Narrative:

During preflight; captain noticed and wrote-up the yellow system hydraulic rsvr ovht light being on. This was a repeat write up and had been worked on and MEL'd [over the last four days]. In hindsight; the deferral input was wrong as the light did in fact work. MEL 29-33-02 just covers the light not operative. This particular issue involves more since the light actually did work. The problem was a malfunction that had been previously repaired with sensor and relay replacement. The crew was lulled into a false sense of completion when the mechanic presented the sign-off and about that time the light extinguished. The thinking by me; the captain; was the deferral required disabling what the mechanic called a malfunctioning light. What I failed to notice was the lack of maintenance action in the actual MEL 29-33-02. The light had simply extinguished on its own. As such; on climbout through approximately 25;000 ft; the light illuminated and ECAM procedures accomplished. First and foremost; resolving all doubt as to the actual malfunction; write-up and the associated MEL sign off. The ovht light did in fact extinguish later in the flight which was pivotal in preventing a multiple hydraulic system inoperative scenario since we were able to recover the yellow system in the wake of a green hydraulic system malfunction. With this green hydraulic low quantity issue; the crew elected to divert and declare an emergency due to loss of nose wheel steering and the potential for closing down a runway with the inability to steer. In the future; I; the captain; will ask more questions and scrutinize sign-offs to the point of ensuring the aircraft is ready to [fly] safely.

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Original NASA ASRS Text

Title: A300 flight crew reports being dispatched with an MEL for an inoperative Yellow Hydraulic System overheat light. Climbing through FL250 the light illuminates with associated ECAM warning and the Yellow System is shut down. During cruise the Green System quantity is noted to be decreasing and eventually triggers a low level light and ECAM warning. The Green System is shut down and the flight diverts. During descent the Yellow System overheat light extinguishes allowing that system to be used for approach and landing.

Narrative: During preflight; Captain noticed and wrote-up the Yellow System Hydraulic RSVR OVHT light being on. This was a repeat write up and had been worked on and MEL'd [over the last four days]. In hindsight; the deferral input was wrong as the light did in fact work. MEL 29-33-02 just covers the light not operative. This particular issue involves more since the light actually did work. The problem was a malfunction that had been previously repaired with sensor and relay replacement. The crew was lulled into a false sense of completion when the mechanic presented the sign-off and about that time the light extinguished. The thinking by me; the Captain; was the deferral required disabling what the mechanic called a malfunctioning light. What I failed to notice was the lack of Maintenance action in the actual MEL 29-33-02. The light had simply extinguished on its own. As such; on climbout through approximately 25;000 FT; the light illuminated and ECAM procedures accomplished. First and foremost; resolving all doubt as to the actual malfunction; write-up and the associated MEL sign off. The OVHT light did in fact extinguish later in the flight which was pivotal in preventing a multiple hydraulic system inoperative scenario since we were able to recover the Yellow System in the wake of a Green Hydraulic System malfunction. With this Green Hydraulic low quantity issue; the crew elected to divert and declare an emergency due to loss of nose wheel steering and the potential for closing down a runway with the inability to steer. In the future; I; The Captain; will ask more questions and scrutinize sign-offs to the point of ensuring the aircraft is ready to [fly] safely.

Data retrieved from NASA's ASRS site as of July 2013 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.