On November 16, 2017, about 1800 Pacific standard time, a Piper PA-28R-201T, N5966V, was substantially damaged when it was involved in an accident near Nipton, California. The pilot and passenger were not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight. The flight departed North Las Vegas Airport (VGT), Las Vegas, Nevada, about 1655, destined for Brackett Field Airport (POC), La Verne, California. The pilot reported that while en route at a cruise altitude of 10,000 ft mean sea level, he began leaning the engine mixture using the JPI-700 onboard engine data monitor; the procedure was routine and uneventful. The pilot stated that after about 5 minutes, both he and the passenger noted an abrupt engine roughness. About that time, the pilot observed that the No. 5 cylinder head temperature (CHT) had spiked, the airplane started slowing, and he was unable to maintain altitude. The pilot reported that during the descent, a slight acrid odor permeated the cockpit for about 1 minute and then dissipated. The pilot confirmed that the electric fuel pump was set to high and switched fuel tanks, but he noticed no change in performance. After the airplane lost about 500 ft in altitude, air traffic control contacted the pilot, who advised of engine trouble. The controller stated that an alternate airport was about 26 miles to the west, but the pilot determined that a highway was the safest landing point. During the descent, the pilot noted that the propeller was still turning but did not “feel” like it was producing power. The pilot stated that the touchdown was hard, and that his visual reference was inhibited by dust, vibration, and the moonless night conditions. The right main landing gear collapsed, and the airplane sustained substantial damage to both wings, with the right main landing gear strut pushed up through the wing. The onboard JPI-700 engine data monitor was sent to the National Transportation Safety Board Vehicle Recorder Laboratory for download of recordable nonvolatile memory data. Data showed a rise in CHT for the No. 5 cylinder just after 1740. Exhaust gas temperature (EGT) values for cylinder Nos. 1, 3, and 5 diverged about the same time, and the device calculated a disparity in EGT differential about the same time. Engine rpm started to drop from 2,400 about 1743:45, with rpm stabilizing shortly thereafter about 2,100. Rpm then began to drop off sharply about 1751:03, and the recording ended at 17:51:15. A postaccident engine examination was conducted, during which the crankshaft was rotated by hand using the propeller. Continuity to the accessory section was confirmed, and spark was produced on all six top spark plug leads. Thumb compression was obtained on all cylinders except the No. 5 cylinder. The top spark plugs were removed, and all except the No. 5 spark plug exhibited dark-colored combustion deposits. The No. 5 spark plug exhibited a molten, metal-like substance around the electrode. The cylinders were examined with a lighted borescope. The No. 2, 4, and 6-cylinder combustion chambers, piston faces, and exhaust and intake valves exhibited normal operating signatures. The No. 5 cylinder combustion chamber exhibited signs consistent with detonation/preignition, with material missing from around the circumference of the piston face, and the exhaust and intake valves showed signs of mechanical damage. Cylinder Nos. 1 and 3 exhibited minor signs of detonation.