THE INJURED PLACED 2 NUTS ON 660 LBS D11N BELLY & HE DECIDED TO BACK OFF 1 NUT, THEN ONLY 1 NUT REMAINED SECURING THE BELLY PAN. HE YELLED TO HIS HELPER WHO WAS INSIDE THE CAB TO LOOSEN THE HO IST LINE & REMOVE THE WEB SLING. HE INTENDED TO BACK OFF THE 2ND NUT WITH IMPACT WRENCH BUT IT CAME COMPLETELY OFF. THE BELLY PAN HAD 2 HINGES. IT SWUNG DOWN & LANCED BY HIS LEFT THIGH.