Pork loins do not need to be covered while they are roasted or broiled in the oven. However, they should be covered during braising, before the meat turns brown when the meat is pre-browned and then cooked slowly in the oven in a small amount of liquid.
The typical method for cooking a pork loin is roasting. This involves placing the pork in a pan without additional liquid; the pan is then put in the oven uncovered, allowing the pork to develop a crisp surface.
However, if the pork loin is cooked via braising, it should be covered after the meat is browned on all sides.