Abstract: At approximately 1:18 p.m. on December 9, 2008, Employee #1 was clearing partially frozen dirt and mud from the mechanism of a Midland belly dump trailer (Model Number MG43CLAM). The trailer was being pulled by a Mack tractor (Model Number RD688S). When the clamshell closed, he was crushed and killed. The weather at the time of the accident was clear, winds at 21 mph out of the North, and 21 degrees. The temperatures had caused problems for the dump trailers throughout the morning and the supervisor advised the drivers to keep the dump mechanisms clean. There were no witnesses to the accident, but the first person to arrive at the scene reported that the Mack tractor's engine was not running as he approached. The clamshell on the trailer requires air pressure to stay open and when the engine is not operating, the clamshell will close. Other drivers indicated that it was common to lock the engine idle on by setting the cruise control to maintain pressure and keep the clamshell open. They would then clean the trailer. This is a violation of the manufacturer's recommended procedures for entering the clamshell area. The operator's manual for the Midlands trailer clearly states that the air valve should be turned off, the system air exhausted, and the gates locked in an open position. It was also found that this model of Mack tractor will automatically shut down after 15 minutes of idling. It appears that the Employee #1 was working in the discharge chute when the tractor shut down. This allowed the clamshell doors on the discharge chute to close and trap him. It could not be determined if the tractor automatically shut down or the engine simply stalled. Neither the tractor nor the trailer exhibited any mechanical problems during the investigation.