On July 23, 2001, at 1830 eastern daylight time, a Cessna 150L, N21964, was substantially damaged during a forced landing to a field after a partial loss of engine power in Lusby, Maryland. The certificated student pilot was not injured. Visual meteorological conditions prevailed for the solo instructional flight that originated at St. Mary's Airport (2W6), California, Maryland. No flight plan was filed for the flight conducted under 14 CFR Part 91. In a written statement, the student pilot said he performed a preflight inspection of the airplane with no deficiencies noted. He said that fuel samples from both tanks revealed an absence of water or other contaminants. The student pilot said he filled both tanks with fuel, and that he took another fuel sample from each tank with no contaminants noted. According to the student pilot: "I taxied to runway 11 and performed a run up. RPM drop with carburetor heat was about 150. The magnetos dropped approximately 80 RPMs. Oil Pressure was in the green and the engine sounded smooth." "I took off from 2W6 runway 11 at approximately 1715 and climbed normally. My intention was to select a smooth air altitude and practice steep turns. This altitude was 2,500ft MSL. I flew for approximately 1 hour, during which I noted low clouds that caused me to decent to 2,300, whereby I resumed practice. Visibility was good but hazy, so I performed a couple of turns then ran with carburetor heat on for about five minutes, but with the mixture leaned for best cruise RPM. I observed no indication of icing. I did a couple more steep turns then again applied carburetor heat and proceeded to another area." "I noted a heavy aircraft approaching from the West towards Patuxent River Air Station, so I moved North to within approximately 5 miles of the offshore platform that demarcates Patuxent's restricted airspace. As clouds appeared to be lowering, I returned to 2W6 to over-fly and check cross winds and ceiling. The conditions were the same as where I had been practicing, so I returned to the location near the platform and performed 3 additional steep turns in each direction at 2,300 ft MSL. At that time, I noticed still lower clouds coming from the South so I decided to end my practice." "The Clouds [were] approximately 2000ft MSL and were between myself and 2W6, so I applied carburetor heat and started an immediate decent. As I leveled off, the engine started losing RPMs. I checked that my carburetor heat was on and tried to readjust the mixture; the engine continued to lose RPMs and began to miss. I verified ignition and fuel switches and gauges. I did not think to cycle the magneto switches. I then tried re-adjusting mixture and throttle, but the engine continued to lose RPMs. At that point, I called 2W6 UNICOM and reported that I was losing power. UNICOM advised me to call Patuxent approach." "I changed frequency, declared an emergency to Patuxent Approach, entered the requested transponder code, and noted that I had descended to 1100ft MSL. Patuxent Approach located me and offered a vector to Chesapeake Ranch private field, but I felt I did not have the altitude to make it. I had noticed a cleared field and began to circle it, expressing my intentions to land." "As I turned on final, I noticed the engine was still running but had slowed to an idle. I did not think to cycle the magneto switches or to re-apply power to see if possible icing had started to clear. I was still at about 75 KIAS, so I extended my flaps fully. There was a fence across the near end of the field, and in my effort to flare immediately beyond the fence, I was unable to reduce airspeed below 65 KIAS. The aircraft touched down smoothly and I got hard on the brakes. I did not raise the flaps or nose. Braking was ineffective, but the aircraft tracked straight until impacting the trees at 10 or 15 mph." In a telephone interview, the student pilot gave a similar accounting of events. When he described his descent below the clouds during his return to St. Mary's Airport, he said: "As I was descending, I was trying to get a steep angle because it was kind of gray underneath the clouds. I was a couple of minutes into my cruise setting when it started to act up. I didn't notice the RPM was dropping, and I was probably late in getting the carb heat back in. I pulled the carburetor heat all the way on, and it started to run real rough, so I pushed it back in." The pilot said he selected the field for a forced landing, and on final approach, the engine ran more smoothly. He said: "The engine started to smooth out, but I didn't think to put the power back in. I had given up too much altitude, and I didn't want to try to get it back in the air. I just wanted to get it into that field." The student pilot reported 108.5 hours of flight experience, all of which was in the Cessna 150L. The pilot said that other than the partial loss of engine power, there were no mechanical deficiencies with the airplane. The airplane was examined at the St. Mary's Airport on July 27, 2001. The airplane was upright on its landing gear, with the wings and the engine cowling removed. Prior to the examination, the propeller had been removed, straightened, and reinstalled. A small fuel can was plumbed into the fuel system. Examination of the engine revealed that the power controls were attached and intact, and that the throttle, mixture, and carburetor heat controls all operated throughout their respective ranges without binding. The engine started and ran. The engine was accelerated to cruise power and ran without interruption. The engine was then decelerated to idle power. The engine decelerated and idled smoothly without interruption. The weather reported at the Patuxent River Naval Air Station, at 1855, included scattered clouds at 5,000 feet with 6 miles of visibility in haze. The winds were from 190 degrees at 9 knots. The temperature was 82 degrees and the dewpoint was 73 degrees. According to the Federal Aviation Administration (FAA) Advisory Circular (AC) 20-113, Pilot Precautions and Procedures to be Taken In Preventing Aircraft Reciprocating Engine Induction System and Fuel System Icing Problems: "If induction system ice is suspected of causing a power loss, apply full heat or alternate air. Do not disturb the throttle until improvement is noted. Expect a further power loss momentarily and then a rise in power as the ice is melted." Interpolation of an FAA Carburetor Icing Probability Chart revealed that atmospheric conditions at the time of the accident were conducive to: "Serious Icing at Glide Power."