Narrative:

I was under an IFR flight plan and was making an approach into gjt; co. WX condition was clear; light to moderate turbulence on the descent from 16000 ft to my new assigned altitude of 12000 ft and I was slowing the descent rate and speed anticipating the usual increase in turbulence at the lower altitude. I was on a heading of approximately 330 degrees with gjt approximately 20 mi ahead. Either before or after my autoplt captured 12000 ft I had preset into the autoplt computer; the aircraft pitched up suddenly. I began an instrument scan and saw the airspeed dropping. I simultaneously disconnected the autoplt and pushed the yoke forward while trimming nose down. Turbulence strongly increased and I tried to get the aircraft speed into the 'maneuvering speed' area to minimize structural loads and also worked to maintain pitch and roll stability. (The aircraft pitched up because I entered a downdraft and the autoplt; attempting to correct and maintain altitude; that was preset; trimmed the aircraft into a nose-up attitude. This is an event I have trained for and am familiar with.) I immediately used outside visual references to ensure terrain and aircraft separation; and orientation for roll control and pitch control and referenced the airspeed to avoid stall/spin by maintaining sufficient airspeed by pitching down. Because of the strong turbulence and my concern of excessively loading the aircraft structure; and because I was looking at terrain and the terrain was not sufficiently close to pose any threat; I did not increase g-loading to climb and concentrated on maintaining the aircraft in controled flight. Momentarily approach control called and asked my altitude. I replied 10000 ft and I had encountered a downdraft. The controller advised me to turn 90 degrees left. I complied immediately and flew out of the strong turbulence.

Related reports:

Google
 

Original NASA ASRS Text

Title: PIPER P46 PLT HAS AN ALT EXCURSION DURING DSCNT FOR LNDG AT GJT.

Narrative: I WAS UNDER AN IFR FLT PLAN AND WAS MAKING AN APCH INTO GJT; CO. WX CONDITION WAS CLR; LIGHT TO MODERATE TURB ON THE DSCNT FROM 16000 FT TO MY NEW ASSIGNED ALT OF 12000 FT AND I WAS SLOWING THE DSCNT RATE AND SPD ANTICIPATING THE USUAL INCREASE IN TURB AT THE LOWER ALT. I WAS ON A HDG OF APPROX 330 DEGS WITH GJT APPROX 20 MI AHEAD. EITHER BEFORE OR AFTER MY AUTOPLT CAPTURED 12000 FT I HAD PRESET INTO THE AUTOPLT COMPUTER; THE ACFT PITCHED UP SUDDENLY. I BEGAN AN INST SCAN AND SAW THE AIRSPD DROPPING. I SIMULTANEOUSLY DISCONNECTED THE AUTOPLT AND PUSHED THE YOKE FORWARD WHILE TRIMMING NOSE DOWN. TURB STRONGLY INCREASED AND I TRIED TO GET THE ACFT SPD INTO THE 'MANEUVERING SPD' AREA TO MINIMIZE STRUCTURAL LOADS AND ALSO WORKED TO MAINTAIN PITCH AND ROLL STABILITY. (THE ACFT PITCHED UP BECAUSE I ENTERED A DOWNDRAFT AND THE AUTOPLT; ATTEMPTING TO CORRECT AND MAINTAIN ALT; THAT WAS PRESET; TRIMMED THE ACFT INTO A NOSE-UP ATTITUDE. THIS IS AN EVENT I HAVE TRAINED FOR AND AM FAMILIAR WITH.) I IMMEDIATELY USED OUTSIDE VISUAL REFS TO ENSURE TERRAIN AND ACFT SEPARATION; AND ORIENTATION FOR ROLL CTL AND PITCH CTL AND REFED THE AIRSPD TO AVOID STALL/SPIN BY MAINTAINING SUFFICIENT AIRSPD BY PITCHING DOWN. BECAUSE OF THE STRONG TURB AND MY CONCERN OF EXCESSIVELY LOADING THE ACFT STRUCTURE; AND BECAUSE I WAS LOOKING AT TERRAIN AND THE TERRAIN WAS NOT SUFFICIENTLY CLOSE TO POSE ANY THREAT; I DID NOT INCREASE G-LOADING TO CLB AND CONCENTRATED ON MAINTAINING THE ACFT IN CTLED FLT. MOMENTARILY APCH CTL CALLED AND ASKED MY ALT. I REPLIED 10000 FT AND I HAD ENCOUNTERED A DOWNDRAFT. THE CTLR ADVISED ME TO TURN 90 DEGS L. I COMPLIED IMMEDIATELY AND FLEW OUT OF THE STRONG TURB.

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.