Narrative:

A student; working on his commercial license; and I were conducting a night cross country of 100 miles point to point. We had an extremely slow [ground speed] due to high winds. There were intermittent clouds at our altitude.I contacted flight service to show the student how to use the service to receive updated weather information. We were cruising at 4;200 ft MSL with an airspeed of approx 80 knots. I then noticed that the altitude was below 4;000 ft and descending. I asked the student about this and he replied that he didn't know why we were descending. The descent continued and I took control of the aircraft. I applied full power; full mixture and carburetor heat. I radioed that we had a problem and that I was switching back to approach. Approach had apparently noticed our descent and called us on the radio. I informed them that we could not hold altitude and asked for vectors to the nearest airport. We were vectored to [an airport] approximately 9 miles away. The aircraft continued to descend as we made a 180 degree turn back towards [the airport]. The airplane stopped descending at approximately 3;000 ft and we were able to slowly climb back up to 4;000 ft. As the aircraft stabilized at 4;000 ft; I handed the controls back to the student. We maintained 4;000 ft until we were over [the airport]. We performed a spiraling descent and landed.on the ground; we checked the aircraft and found no indication of airframe icing or any other obvious problems. We believe that the descent was probably caused by carburetor ice.

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Original NASA ASRS Text

Title: C152 instructor pilot reported a precautionary landing with a student after experiencing a power loss possibly related to carburetor ice.

Narrative: A student; working on his Commercial license; and I were conducting a night cross country of 100 miles point to point. We had an extremely slow [ground speed] due to high winds. There were intermittent clouds at our altitude.I contacted Flight Service to show the student how to use the service to receive updated weather information. We were cruising at 4;200 ft MSL with an airspeed of approx 80 knots. I then noticed that the altitude was below 4;000 ft and descending. I asked the student about this and he replied that he didn't know why we were descending. The descent continued and I took control of the aircraft. I applied full power; full mixture and carburetor heat. I radioed that we had a problem and that I was switching back to Approach. Approach had apparently noticed our descent and called us on the radio. I informed them that we could not hold altitude and asked for vectors to the nearest airport. We were vectored to [an airport] approximately 9 miles away. The aircraft continued to descend as we made a 180 degree turn back towards [the airport]. The airplane stopped descending at approximately 3;000 ft and we were able to slowly climb back up to 4;000 ft. As the aircraft stabilized at 4;000 ft; I handed the controls back to the student. We maintained 4;000 ft until we were over [the airport]. We performed a spiraling descent and landed.On the ground; we checked the aircraft and found no indication of airframe icing or any other obvious problems. We believe that the descent was probably caused by carburetor ice.

Data retrieved from NASA's ASRS site 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.