Narrative:

Student did preflight of aircraft; nav log; and weight and balance for VFR cross country training flight. Student was in left seat and I was the instructor and was seated in the right seat. After startup; and with full fuel; we got the current WX at ZZZ which was calm winds; and -15 C OAT; above SOP limit of 0 F. After the weather we tested the brakes and taxied to runway. Holding short of runway we completed the run up and no anomalies were seen so we took off; student flying; and headed south until we established contact with approach. After contact we proceeded on course direct to ZZZ1. [While enroute]; I noticed a random; very light; what seemed to be misfire in the engine. Since it was so cold my first thought went to carb ice. I turned on carb heat and noted the RPM drop but no engine roughness usually associated with carb ice. I then asked my student what airspeed he was maintaining and if he was able to climb. He told me about 60-65 kts and he was only able to maintain altitude with full power. After about 3-5 mins of carb heat and full power; I turned it off and; still and full power; noted the RPM only came up to around 2000; bottom of the green arc. At that point I told my student to head back to ZZZ and I advised approach of our intentions making a 180 degree turn back. We were cleared as requested and while my student flew; I trouble shot the issue and reviewed the emergency checklists. Not finding any applicable items in the checklists I began leaning the mixture and adjusting the throttle to get any better power. I noticed no RPM climb as I leaned the mixture until I hit engine roughness associated with an excessively lean mixture. I then returned the mixture to full rich and reduced the throttle. When I pulled the throttle about an inch back I noticed the RPM climbed about 200. I elected to leave the throttle there since it was generating the best power. About 2-3 minutes later we got extreme engine roughness that felt like a dead cylinder. At this point I took over flying and [advised] approach. I then decided that I was going to head for ZZZ2 since [terrain] was between me and ZZZ and the city was between me and ZZZ3 and also not confident that the engine would make it to either airport. As we turned to fly to ZZZ2 we were losing altitude and were unable to see ZZZ2. With the river between us and also no good landing areas between us and ZZZ2 I decided I wasn't going to try and make it to ZZZ2. I began looking for a suitable landing area and saw a flat soybean field right under the aircraft. I continued to fly the aircraft and landed in the field with no injuries to me or my student and no visible damage to the aircraft from the off airport landing. After [advising ATC] I was communicating with approach but after a short while I was unable to understand them due to some loud beeping tones in the background when approach transmitted. I did try switching radios but it did not improve. I made one last final radio call advising them of my intended landing area and did not speak to approach again until I landed and called them on my cell phone.

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Original NASA ASRS Text

Title: C150 instructor reported engine troubles led to a forced landing in a field.

Narrative: Student did preflight of aircraft; nav log; and weight and balance for VFR cross country training flight. Student was in left seat and I was the instructor and was seated in the right seat. After startup; and with full fuel; we got the current WX at ZZZ which was calm winds; and -15 C OAT; above SOP limit of 0 F. After the weather we tested the brakes and taxied to runway. Holding short of runway we completed the run up and no anomalies were seen so we took off; student flying; and headed south until we established contact with Approach. After contact we proceeded on course direct to ZZZ1. [While enroute]; I noticed a random; very light; what seemed to be misfire in the engine. Since it was so cold my first thought went to carb ice. I turned on carb heat and noted the RPM drop but no engine roughness usually associated with carb ice. I then asked my student what airspeed he was maintaining and if he was able to climb. He told me about 60-65 kts and he was only able to maintain altitude with full power. After about 3-5 mins of carb heat and full power; I turned it off and; still and full power; noted the RPM only came up to around 2000; bottom of the green arc. At that point I told my student to head back to ZZZ and I advised Approach of our intentions making a 180 degree turn back. We were cleared as requested and while my student flew; I trouble shot the issue and reviewed the emergency checklists. Not finding any applicable items in the checklists I began leaning the mixture and adjusting the throttle to get any better power. I noticed no RPM climb as I leaned the mixture until I hit engine roughness associated with an excessively lean mixture. I then returned the mixture to full rich and reduced the throttle. When I pulled the throttle about an inch back I noticed the RPM climbed about 200. I elected to leave the throttle there since it was generating the best power. About 2-3 minutes later we got extreme engine roughness that felt like a dead cylinder. At this point I took over flying and [advised] Approach. I then decided that I was going to head for ZZZ2 since [terrain] was between me and ZZZ and the city was between me and ZZZ3 and also not confident that the engine would make it to either airport. As we turned to fly to ZZZ2 we were losing altitude and were unable to see ZZZ2. With the river between us and also no good landing areas between us and ZZZ2 I decided I wasn't going to try and make it to ZZZ2. I began looking for a suitable landing area and saw a flat soybean field right under the aircraft. I continued to fly the aircraft and landed in the field with no injuries to me or my student and no visible damage to the aircraft from the off airport landing. After [advising ATC] I was communicating with Approach but after a short while I was unable to understand them due to some loud beeping tones in the background when Approach transmitted. I did try switching radios but it did not improve. I made one last final radio call advising them of my intended landing area and did not speak to Approach again until I landed and called them on my cell phone.

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.