A US Airways Boeing 757 lost a left wing cover panel on a flight from Orlando, FL, to Philadelphia, PA, last week. As the composite panel departed, it struck the fuselage, but didn't do any real damage and the plane landed safely in Philly.  From the picture taken by a passenger, it looks like the panel was located well inboard (close to the fuselage), and farther back, on the wing. I would speculate that at this location the panel is not a main load-bearing part of the wing, and is there instead to cover internal wing structure to reduce drag. Also, that section of the wing is not really on the main part of the wing that is producing aerodynamic lift. So other than increased drag caused by the missing panel, I don't believe that controllability or structural integrity of the 757 was compromised. In fact the pilot continued on to the original destination in Philly after the panel departed over Maryland. Of more concern would be the potential for further damage caused by panel as it flew off. Besides the obvious unsettling sound that it must have made as it struck the cabin, as a pilot I would be most worried if it hit and damaged the horizontal stabilizer on the 757. If that had occurred, then you might have some real control problems, since the elevator controls the pitch of the aircraft. Fortunately on the 757 the horizontal stabilizer is well aft of the trailing edge of the wing, where the wayward panel was originally located. After striking the fuselage, it probably continued its plummet to earth and passed well beneath the stabilizer.  All in all, better to have a relatively small wing panel fall off than an entire engine, as I recall happened on a USAir 737 leaving Philly back in 1987.

I remember attending a safety meeting once for the Rockwell OV-10 Bronco flown by the U.S. Marines (no longer in service) where one of the topics covered was Things that Fall Off Aircraft (TFOA - it wouldn't be a government program without an acronym). I didn't realize that stuff fell off airplanes that often, let alone frequently enough to have a whole area of study devoted to it. Although still very rare in commercial aviation, apparently things falling off military aircraft is a little more common. Probably because military aircraft have more things hanging off the underside that can fall off, such as external fuel tanks, rockets, and of course, bombs. The trick is to have them depart only when you want them too, not inadvertently, which can be embarrassing.