Since Columbia was a test vehicle it was equipped with the Orbiter EXperiment (OEX) recorder a/k/a MADS (Modular Auxiliary Data System). A variety of sensors throughout Columbia's structure monitored temperature and pressure during launch and reentry. Based on 1970s technology, it stored data on a 10,000-foot magnetic tape. It has the capacity to store up to two hours of data with 28 channels. OEX's data was used to refine Columbia’s flight characteristics and make improvements to the later shuttles.
OEX was found near Hemphill, Texas, on March 20. It was in good shape, just sitting on the ground partially buried. The box was taken to NASA, where its cover was removed, and the inside appeared even better. CAIB member Scott Hubbard said, "This could be a gold mine of information. If it was a normal mission it would have 30 minutes of data from ascent and an hour from entry." However, if the OEX tape was heated too high, its data would be erased.
Amazingly the OEX box was in excellent condition and its magnetic tape inside was unharmed. The OEX data showed unexpected heating in the leading edge of the left wing during Columbia’s launch, confirmation the damage occurred shortly after launch.
There was a pair of OEX sensors on Columbia's left wing behind RCC panel #9, close to the location where the bipod foam hit. The sensors showed a temperature spike eight minutes after Entry Interface, and then the sensor went dead. It was clear evidence Columbia had been damaged before reentry and when the reentry heat destroyed the sensor or its wiring. Data from the other OEX sensors also helped verify what happened to Columbia.
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