Cessna 140 accident near Arlington, Texas, July 18, 2020

The NTSB's narrative final · quoted from the NTSB record

The flight instructor reported that he and the student pilot departed on a cross-country flight. The airplane's engine performed as expected during the run up, and the student pilot set the carburetor heat on cold for takeoff and departed without issues.  About 2 minutes into the flight, the engine started to run a "little bit rough and shaky." The instructor pulled the carburetor heat on about 10% hot and left it in this position in accordance with his training in this airplane. The engine roughness disappeared, and the airplane kept climbing at the same rate. About 1 minute after this event, the roughness returned more severe this time. The instructor pulled the carburetor heat all the way on to full hot and waited. The roughness continued. The power available did not allow the airplane to continue the climb at the same rate and subsequently was not sufficient to keep a level flight attitude at 1,200 ft. There was no suitable area ahead for about the next 8 to 10 miles to perform an off-field emergency landing so the instructor elected to turn back to the departure airport. He was not confident that the airplane could maintain altitude in the traffic pattern, so he elected to land on a closer runway with a quartering tailwind. As the airplane slowed down during the landing roll, he retracted the flaps to remove any residual lift and applied the yoke "slightly diving" away from the wind. While the airplane was still moving forward at a low ground speed, the nose "suddenly" yawed very "quickly and strongly" to the right. The instructor pilot applied ailerons, elevators, and full left rudder and brake, but these did not stop the yaw. The airplane subsequently ground looped and came to rest in the grass resulting in substantial damage to the aft fuselage and tailwheel.  Using the pilot's reported departure weather, the plotted relative humidity was about 80%. Review of an icing probability chart revealed that the weather conditions at the time of the accident were "conducive to serious icing at glide power."  The operator of the airplane was asked to run the engine after the accident. The operator reported the engine started and operated "normally."  The airplane's engine likely encountered carburetor icing conditions during the climb, which resulted in a partial loss of engine power and forced landing where the flight instructor was not able to maintain directional control during the roll out.

The complete narrative as the NTSB published it. The NTSB's docket holds the report as a PDF and any photographs, statements and other documents from the investigation.