A cessna 172-rg has an altitude excursion while flying over the mountains south of lake tahoe.

Attributes
ACN	345210
Time
Date	199608
Day	Mon
Local Time Of Day	1201 To 1800
Place
Locale Reference	airport : tvl
State Reference	CA
Altitude	msl bound lower : 12000 msl bound upper : 13000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zoa tracon : csg
Operator	general aviation : personal
Make Model Name	Skyhawk 172/Cutlass 172
Operating Under FAR Part	Part 91
Flight Phase	cruise other
Flight Plan	IFR
Person 1
Affiliation	Other
Function	instruction : instructor
Qualification	pilot : cfi
Experience	flight time last 90 days : 140 flight time total : 1108 flight time type : 50
ASRS Report	345210
Person 2
Affiliation	Other
Function	instruction : trainee
Qualification	pilot : private
Events
Anomaly	altitude deviation : excursion from assigned altitude inflight encounter : weather non adherence : clearance other anomaly other
Independent Detector	other controllera other flight crewa
Resolutory Action	controller : issued new clearance
Consequence	Other
Supplementary
Primary Problem	Weather
Air Traffic Incident	Pilot Deviation other

Narrative:

On aug/xx/96, pilot a (cfii) and pilot B (student instrument pilot) filed an IFR flight plan from modesto, ca, to south lake tahoe, ca, and were assigned the route of radar vectors linden- V28-113-spook-direct south lake tahoe and to maintain 13000 ft. After passing spook intersection, we experienced a down- draft and were unable to maintain 13000 ft MSL. We made 2 attempts to radio ZOA to inform them of our loss in altitude, but we were blocked by other radio calls to ATC. Upon reaching 12000 ft MSL, ZOA inquired about our altitude and we informed ATC that we were unable to maintain our assigned altitude of 13000 ft due to sink and turbulence. We also informed ATC that we tried twice to radio them of our loss of altitude and were blocked. ZOA then cleared us for a visual descent into tahoe where we landed without any further incident. Upon review of the situation described, I have found that under certain WX conditions (ie, in turbulence and down-drafts) it may be impossible to maintain altitude even when full power is applied as was the case with our C172RG. Careful consideration was given for the WX along our route of flight. The WX was forecast to be clear along our route of flight with a chance of thunderstorms after XA00 local time. Near our destination, we encountered unforecasted turbulence which caused our loss in altitude. If current PIREPS were available we could have avoided the unforecast WX and possible avoided our loss in altitude by changing our route of flight.

Original NASA ASRS Text

Title: A CESSNA 172-RG HAS AN ALT EXCURSION WHILE FLYING OVER THE MOUNTAINS S OF LAKE TAHOE.

Narrative: ON AUG/XX/96, PLT A (CFII) AND PLT B (STUDENT INST PLT) FILED AN IFR FLT PLAN FROM MODESTO, CA, TO S LAKE TAHOE, CA, AND WERE ASSIGNED THE RTE OF RADAR VECTORS LINDEN- V28-113-SPOOK-DIRECT S LAKE TAHOE AND TO MAINTAIN 13000 FT. AFTER PASSING SPOOK INTXN, WE EXPERIENCED A DOWN- DRAFT AND WERE UNABLE TO MAINTAIN 13000 FT MSL. WE MADE 2 ATTEMPTS TO RADIO ZOA TO INFORM THEM OF OUR LOSS IN ALT, BUT WE WERE BLOCKED BY OTHER RADIO CALLS TO ATC. UPON REACHING 12000 FT MSL, ZOA INQUIRED ABOUT OUR ALT AND WE INFORMED ATC THAT WE WERE UNABLE TO MAINTAIN OUR ASSIGNED ALT OF 13000 FT DUE TO SINK AND TURB. WE ALSO INFORMED ATC THAT WE TRIED TWICE TO RADIO THEM OF OUR LOSS OF ALT AND WERE BLOCKED. ZOA THEN CLRED US FOR A VISUAL DSCNT INTO TAHOE WHERE WE LANDED WITHOUT ANY FURTHER INCIDENT. UPON REVIEW OF THE SIT DESCRIBED, I HAVE FOUND THAT UNDER CERTAIN WX CONDITIONS (IE, IN TURB AND DOWN-DRAFTS) IT MAY BE IMPOSSIBLE TO MAINTAIN ALT EVEN WHEN FULL PWR IS APPLIED AS WAS THE CASE WITH OUR C172RG. CAREFUL CONSIDERATION WAS GIVEN FOR THE WX ALONG OUR RTE OF FLT. THE WX WAS FORECAST TO BE CLR ALONG OUR RTE OF FLT WITH A CHANCE OF TSTMS AFTER XA00 LCL TIME. NEAR OUR DEST, WE ENCOUNTERED UNFORECASTED TURB WHICH CAUSED OUR LOSS IN ALT. IF CURRENT PIREPS WERE AVAILABLE WE COULD HAVE AVOIDED THE UNFORECAST WX AND POSSIBLE AVOIDED OUR LOSS IN ALT BY CHANGING OUR RTE OF FLT.

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.