Narrative:

Just after receiving clearance to enter the class D airspace and enter a left downwind I noticed a decrease in power and roughness to the engine. I immediately pitched for best glide speed and began troubleshooting as I turned southbound towards a field I determined I could land in. After exhausting my troubleshooting options; I informed tower that I was setting down in a field north of [nearby lake]; that I had quarter to half tanks and two souls were on board. After landing successfully I informed the tower that I was on the ground and everyone was ok. I called the FBO and asked that a ride for my student head our direction. When the fire truck arrived; I told them that everything was ok; and that our mechanic was heading out to check the airplane. After they left I spoke with the sheriff when he arrived and gave him my information. He then left; and the ride for my student showed up and took him back to the airport.once the student had left; I began investigating the situation with the aircraft and determined that there may have been a fuel blockage in the system because there was still fuel in both tanks and the fuel strainer/sump showed fuel when drained. After becoming convinced that the fuel system was operating properly; I added a little more fuel to the aircraft and removed the passenger seat to keep the aircraft lighter. I then performed a soft/short field takeoff and flew it back to our home airport where I had another mechanic further check it.before I departed for the flight; I had preflight a piper aircraft which I was going to take. However; as I was pulling the aircraft out to be flown; the cessna returned from its flight with the senior instructor aboard. He asked me if I would take the cessna; which I agreed to do. However; I did not preflight it; I took on faith that it was in an airworthy condition since it had just been flown. I also did not check the fuel level visually; rather I trusted the gauges which were reading just under a half tank per side. The flight was going to be only half an hour so I figured that it would be fine. That was my mistake. When the engine began to show signs of lost power I did not take time to squawk 7700 or call on 121.5 since I was already in contact with the tower. I just told them I was putting down and focused on getting to a suitable spot.

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Original NASA ASRS Text

Title: A Cessna 150 has a forced landing into a field soon after takeoff after the instructor and student pilot failed to conduct a thorough preflight. They contribute a last minute aircraft swap from a Piper as potential cause.

Narrative: Just after receiving clearance to enter the Class D airspace and enter a left downwind I noticed a decrease in power and roughness to the engine. I immediately pitched for best glide speed and began troubleshooting as I turned southbound towards a field I determined I could land in. After exhausting my troubleshooting options; I informed Tower that I was setting down in a field north of [nearby lake]; that I had quarter to half tanks and two souls were on board. After landing successfully I informed the Tower that I was on the ground and everyone was OK. I called the FBO and asked that a ride for my student head our direction. When the fire truck arrived; I told them that everything was OK; and that our Mechanic was heading out to check the airplane. After they left I spoke with the Sheriff when he arrived and gave him my information. He then left; and the ride for my student showed up and took him back to the airport.Once the student had left; I began investigating the situation with the aircraft and determined that there may have been a fuel blockage in the system because there was still fuel in both tanks and the fuel strainer/sump showed fuel when drained. After becoming convinced that the fuel system was operating properly; I added a little more fuel to the aircraft and removed the passenger seat to keep the aircraft lighter. I then performed a soft/short field takeoff and flew it back to our home airport where I had another Mechanic further check it.Before I departed for the flight; I had preflight a Piper aircraft which I was going to take. However; as I was pulling the aircraft out to be flown; the Cessna returned from its flight with the Senior Instructor aboard. He asked me if I would take the Cessna; which I agreed to do. However; I did not preflight it; I took on faith that it was in an airworthy condition since it had just been flown. I also did not check the fuel level visually; rather I trusted the gauges which were reading just under a half tank per side. The flight was going to be only half an hour so I figured that it would be fine. That was my mistake. When the engine began to show signs of lost power I did not take time to squawk 7700 or call on 121.5 since I was already in contact with the Tower. I just told them I was putting down and focused on getting to a suitable spot.

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.