Narrative:

The aircraft involved is an MD11 freighter. The aircraft was flown by the first officer. The flight leg was mine, but due to the WX at anc we were executing a standard monitored approach. Prior to the top of descent I transferred aircraft control to the first officer. Zan cleared our flight to 'descend and maintain 16000 ft MSL.' during the descent we were cleared direct to the anc VOR. The aircraft was being controled by the autoflt system and the FMS and the autoplt was manipulated by the first officer, while I was making inputs to the FMS as necessary. During the descent the aircraft entered IMC. The WX radar was on and operating. A small cell was being depicted on the radar. Descending from approximately FL180, the first officer selected heading on the autoplt to execute a minor deviation to the south (left) of course to circumnav a cell. As the turn to the left was selected with the autoplt engaged, the aircraft entered a pocket of severe turbulence. This encounter resulted in a hard left, steep, descending flight path. The first officer selected the heading selector of the autoplt to return to the course to the anc VOR. The autoplt was not responding appropriately. The autoplt limits were being exceeded. During this period, the aircraft did not honor the selected altitude of 16000 ft MSL which was in the altitude window of the flight control panel and continued descent to approximately 15500 ft MSL. I commanded the first officer to disengage the autoplt and manually fly the airplane. The first officer appropriately disconnected and made proper inputs manually. The aircraft was still being affected by turbulence. By the time the first officer returned the aircraft to level flight at 16000 ft MSL and proceeding normally to the anc VOR, the aircraft had climbed to approximately 17000 ft MSL. We received a transmission from another air carrier stating ATC wanted him to inform us ATC viewed our aircraft at 15500 ft MSL. A short time after, once the aircraft was stabilized (1 or 2 mins?), a center controller asked 'if we were having problems.' he further added it appeared on the radar that the aircraft was making a 180 degree turn. I replied 'the aircraft was under control.' so much was happening I did not report the turbulence or the reason for this event. We were cleared direct to the anc VOR for the ILS runway 6R. Once we stabilized at 16000 ft, I was then required to make amendments to the FMS for the approach and brief the approach. In addition, the first officer and I were continuing to be cautious in the event we should encounter additional turbulence. Due to these described factors, I forgot to report the turbulence. This event or incident was unanticipated and not avoidable as best as the radar and no available reports of turbulence via PIREPS or another source. The altitude diversion was wholly induced by atmospheric conditions.

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Original NASA ASRS Text

Title: AN ACR MD11 FREIGHTER LOST ALT IN TURB NEAR ANC. NIGHT OP. WX FACTORS SMALL TSTM CELL WITH VERY LARGE TURB. ALTDEV. TRACK HDG DEV.

Narrative: THE ACFT INVOLVED IS AN MD11 FREIGHTER. THE ACFT WAS FLOWN BY THE FO. THE FLT LEG WAS MINE, BUT DUE TO THE WX AT ANC WE WERE EXECUTING A STANDARD MONITORED APCH. PRIOR TO THE TOP OF DSCNT I TRANSFERRED ACFT CTL TO THE FO. ZAN CLRED OUR FLT TO 'DSND AND MAINTAIN 16000 FT MSL.' DURING THE DSCNT WE WERE CLRED DIRECT TO THE ANC VOR. THE ACFT WAS BEING CTLED BY THE AUTOFLT SYS AND THE FMS AND THE AUTOPLT WAS MANIPULATED BY THE FO, WHILE I WAS MAKING INPUTS TO THE FMS AS NECESSARY. DURING THE DSCNT THE ACFT ENTERED IMC. THE WX RADAR WAS ON AND OPERATING. A SMALL CELL WAS BEING DEPICTED ON THE RADAR. DSNDING FROM APPROX FL180, THE FO SELECTED HDG ON THE AUTOPLT TO EXECUTE A MINOR DEV TO THE S (L) OF COURSE TO CIRCUMNAV A CELL. AS THE TURN TO THE L WAS SELECTED WITH THE AUTOPLT ENGAGED, THE ACFT ENTERED A POCKET OF SEVERE TURB. THIS ENCOUNTER RESULTED IN A HARD L, STEEP, DSNDING FLT PATH. THE FO SELECTED THE HDG SELECTOR OF THE AUTOPLT TO RETURN TO THE COURSE TO THE ANC VOR. THE AUTOPLT WAS NOT RESPONDING APPROPRIATELY. THE AUTOPLT LIMITS WERE BEING EXCEEDED. DURING THIS PERIOD, THE ACFT DID NOT HONOR THE SELECTED ALT OF 16000 FT MSL WHICH WAS IN THE ALT WINDOW OF THE FLT CTL PANEL AND CONTINUED DSCNT TO APPROX 15500 FT MSL. I COMMANDED THE FO TO DISENGAGE THE AUTOPLT AND MANUALLY FLY THE AIRPLANE. THE FO APPROPRIATELY DISCONNECTED AND MADE PROPER INPUTS MANUALLY. THE ACFT WAS STILL BEING AFFECTED BY TURB. BY THE TIME THE FO RETURNED THE ACFT TO LEVEL FLT AT 16000 FT MSL AND PROCEEDING NORMALLY TO THE ANC VOR, THE ACFT HAD CLBED TO APPROX 17000 FT MSL. WE RECEIVED A XMISSION FROM ANOTHER ACR STATING ATC WANTED HIM TO INFORM US ATC VIEWED OUR ACFT AT 15500 FT MSL. A SHORT TIME AFTER, ONCE THE ACFT WAS STABILIZED (1 OR 2 MINS?), A CTR CTLR ASKED 'IF WE WERE HAVING PROBS.' HE FURTHER ADDED IT APPEARED ON THE RADAR THAT THE ACFT WAS MAKING A 180 DEG TURN. I REPLIED 'THE ACFT WAS UNDER CTL.' SO MUCH WAS HAPPENING I DID NOT RPT THE TURB OR THE REASON FOR THIS EVENT. WE WERE CLRED DIRECT TO THE ANC VOR FOR THE ILS RWY 6R. ONCE WE STABILIZED AT 16000 FT, I WAS THEN REQUIRED TO MAKE AMENDMENTS TO THE FMS FOR THE APCH AND BRIEF THE APCH. IN ADDITION, THE FO AND I WERE CONTINUING TO BE CAUTIOUS IN THE EVENT WE SHOULD ENCOUNTER ADDITIONAL TURB. DUE TO THESE DESCRIBED FACTORS, I FORGOT TO RPT THE TURB. THIS EVENT OR INCIDENT WAS UNANTICIPATED AND NOT AVOIDABLE AS BEST AS THE RADAR AND NO AVAILABLE RPTS OF TURB VIA PIREPS OR ANOTHER SOURCE. THE ALT DIVERSION WAS WHOLLY INDUCED BY ATMOSPHERIC CONDITIONS.

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.