Altitude excursion of -500 ft by single engine cessna flown by private pilot.

Attributes
ACN	435241
Time
Date	199904
Day	Mon
Local Time Of Day	1201 To 1800
Place
Locale Reference	airport : las.airport
State Reference	NV
Altitude	msl bound lower : 5000 msl bound upper : 5500
Environment
Flight Conditions	VMC
Weather Elements	Turbulence Windshear
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : l30.tracon
Operator	general aviation : personal
Make Model Name	Cessna Stationair/Turbo Stationair 6
Operating Under FAR Part	Part 91
Route In Use	arrival : on vectors
Flight Plan	None
Person 1
Affiliation	other
Function	flight crew : single pilot
Qualification	pilot : instrument pilot : private
Experience	flight time last 90 days : 50 flight time total : 1050 flight time type : 600
ASRS Report	435241
Person 2
Affiliation	government : faa
Function	controller : approach
Qualification	controller : radar
Events
Anomaly	altitude deviation : excursion from assigned altitude inflight encounter : turbulence non adherence : clearance other anomaly
Independent Detector	other controllera other flight crewa
Resolutory Action	flight crew : returned to original clearance flight crew : regained aircraft control
Supplementary
Problem Areas	Aircraft Weather Flight Crew Human Performance
Primary Problem	Weather
Air Traffic Incident	Pilot Deviation

Narrative:

I was being vectored over las on my way to vgt at 4500 ft when I encountered a severe downdraft in clear air. At full power and full RPM, I was unable to maintain altitude. I lost approximately 500 ft before I was able to arrest the descent. Immediately after arresting the descent, I encountered a 1500-2000 FPM updraft and moderate turbulence. The aircraft rose approximately 1000 ft as I was reducing power. Once through the updraft I was able to resume assigned altitude and heading. When contacted by ATC when the aircraft descended, all I was able to say was that I couldn't maintain altitude as I was in a downdraft. I do not know if event was the result of windshear, standing wave, or wake turbulence, but I have never before experienced a situation, even at high density altitudes, where maximum power could not overcome a downdraft.

Original NASA ASRS Text

Title: ALT EXCURSION OF -500 FT BY SINGLE ENG CESSNA FLOWN BY PVT PLT.

Narrative: I WAS BEING VECTORED OVER LAS ON MY WAY TO VGT AT 4500 FT WHEN I ENCOUNTERED A SEVERE DOWNDRAFT IN CLR AIR. AT FULL PWR AND FULL RPM, I WAS UNABLE TO MAINTAIN ALT. I LOST APPROX 500 FT BEFORE I WAS ABLE TO ARREST THE DSCNT. IMMEDIATELY AFTER ARRESTING THE DSCNT, I ENCOUNTERED A 1500-2000 FPM UPDRAFT AND MODERATE TURB. THE ACFT ROSE APPROX 1000 FT AS I WAS REDUCING PWR. ONCE THROUGH THE UPDRAFT I WAS ABLE TO RESUME ASSIGNED ALT AND HDG. WHEN CONTACTED BY ATC WHEN THE ACFT DSNDED, ALL I WAS ABLE TO SAY WAS THAT I COULDN'T MAINTAIN ALT AS I WAS IN A DOWNDRAFT. I DO NOT KNOW IF EVENT WAS THE RESULT OF WINDSHEAR, STANDING WAVE, OR WAKE TURB, BUT I HAVE NEVER BEFORE EXPERIENCED A SIT, EVEN AT HIGH DENSITY ALTS, WHERE MAX PWR COULD NOT OVERCOME A DOWNDRAFT.

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.