What happened
On December 19, 2011, a Boeing 737-400 was performing a landing at Krakow Airport (EPKT). During the final approach, the flight crew observed several simultaneous system warnings. These included a low-pressure alert on the electric hydraulic pump for system A, loss of hydraulic fluid quantity indications for system B, and loss of pressure indications for both systems A and B. Additionally, the crew noted "ANTI SKID INOP" and "AUTOBREAKE DISARM" alerts, as well as low-pressure indications on the left rear and right front fuel pumps.
The commander elected to continue the approach, as the fuel pump warnings suggested a potential for engine power fluctuations during a go-around. Following touchdown, all warnings ceased and instrument indications returned to normal. However, upon taxiing to the stand, it was discovered that the left main landing gear tire had experienced significant wear/rubbing, necessitating a wheel replacement. The crew reported no unusual aircraft handling during the landing or braking phase.
The investigation
The investigation examined a series of electrical issues involving the aircraft's DC power supply that had occurred on several dates between November 2011 and January 2012. Previous attempts to rectify the issue included replacing a fuse in Transformer Rectifier Unit 1 and replacing the R9 TR3 Disconnect Relay.
Technical analysis of the aircraft's maintenance records and component testing revealed that the R3 Transfer Relay-1 was defective, with internal testing confirming that contacts B1 and B2 were non-functional. The investigation established that when the aircraft intercepts the Glide Slope, the R9 TR3 Disconnect Relay operates, disconnecting DC Bus 1 from DC Bus 2. This specific electrical transition revealed the underlying fault in the R3 Transfer Relay-1, which in turn triggered the erroneous warnings in the hydraulic and fuel systems.
Findings
- The primary cause of the various system warnings was a faulty R3 Transfer Relay-1.
- The electrical malfunction caused transient loss of indications in the hydraulic and fuel systems during specific flight phases.
- The tire wear on the left main gear was a result of the landing conditions but was not caused by a failure in the anti-skid system itself.
- Previous maintenance actions, such as replacing the TRU 1 fuse, had failed to resolve the underlying issue because the root cause remained unaddressed.