Narrative:

I was descending on the arrival. Autopilot and autothrottles were engaged. Aircraft was flying VNAV descent and doing a good job of meeting the initial crossing restrictions. I don't have a chart handy; but I think the incident occurred around the first intersection with a speed and altitude restriction. I initially noticed that the aircraft wasn't slowing in a timely manner to meet the speed restriction; so I deployed the speedbrakes to assist. As the aircraft slowed I ordered the flaps extended to the 1 and 5 positions. When I stowed the speedbrakes I noticed that the aircraft wanted to accelerate again. At this time we were approaching the intersection and the aircraft was approaching the limit altitude. Around this time I noticed the autopilot disconnect advisory illuminated temporarily on the EICAS; but there was no aural warning. I also looked at the autopilot engagement button on the MCP and noticed that it was still illuminated; which meant that the autopilot should still be engaged. At this point the aircraft began descending below the minimum altitude at the intersection. I repeatedly pressed the autopilot disconnect button on the yoke and tried to level the aircraft. I also tried to trim the aircraft with no noticeable effect. I looked at the trim indicator and noticed that it was in the normal range. When I attempted to manually fly the aircraft there was an extremely heavy nose down force; and I initially thought the autopilot didn't disconnect. I was having trouble holding enough force on the yoke to level the aircraft; so I asked the captain to assist me. With both of us pulling; and as power was applied; we were able to level the aircraft. At that point; we declared an emergency; and asked for vectors so we could figure out what we were dealing with. We ran the jammed flight controls checklist; which didn't seem to help. As we were vectored for the approach the captain advised that the control forces were returning to normal. This lead us to believe that maybe something involved with the elevator had frozen. Before we departed that evening the aircraft was exposed to several hours of very heavy rain; and also heavy rain on the climb-out. The captain made a normal landing; and we taxied to the gate without any more issues.

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Original NASA ASRS Text

Title: A B767-300 crew reported an apparent ice frozen elevator system after exposure to heavy precipitation prior to departure followed by a long high altitude flight. The autopilot failed to operate the elevator and two pilots had difficulty leveling the aircraft during descent until the apparent ice melted at lower altitudes.

Narrative: I was descending on the arrival. Autopilot and autothrottles were engaged. Aircraft was flying VNAV descent and doing a good job of meeting the initial crossing restrictions. I don't have a chart handy; but I think the incident occurred around the first intersection with a speed and altitude restriction. I initially noticed that the aircraft wasn't slowing in a timely manner to meet the speed restriction; so I deployed the speedbrakes to assist. As the aircraft slowed I ordered the flaps extended to the 1 and 5 positions. When I stowed the speedbrakes I noticed that the aircraft wanted to accelerate again. At this time we were approaching the intersection and the aircraft was approaching the limit altitude. Around this time I noticed the Autopilot Disconnect advisory illuminated temporarily on the EICAS; but there was no aural warning. I also looked at the autopilot engagement button on the MCP and noticed that it was still illuminated; which meant that the autopilot should still be engaged. At this point the aircraft began descending below the minimum altitude at the intersection. I repeatedly pressed the autopilot disconnect button on the yoke and tried to level the aircraft. I also tried to trim the aircraft with no noticeable effect. I looked at the trim indicator and noticed that it was in the normal range. When I attempted to manually fly the aircraft there was an extremely heavy nose down force; and I initially thought the autopilot didn't disconnect. I was having trouble holding enough force on the yoke to level the aircraft; so I asked the Captain to assist me. With both of us pulling; and as power was applied; we were able to level the aircraft. At that point; we declared an emergency; and asked for vectors so we could figure out what we were dealing with. We ran the Jammed Flight Controls Checklist; which didn't seem to help. As we were vectored for the approach the Captain advised that the control forces were returning to normal. This lead us to believe that maybe something involved with the elevator had frozen. Before we departed that evening the aircraft was exposed to several hours of very heavy rain; and also heavy rain on the climb-out. The Captain made a normal landing; and we taxied to the gate without any more issues.

Data retrieved from NASA's ASRS site as of July 2013 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.