On June 24, 2017, about 1300 central daylight time, an Aero Commander 200D, N929DM, sustained substantial damage during a forced landing while approaching Northwest Florida Beaches International Airport (ECP), Panama City, Florida. The pilot and the passenger sustained minor injuries. The airplane was registered to a private company and operated by the pilot as a Title 14 Code of Federal Regulations Part 91 personal flight. Visual meteorological conditions prevailed, and the flight was operated on an instrument flight rules flight plan. The flight originated from the Malden Regional Airport (MAW), Malden, Missouri, about 1030.The pilot stated that he topped off the airplane's four fuel tanks (20 gallons each, 18.5 gallons usable) prior to the estimated 2 hour and 20-minute flight. He started the engine, taxied, and took off on the left main fuel tank and climbed to 7,000 ft mean sea level (msl), burning about 18 gph. When he leveled off at 7,000 ft msl, he set engine power to a 15.5 gph fuel burn rate. The pilot said he had to deviate around weather but used a stop watch to remind him to switch tanks every 30 minutes, which he said he did during the entire flight. He cycled from the left main tank over to the left auxiliary tank after he leveled off at 7,000 ft msl. He then switched to the right auxiliary tank, then the right main tank, before switching back to the left main fuel tank for the remainder of the flight. About 20 minutes after switching to the left main tank, the engine lost power when he was five miles from the airport at an altitude of about 2,000 ft mean sea level (msl) (about 1,930 above ground level). The pilot said the "engine was still running but not producing hardly any power, if any" and he was unable to maintain altitude, so he prepared to make a forced landing on a highway. During this time, he adjusted the mixture control and brought the throttle full forward, but to no avail. The pilot said he did not turn on the auxiliary fuel boost pump or switch fuel tanks as per the emergency checklist, and instead focused on trying to make a safe landing in a clearing between a set of power lines and a forest. He also thought he had about 20 minutes of fuel remaining in the left tank and did not consider the loss of power to be fuel related. The airplane stalled right before touchdown and landed hard on the right main landing gear. The pilot said the engine lost complete power before touchdown. The airplane skidded, turned around and came to rest upright on a southwesterly heading resulting in substantial damage to both wings and the fuselage. A postaccident examination of the airplane revealed that the left main fuel tank was intact and absent of fuel. The outboard section of the left wing, that housed the left auxiliary fuel tank, was bent up and partially separated. A small amount of fuel was observed in the left auxiliary tank and a sheen of fuel was observed under this portion of wing when it was recovered. The right wing also sustained impact damage, but the fuel tanks were not compromised. About 13 gallons of fuel were drained from the right wing auxiliary tank and about 11.5 gallons were drained from the right main tank. Postaccident examination of the engine revealed it had sufficient oil and the spark plugs exhibited normal wear as per the Champion Check-A-Plug chart. An internal inspection of each cylinder via a lighted borescope revealed each of the cylinder's valves were intact and exhibited normal combustion deposits. The engine was rotated via manual rotation of the propeller and valve train continuity and thumb-compression were established on each cylinder. The spark plugs were re-installed, and the engine was prepped to be test-run on the airframe utilizing the airplane's own fuel system. About 5-gallons of fuel were placed in the left main fuel tank and no leaks from the tank or its associated plumbing were observed. However, in preparation to start the engine, when the electrical fuel boost-pump was turned on to pressurize the fuel system, fuel sprayed from the b-nut that connected the main fuel supply line to the fuel manifold fitting. The b-nut was cross-threaded on the fitting and connected by about three threads. The fuel supply line was loose and not properly seated to the fitting. About six threads on the fitting were exposed, of which, two of the threads were shiny (clean) in appearance and the other four were dark from exposure to dirt and oil. There was no anti-tamper putty observed on the b-nut and fitting. No fuel stains were observed on the b-nut, fire sleeve, or surrounding area. The b-nut was tight on the fitting and had to be removed using a wrench. The b-nut was re-installed, and the engine started immediately and ran through its entire power-band without hesitation. A review of the maintenance logbooks revealed the last annual inspection on the engine was performed on September 13, 2016, at a total time of 774.78 hours. At the time of the accident, the engine had accrued about 25.22 hours since the annual inspection. The airplane's last annual inspection was completed on February 2, 2017, at a total time of 2,850.68 flight hours. The pilot held a private pilot certificate with ratings for airplane single-engine land, and instrument airplane. His last Federal Aviation Administration (FAA) third-class medical was issued on September 11, 2015. The pilot reported a total of 252.2 flight hours, of which, 20.2 hours, were in the accident airplane. The weather conditions reported at ECP at 1328 included wind from 220 degrees at 8 knots, gusting to 18 knots, visibility 10 miles, scattered clouds 2,900 ft, broken clouds 3,800 ft, temperature 30° degrees C, dewpoint 23° C, and a barometric altimeter setting of 30.04 inches of mercury. The field elevation at ECP is 64 feet msl. Per the airplane's Owner's Manual, page 4-2, ENGINE FAILURE, it states, "Should engine failure occur, the first step in any procedure is to lower the nose, set up and maintain a safe flying airspeed. Some of the most probable causes of engine failure are: fuel starvation, ignition malfunction, loss of oil and engine seizure. Some of the symptoms usually appear prior to complete engine failure. If the engine shows a decrease in power, starts running rough, or completely loses power, perform the following check: 1) Check that the fuel selector is on a usable tank. If necessary to switch tanks, retard the throttle, switch to desired tank. 2). Turn fuel boost pump "ON", advance throttle slowly until power is regained, and then turn OFF fuel boost pump. 3). Place mixture full rich and then lean to altitude if necessary. 4) Check ignition switch to "BOTH" position 5) Check all engine instruments.