On January 12, 2022, about 1427 eastern standard time, a Piper J3C-65 airplane, N88509, was substantially damaged when it was involved in an accident near Winter Haven, Florida. The flight instructor and pilot receiving instruction sustained serious injuries. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 instructional flight. The flight instructor reported that the airline transport pilot receiving instruction had not flown a float-equipped airplane in 3 years, so the flight instructor considered the training flights as review. The flight instructor stated that, during their 2-hour morning flight, the pilot receiving instruction sat in the front seat and that he tended to “fly too slow” after takeoff and “turn sharply.” The instructor further stated that he had to “push the nose down on several occasions.” After eating lunch and refueling the airplane, the afternoon flight began, with the pilot receiving instruction sitting in the rear seat. The instructor estimated that they flew about 1 hour when the accident occurred. Although the flight instructor did not recall the details of the accident, he did recall thinking that the pilot receiving instruction might not have been able to see the instruments well from the rear seat or that he was being inattentive. The pilot receiving instruction had no recollection of the accident flight. A witness near the accident site stated that he saw the accident airplane fly one low pass over the lake, circle to the right, and then come around and descend again toward the lake. The witness stated that airplanes typically either land on the lake or fly a low pass but that the accident airplane made a small splash on the second pass when the right float touched the water. The airplane then “accelerated up” a couple hundred feet and started veering to the right in a “big arc.” The witness also stated that the turn “seemed like a steep angle” and that, at the top of the arc, he saw “the whole top of the airplane and it just continued downward.” The witness could not hear the engine when the airplane was descending. He stated that the airplane impacted the ground in a near-vertical nose-down attitude and that the airplane stayed in that position for a short time before the tail settled toward the ground. During a postaccident interview, the flight instructor stated that the witness’ description of the second pass sounded as if it were a go-around maneuver. The flight instructor also stated that the pilot receiving instruction had been “making really steep turns and he was too slow on airspeed.” The instructor added that he did not correct it in time. Postaccident examination of the wreckage revealed that the airplane came to rest in a near-vertical nose-down attitude with no ground scars or tree damage leading toward the wreckage, consistent with a near-vertical flightpath angle. The left wing sustained substantial damage. Control cable continuity from the cockpit area to the respective ailerons, rudder, and elevator control surfaces were confirmed. Further examination of the airplane revealed that the engine remained attached to the airframe with the propeller attached and canted left. Both propeller blades exhibited leading-edge abrasion. One propeller blade was cut to facilitate propeller rotation. All the spark plugs were removed and visually examined, and their condition was noted as normal-to-worn-out based on Champion Aviation’s Check-A-Plug Guide. Rotation of the engine’s crankshaft produced compression on all cylinders, and normal valvetrain movement was observed when the crankshaft was rotated. Examination of the engine’s cylinders with a lighted borescope revealed no damage to the cylinders or pistons. Both magnetos produced spark at all towers when their input drives were rotated. Borescope examination of all cylinders revealed carbon deposits consistent with a rich mixture and/or the use of automotive fuel. All cylinders passed a cold compression check. Fuel was present in the carburetor bowl and was absent of water or debris. Examination of the engine revealed no preimpact mechanical malfunctions or anomalies that would have precluded normal operation. Review of the icing probability chart contained within Federal Aviation Administration Special Airworthiness Information Bulletin CE-09-35 revealed that the atmospheric conditions at the time of the accident were conducive to “serious icing at glide power.” The flight instructor explained, during a postaccident interview, that carburetor heat was used on all approaches and would have been turned off at the initiation of the go-around maneuver.