CRJ200 Captain discovers a 2;600 LB error in weight and balance calculations performed by Central Loading Planning after the flight is airborne. An incorrect passenger weight of two pounds was found to have been entered for Section C at some point in the load planning computer.

Narrative

After initially inputting data for weight and balance via ACARS; we received instructions to add 700 LBS of ballast. We re-submitted passenger information utilizing the children on board which were located in section A-2. The takeoff data that we received still required 300 LBS of ballast to be loaded. After instructing the ground crew to load the ballast; we submitted the new information and received takeoff data. I mentioned that it still seemed odd that we would need ballast since the passengers were evenly distributed with sufficient bags in the cargo area. Not having any reason to question the accuracy of the load planning system; we continued with the flight. After take-off; which occurred normally; I still felt that it was odd that ballast was needed; so I calculated the weight and balance manually. I determined that the actual weight of the aircraft as it was loaded; was 2;600 LBS greater than what Load Planning had provided. The takeoff data provided by the load planning system was incorrect for an unknown reason at the time. I sent an ACARS message to the controlling Dispatcher that the error had occurred and asked him to investigate it immediately. When he did not respond after approximately 10 minutes; I sent another message asking if he had any explanation. He responded simply; 'nope.' I sent another message asking if the Coordinator had called Load Planning; because if it was a system wide problem; it presented a serious safety issue. He responded back that our flight was the only one that was experiencing any problems. The landing data provided by Load Planning had the same 2;600 LBS error; so we calculated the landing weight manually and utilized the correct speeds from the speed book. The landing was normal. I contacted Mr. X after landing and he said that he would investigate what happened. He contacted me the next day; and he related that he went through the information with Load Planning; and it was determined that at some point during the day; 2 LBS had been entered for passengers weight in section C. The entry did not drop out after the flight as it should have. This entry was undetected in the normal course of operations since the passenger numbers entered in section C on each subsequent flight appeared normally on the passenger loading page. During the course of the review with load planning; it was determined that the CG was within limits; but the V speeds were incorrect for the actual weight of the aircraft.Given the dynamic; fast paced environment that we operate in; we rely heavily on the accuracy of the information provided by Load Planning. The expectation is that all data entered from previous flights will drop out so that when new data is entered; it relates solely to the present flight. The Load Planning Vendor should be tasked to correct the software error so that discrepancies like this do not occur again. In light of this event; if the takeoff data just doesn't seem correct; manual verification should be performed.