On June 16, 2024, about 0741 mountain daylight time, a Tecnam P2006T airplane, N968GV, was substantially damaged when it was involved in an accident near Larkspur, Colorado. The flight instructor and pilot receiving instruction were seriously injured. The airplane was operated as a Title 14 Code of Federal Regulations (CFR) Part 91 instructional flight. The flight instructor stated that he and the commercial-rated pilot receiving instruction (pilot) were completing a multi-engine training flight, which was their first multi-engine training flight together. The flight instructor had accumulated 115 flight hours in the accident airplane make and model, all of which were completed while acting as flight instructor. During the first portion of the flight, they completed five full-stop, taxi-back landings at Centennial Airport (APA), Englewood, Colorado. After the final landing, they proceeded to the southwest and climbed to 10,800 ft mean sea level (about 4,500 ft above ground level [agl]), where they intended to practice OEI training procedures in the North Perry practice area designated for multi-engine training. They initially completed directional control exercises and multiple simulated engine-out emergencies so the pilot receiving instruction could understand the step-by-step process. Next, they shut down the left engine by turning the left ignition switches to OFF. With the engine secured and the propeller feathered, the pilot maneuvered the airplane for “a little bit” until they attempted to the restart the engine. They referenced the engine restart checklist and attempted to restart the engine but were unsuccessful. They waited about 5 seconds, then engaged the starter buttons again and the engine restarted successfully. They slowly increased the left engine throttle and completed the cruise checklist before proceeding back toward APA. While en route, the flight instructor noted that the airplane required additional right rudder to maintain control. The pilot took over the controls and confirmed that more right rudder was required. The flight instructor noted that the left engine coolant temperature and rpm had decreased so he took back the controls and proceeded to Perry Park Airport (CO93), Larkspur, Colorado, which was about 4 nm south of their location. After completing a 180° turn to south, he secured the left engine and feathered the propeller. The airplane was difficult to control and the airplane was rapidly losing altitude. When he realized that they could not reach CO93, he set up to land on Interstate 25 (I-25). Just before landing in the southbound lanes of I-25, he pitched up and banked left to avoid hitting southbound vehicles. The airplane climbed over the traffic, then impacted a marshy area to the east of I-25. The pilot receiving instruction reported that he had previously accumulated 2 flight hours in the accident airplane make and model, which was his only multi-engine flight experience. He added that when they turned toward CO93 and secured the left engine, they were not sure if the propeller was fully feathered. ADS-B data indicated that they departed APA about 0617 and completed five full-stop landings. At 0715, after the fifth landing, they departed to the southwest and climbed to 10,800 ft mean sea level (msl). The airplane was in the North Perry practice area for about 10 minutes between 10,200 ft msl and 10,800 ft msl, before the airplane turned north toward APA. In the next 3 minutes, the airplane descended about 2,000 ft before making a 180° turn toward CO93. After completing the turn, the airplane was about 1,600 ft agl and 4 nm from CO93. In the next 2 minutes the airplane continued south over I-25 before the forced landing. The final ADS-B point was recorded 1.1 nm from CO93 about 200 ft agl. Witness videos of the accident flight showed that the airplane descended over I-25 with the right propeller rotating and the left propeller not rotating. The left wing struck a road sign, veered left, and climbed over the oncoming traffic. The end of the accident sequence was not captured in the video. The airplane came to rest inverted in a swampy area 0.17 nm east of the runway at CO93 and sustained substantial damage. The fuselage came to rest inverted and folded on top of the wings. The engines remained attached to the nacelles. The right propeller blades were fractured. The left propeller blades were relatively undamaged and appeared to be feathered. Postaccident examination of the airplane revealed that the left propeller lever was found retarded but not in the feathered position. The lever sustained damage and was bent right about 90° and the pedestal upper cover was bent to the right in the same area as the lever was bent over. The left throttle lever was found retarded about mid travel. The carburetor heat levers were positioned to OFF. The left and right engines were examined and prepared for functional test runs; both engines were run successfully at various power settings. There were no preimpact mechanical malfunctions or failures found during the examination or engine functional test runs that would have precluded normal operation. The airplane’s Garmin G1000 avionics were removed and sent to the NTSB Vehicle Recorder Laboratory for data extraction. The units did not record any engine parameter data. A review of the Carburetor Icing Probability Chart located in the Federal Aviation Administration’s Special Airworthiness Information Bulletin CE-09-35, Carburetor Icing Prevention, dated June 30, 2009, indicated that the atmospheric conditions in which the airplane was operating were conducive to the formation of serious icing at glide power settings. The aircraft flight manual, Section 4 – Normal procedures (2.2 Single engine training), stated in part: The best practice to perform single engine training is to retard one engine to the flight parameters equivalent to a dead engine. A simulated feather condition is obtained with propeller lever full forward and throttle lever set at 13.5 in Hg MAP [manifold pressure] at 70-90 KIAS [knots indicated airspeed] and 2,000-4,000 ft (density altitude). In normal operations, shutting down an engine for training shall not become a habit, in particular for safety reasons and in order to optimise training; engine shutdown to perform OEI shall be executed only when required by regulations (e.g. during flight check, skill tests or demonstration as per 14 CFR Part 61 or equivalent rule). The continuous operation of engine securing for training may indeed cause long term damages to the engine itself due to the high load coming from propeller (which is in feathering angle during the engine re-starting).