (Turbulence reports at all altitudes.) no tp's had been recently issued on our route of flight; but there was tp activity along our route in the WX packet. ATC had been queried about turbulence and stated continuous light chop ahead. As we neared den; about 150 mi or so; our airspeed began to increase. We pulled the thrust levers back as the airspeed continued to increase; finally setting in at .85 mach and the thrust levers at idle. ATC informed us that an aircraft ahead of us had encountered strong mountain wave and would we like a lower altitude. We agreed and were assigned FL390. As we began our descent we encountered what I will call moderate turbulence; and increased banking of the aircraft left and right. As I was trying to keep the aircraft wings level and descend quickly out of the turbulence; we came out the back side of the mountain wave. The airspeed fell off rapidly and I added power. I noticed that the airspeed was not responding to the descent or increased power; and we were nearing 200 KTS. More power was added; but I was reluctant to raise the nose as we were nearing a stall. The stall warning actually went off twice as we requested a block altitude and settled at FL370. Remainder of flight was uneventful. No injuries occurred as everyone was in their seats.

Original NASA ASRS Text

Title: B737 FLT CREW ENCOUNTERS TURB AND MOUNTAIN WAVE DURING CRUISE FLT AT FL410; ELECTS TO DSND TO A LOWER ALT; AIRSPD DECREASES RAPIDLY AND STALL WARNING IS ACTIVATED.

Narrative: CRUISING AT FL410; WITH CONTINUOUS LIGHT CHOP; SEATBELT SIGN WAS ON. (TURB RPTS AT ALL ALTS.) NO TP'S HAD BEEN RECENTLY ISSUED ON OUR RTE OF FLT; BUT THERE WAS TP ACTIVITY ALONG OUR RTE IN THE WX PACKET. ATC HAD BEEN QUERIED ABOUT TURB AND STATED CONTINUOUS LIGHT CHOP AHEAD. AS WE NEARED DEN; ABOUT 150 MI OR SO; OUR AIRSPD BEGAN TO INCREASE. WE PULLED THE THRUST LEVERS BACK AS THE AIRSPD CONTINUED TO INCREASE; FINALLY SETTING IN AT .85 MACH AND THE THRUST LEVERS AT IDLE. ATC INFORMED US THAT AN ACFT AHEAD OF US HAD ENCOUNTERED STRONG MOUNTAIN WAVE AND WOULD WE LIKE A LOWER ALT. WE AGREED AND WERE ASSIGNED FL390. AS WE BEGAN OUR DSCNT WE ENCOUNTERED WHAT I WILL CALL MODERATE TURB; AND INCREASED BANKING OF THE ACFT L AND R. AS I WAS TRYING TO KEEP THE ACFT WINGS LEVEL AND DSND QUICKLY OUT OF THE TURB; WE CAME OUT THE BACK SIDE OF THE MOUNTAIN WAVE. THE AIRSPD FELL OFF RAPIDLY AND I ADDED PWR. I NOTICED THAT THE AIRSPD WAS NOT RESPONDING TO THE DSCNT OR INCREASED PWR; AND WE WERE NEARING 200 KTS. MORE PWR WAS ADDED; BUT I WAS RELUCTANT TO RAISE THE NOSE AS WE WERE NEARING A STALL. THE STALL WARNING ACTUALLY WENT OFF TWICE AS WE REQUESTED A BLOCK ALT AND SETTLED AT FL370. REMAINDER OF FLT WAS UNEVENTFUL. NO INJURIES OCCURRED AS EVERYONE WAS IN THEIR SEATS.

Data retrieved from NASA's ASRS site as of January 2009 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.

B737 flight crew encounters turbulence and mountain wave during cruise flight at FL410; elects to descend to a lower altitude; airspeed decreases rapidly and stall warning is activated.