Narrative:

On a trans oceanic flight our aileron controls froze up. During cruise the aileron trim annunciator light kept coming on. Towards the end of the crossing and during the descent autopilot tracking became unreliable without aileron trim assistance. It was during vectors to approach that a left hand turn exceeded 30 degrees bank whereupon I disconnected the autopilot and realized that we had frozen ailerons. The bank angle at the time of disconnect was about 45 degrees and required wings level recovery with rudder. Both pilots working together could not achieve any aileron authority. An emergency was declared at this point. All remaining turns were accomplished using rudder only including ILS intercept and landing. No damage to aircraft; passenger; or crew. Cause seems to be a quantity of water accumulating during the flight and freezing in the belly; eventually building up to the point of binding the aileron control system. I believe that the short duration of the two stops were not long or warm enough to allow thawing of the accumulated ice inside the aircraft; which kept building up as we flew. The actual source of the water is still being investigated. Also apparently we had a failure of the automatic drain on the belly. During the descent we thought the autopilot servo or aileron trim to be the problem; possibly freezing or sticking; as we could assist with trim inputs to help with tracking. I did disconnect once at altitude and the aileron controls felt stiff but free. Thinking it was still an icing valve or sticky actuator I turned on the bleed air wing heat for the descent hoping to help thaw whatever the problem was. It made no noticeable improvement. This occurrence was at the end of a long day for the crew.callback conversation with reporter revealed the following information: the reporter stated that an ice block was found on an aileron control arm common to both the hydraulically boosted and manual reversion control of the ailerons. The aircraft had made two previous stops during the long international flight and at each stop ice in the 2 refreshment cooling drawers was replenished. Because the under fuselage drain mast usually allows thawed liquid to drain from the refreshment drawer area; there was nothing unusual noted at either stop. However; it is now suspected that the drain mast had become clogged with frozen material. During the final flight enough fluid had accumulated around the aileron control rod because of the aircraft's normal 5 degree nose up flight attitude; that upon freezing and being unable to drain in the frozen drain mast; the ice locked the ailerons.

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Original NASA ASRS Text

Title: A FALCON 20'S AILERON CTL LEVERS IN THE ACFT'S FUSELAGE BECAME FROZEN WITH MELTED GALLEY ICE APPARENTLY UNABLE TO DRAIN FROM A FROZEN DRAIN MAST.

Narrative: ON A TRANS OCEANIC FLT OUR AILERON CONTROLS FROZE UP. DURING CRUISE THE AILERON TRIM ANNUNCIATOR LIGHT KEPT COMING ON. TOWARDS THE END OF THE CROSSING AND DURING THE DSCNT AUTOPILOT TRACKING BECAME UNRELIABLE WITHOUT AILERON TRIM ASSISTANCE. IT WAS DURING VECTORS TO APCH THAT A LEFT HAND TURN EXCEEDED 30 DEGS BANK WHEREUPON I DISCONNECTED THE AUTOPILOT AND REALIZED THAT WE HAD FROZEN AILERONS. THE BANK ANGLE AT THE TIME OF DISCONNECT WAS ABOUT 45 DEGS AND REQUIRED WINGS LEVEL RECOVERY WITH RUDDER. BOTH PILOTS WORKING TOGETHER COULD NOT ACHIEVE ANY AILERON AUTHORITY. AN EMER WAS DECLARED AT THIS POINT. ALL REMAINING TURNS WERE ACCOMPLISHED USING RUDDER ONLY INCLUDING ILS INTERCEPT AND LNDG. NO DAMAGE TO ACFT; PAX; OR CREW. CAUSE SEEMS TO BE A QUANTITY OF WATER ACCUMULATING DURING THE FLT AND FREEZING IN THE BELLY; EVENTUALLY BUILDING UP TO THE POINT OF BINDING THE AILERON CONTROL SYSTEM. I BELIEVE THAT THE SHORT DURATION OF THE TWO STOPS WERE NOT LONG OR WARM ENOUGH TO ALLOW THAWING OF THE ACCUMULATED ICE INSIDE THE ACFT; WHICH KEPT BUILDING UP AS WE FLEW. THE ACTUAL SOURCE OF THE WATER IS STILL BEING INVESTIGATED. ALSO APPARENTLY WE HAD A FAILURE OF THE AUTOMATIC DRAIN ON THE BELLY. DURING THE DESCENT WE THOUGHT THE AUTOPILOT SERVO OR AILERON TRIM TO BE THE PROBLEM; POSSIBLY FREEZING OR STICKING; AS WE COULD ASSIST WITH TRIM INPUTS TO HELP WITH TRACKING. I DID DISCONNECT ONCE AT ALTITUDE AND THE AILERON CONTROLS FELT STIFF BUT FREE. THINKING IT WAS STILL AN ICING VALVE OR STICKY ACTUATOR I TURNED ON THE BLEED AIR WING HEAT FOR THE DESCENT HOPING TO HELP THAW WHATEVER THE PROBLEM WAS. IT MADE NO NOTICEABLE IMPROVEMENT. THIS OCCURRENCE WAS AT THE END OF A LONG DAY FOR THE CREW.CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THAT AN ICE BLOCK WAS FOUND ON AN AILERON CTL ARM COMMON TO BOTH THE HYDRAULICALLY BOOSTED AND MANUAL REVERSION CTL OF THE AILERONS. THE ACFT HAD MADE TWO PREVIOUS STOPS DURING THE LONG INTERNATIONAL FLT AND AT EACH STOP ICE IN THE 2 REFRESHMENT COOLING DRAWERS WAS REPLENISHED. BECAUSE THE UNDER FUSELAGE DRAIN MAST USUALLY ALLOWS THAWED LIQUID TO DRAIN FROM THE REFRESHMENT DRAWER AREA; THERE WAS NOTHING UNUSUAL NOTED AT EITHER STOP. HOWEVER; IT IS NOW SUSPECTED THAT THE DRAIN MAST HAD BECOME CLOGGED WITH FROZEN MATERIAL. DURING THE FINAL FLT ENOUGH FLUID HAD ACCUMULATED AROUND THE AILERON CTL ROD BECAUSE OF THE ACFT'S NORMAL 5 DEG NOSE UP FLT ATTITUDE; THAT UPON FREEZING AND BEING UNABLE TO DRAIN IN THE FROZEN DRAIN MAST; THE ICE LOCKED THE AILERONS.

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.