Narrative:

We were in cruise flight. The first officer was flying - I went off frequency to copy ATIS. Center handed us off, the first officer acknowledged the call and contacted approach. (There was a substantial pressure gradient from the south verses the north side of the mountain range over which we were passing). A portion of the ATIS was garbled, so it was necessary to wait for the tape to play again. While I was off ATC frequency, we encountered a strong updraft while in light to occasionally moderate turbulence. The first officer brought the power back to flight idle, but we were taken off altitude by nearly 300 ft. I came back onto communication #1 as the motion now turned into a strong downdraft with associated (now more consistently) moderate turbulence. As the first officer brought the aircraft level again I added maximum power - we were at full power as the airspeed bled off and we struggled to maintain altitude. I called ATC and requested a block altitude stating we were having difficulty maintaining our altitude, at which time they gave us a pilot's discretion descent. A PIREP was made to the ATC facility regarding the turbulence, up/down drafts and mountain wave action. No moisture returns were painted on our radar in the area of the up/down drafts. Unfortunately we were not both on ATC frequency when the drafts first hit. The first officer was busy flying as I attempted to return to ATC frequency to advise of our status. Had we both been on ATC, perhaps the call could have come sooner. None-the-less, we did our best to advise as soon as possible while 'flying the airplane first.' mother nature is strong and at times flexes her muscles and shows just how powerful she can be. ATC never questioned our altitude at any time.

Related reports:

Google
 

Original NASA ASRS Text

Title: ALTDEV THEN A SPD DEV DURING ATTEMPT TO HOLD ALT.

Narrative: WE WERE IN CRUISE FLT. THE FO WAS FLYING - I WENT OFF FREQ TO COPY ATIS. CTR HANDED US OFF, THE FO ACKNOWLEDGED THE CALL AND CONTACTED APCH. (THERE WAS A SUBSTANTIAL PRESSURE GRADIENT FROM THE S VERSES THE N SIDE OF THE MOUNTAIN RANGE OVER WHICH WE WERE PASSING). A PORTION OF THE ATIS WAS GARBLED, SO IT WAS NECESSARY TO WAIT FOR THE TAPE TO PLAY AGAIN. WHILE I WAS OFF ATC FREQ, WE ENCOUNTERED A STRONG UPDRAFT WHILE IN LIGHT TO OCCASIONALLY MODERATE TURB. THE FO BROUGHT THE PWR BACK TO FLT IDLE, BUT WE WERE TAKEN OFF ALT BY NEARLY 300 FT. I CAME BACK ONTO COM #1 AS THE MOTION NOW TURNED INTO A STRONG DOWNDRAFT WITH ASSOCIATED (NOW MORE CONSISTENTLY) MODERATE TURB. AS THE FO BROUGHT THE ACFT LEVEL AGAIN I ADDED MAX PWR - WE WERE AT FULL PWR AS THE AIRSPD BLED OFF AND WE STRUGGLED TO MAINTAIN ALT. I CALLED ATC AND REQUESTED A BLOCK ALT STATING WE WERE HAVING DIFFICULTY MAINTAINING OUR ALT, AT WHICH TIME THEY GAVE US A PLT'S DISCRETION DSCNT. A PIREP WAS MADE TO THE ATC FACILITY REGARDING THE TURB, UP/DOWN DRAFTS AND MOUNTAIN WAVE ACTION. NO MOISTURE RETURNS WERE PAINTED ON OUR RADAR IN THE AREA OF THE UP/DOWN DRAFTS. UNFORTUNATELY WE WERE NOT BOTH ON ATC FREQ WHEN THE DRAFTS FIRST HIT. THE FO WAS BUSY FLYING AS I ATTEMPTED TO RETURN TO ATC FREQ TO ADVISE OF OUR STATUS. HAD WE BOTH BEEN ON ATC, PERHAPS THE CALL COULD HAVE COME SOONER. NONE-THE-LESS, WE DID OUR BEST TO ADVISE ASAP WHILE 'FLYING THE AIRPLANE FIRST.' MOTHER NATURE IS STRONG AND AT TIMES FLEXES HER MUSCLES AND SHOWS JUST HOW POWERFUL SHE CAN BE. ATC NEVER QUESTIONED OUR ALT AT ANY TIME.

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.