Cessna 150
Flight Crew Private; Flight Crew Multiengine; Flight Crew Flight Instructor; Flight Crew Instrument
Flight Crew Last 90 Days 89; Flight Crew Total 1800; Flight Crew Type 100
Situational Awareness; Time Pressure
Aircraft Equipment Problem Critical
Air Traffic Control Issued New Clearance; Flight Crew Landed in Emergency Condition; Flight Crew Diverted
Human Factors; Aircraft
During pre-flight I estimated the plane had about 30 gallons of fuel.  I had just picked up a banner and was towing it for 2 hours around [the local area]. The weather was VFR: 54* 29.96.  After getting to 1;000 FT AGL; I contacted Approach and let them know I would be in the area for 2 hours.  During the flight the engine had a slight roughness occasionally.  The owner told me that the carburetor; mags; spark plugs had all been changed and that the roughness or hiccup should not be a problem.  On the final leg of the flight the plane began to run rough.   I checked everything and the plane cleared itself up.  I told Approach that the plane had just ran rough; but is okay now.  I told the Controller I would continue to my departure airport.  Shortly after that the engine got rough again and I turned back and told Approach of the engine roughness.  They gave me winds and cleared me to land on any runway.  I noticed I still had fuel pressure.  The engine was not making enough power.  I released the banner and landed in a large bean field.  I contacted Approach and told them I landed safely and that there was no damage to the plane or property.  I feel that you should land as soon as there is any abnormality.
C150 pilot reports a slight engine roughness while towing a banner at 1;000 feet.  Toward the end of the flight more pronounced roughness appears then clears on its own.  When the roughness appears again the decision to land at the nearest airport is made but decreasing power results in dropping the banner and landing in a field.