Date: 201306
Local time of day: 1801-2400
Location: ZZZ.Airport, US
Aircraft: A300
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Cruise
Airspace: Class A ZZZ
ATC / advisory: Center ZZZ
Aircraft component: Hydraulic Syst Pressure/Temp Indication
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Situational Awareness
Second reporter function: Pilot Not Flying; First Officer
Anomaly: Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural Maintenance; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Automation Aircraft Other Automation; Person Flight Crew
When detected: In-flight
Result: Flight Crew Diverted; Flight Crew Landed in Emergency Condition; General Declared Emergency; General Maintenance Action
Contributing factors: Aircraft; Human Factors; Procedure
Primary problem: Ambiguous

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.


Second narrative: During preflight the Yellow Hydraulic Reservoir OVHT light came on with ECAM message which was a repeat write up. Maintenance used MEL 29-33-02 and light and ECAM were extinguished. During climbout the Yellow Reservoir OVHT light came on with ECAM warning. Since MEL referred to inoperative annunciator and the annunciator was operating with ECAM warning we complied with ECAM and QRH and continued. During cruise while notifying Maintenance Control the Captain noticed the Green Hydraulic Reservoir was in the yellow arc and approaching the red dot. We then received the Green Hydraulic Reservoir low warning and complied with ECAM and QRH. This caused the autopilots to be inoperative and advised ATC 'unable RVSM' due to autopilot and was given a descent to FL280. During descent the Yellow Reservoir OVHT extinguished with normal ECAM and gauge readings so we recovered the Yellow System without any further indications. We recovered the Number 2 autopilot and advised ATC and were given FL310. We advised the company of our situation and were diverted to ZZZ. We declared an emergency with Approach Control and coordinated an ILS approach and our need to be towed to the gate. We manually extended the landing gear and completed all checklists. Tower could not get the ILS to operate but we were already established on a visual approach on the PAPI. We landed without incident and shut down on taxiway. We were inspected and towed to the gate. Maintenance Control stated that MEL 29-33-02 was not to be used for this type of malfunction.


Synopsis: 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.