Giv flight crew encounters mountain wave activity during cruise at FL430. Aircraft descends and airspeed increases. Flight crew takes precautionary action and returns to altitude.

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Attributes
ACN	688948
Time
Date	200602
Local Time Of Day	1201 To 1800
Place
Locale Reference	navaid : den.vor
State Reference	CO
Altitude	msl single value : 43000
Environment
Flight Conditions	IMC
Weather Elements	Turbulence
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zdv.artcc
Operator	general aviation : corporate
Make Model Name	Gulfstream IV
Operating Under FAR Part	Part 91
Flight Phase	cruise : enroute altitude change cruise : level
Flight Plan	IFR
Person 1
Affiliation	company : corporate
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 50 flight time total : 20000 flight time type : 2000
ASRS Report	688948
Person 2
Affiliation	company : corporate
Function	flight crew : first officer
Events
Anomaly	altitude deviation : excursion from assigned altitude conflict : ground less severe inflight encounter : turbulence other anomaly other
Independent Detector	other flight crewa other flight crewb
Resolutory Action	flight crew : returned to assigned altitude
Supplementary
Problem Areas	Environmental Factor Flight Crew Human Performance Aircraft
Primary Problem	Ambiguous

Narrative:

FL430 eastbound. Requested the altitude due to continuous light turbulence. At FL410 aircraft too heavy for FL450. (Just on top of clouds layer; cruise speed mach .80; outside air temperature -62 degrees C; smooth ride at FL430.) approximately 30 mi east of aircraft position saw cloud layer increase (no mountain wave indication). Determined through HUD; we would enter cloud formation. Put on seatbelt sign. Just as we entered clouds; we heard on ZDV an aircraft report severe mountain wave activity. Within 1 min; airspeed went from mach .80 to mach .88. Outside air temperature went from -62 degrees C to -70 degrees C. Started losing altitude; slowed aircraft; autoplt off advised ZDV of altitude change from FL430 descending. Stopped altitude loss at approximately FL422. Climbed back to FL430 and advised ZDV that we were level. About 1 1/2 mins to level back at FL430 (as soon as possible). (No TCAS alerts.) if severe mountain wave activity or turbulence was reported to the flight crew; we would have changed our course or altitude. Continued flight to destination (normal flight).

Original NASA ASRS Text

Title: GIV FLT CREW ENCOUNTERS MOUNTAIN WAVE ACTIVITY DURING CRUISE AT FL430. ACFT DSNDS AND AIRSPD INCREASES. FLT CREW TAKES PRECAUTIONARY ACTION AND RETURNS TO ALT.

Narrative: FL430 EBOUND. REQUESTED THE ALT DUE TO CONTINUOUS LIGHT TURB. AT FL410 ACFT TOO HVY FOR FL450. (JUST ON TOP OF CLOUDS LAYER; CRUISE SPD MACH .80; OUTSIDE AIR TEMP -62 DEGS C; SMOOTH RIDE AT FL430.) APPROX 30 MI E OF ACFT POS SAW CLOUD LAYER INCREASE (NO MOUNTAIN WAVE INDICATION). DETERMINED THROUGH HUD; WE WOULD ENTER CLOUD FORMATION. PUT ON SEATBELT SIGN. JUST AS WE ENTERED CLOUDS; WE HEARD ON ZDV AN ACFT RPT SEVERE MOUNTAIN WAVE ACTIVITY. WITHIN 1 MIN; AIRSPD WENT FROM MACH .80 TO MACH .88. OUTSIDE AIR TEMP WENT FROM -62 DEGS C TO -70 DEGS C. STARTED LOSING ALT; SLOWED ACFT; AUTOPLT OFF ADVISED ZDV OF ALT CHANGE FROM FL430 DSNDING. STOPPED ALT LOSS AT APPROX FL422. CLBED BACK TO FL430 AND ADVISED ZDV THAT WE WERE LEVEL. ABOUT 1 1/2 MINS TO LEVEL BACK AT FL430 (ASAP). (NO TCAS ALERTS.) IF SEVERE MOUNTAIN WAVE ACTIVITY OR TURB WAS RPTED TO THE FLT CREW; WE WOULD HAVE CHANGED OUR COURSE OR ALT. CONTINUED FLT TO DEST (NORMAL FLT).

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.