Shuttle On Approach

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There was one small glitch in the pre-landing check out. One of three Auxiliary Power Units (APUs) in Discovery's rear compartment has a small leak - about six drops per hour. The APUs power the hydraulic pump, which controls some of the systems needed for landing, such as the rudder/speed brake and the elevon. Flight controllers are not sure whether the leak is hydrazine fuel or the nitrogen that is used to pressurise the fuel tank. A hydrazine leak would be more serious than a nitrogen leak. Before the de-orbit engine firing, Discovery turned on that APU and it appeared to be working fine.