Incident Overview
On 19 December 2011, a Boeing 737-400 was on final approach to Katowice Airport (EPKT) when multiple system warnings appeared. These included LOW PRESSURE indications on the hydraulic electric pump of system A, loss of hydraulic fluid quantity indications for system B, loss of pressure indications for systems A and B, and the activation of ANTI SKID INOP and AUTOBRAKE DISARM warnings. Additionally, LOW PRESSURE indications appeared on the left aft and right forward fuel pumps. The captain decided to continue the approach, as low fuel pressure could have caused engine disruptions during power increase. After landing, all warnings disappeared and normal instrument readings returned. Upon taxiing to the parking stand, scuffing was found on the tires of the left main landing gear strut, necessitating wheel replacement. The crew did not notice any abnormal aircraft behavior during touchdown or braking. Post-flight system tests by maintenance personnel were successful, and no aircraft faults were identified. The damaged wheels were replaced, and the aircraft was returned to service.
Supplementary Information
Additional information from the operator, dated 23 July 2013, indicated a similar event on 28 December 2011 during a flight from Warsaw to Madrid to Warsaw. In that case, circuit breaker C308 for Transformer Rectifier Unit 1 was replaced. The breaker was later tested and found serviceable. On 30 December 2011, another similar event occurred on a London to Warsaw flight, where the Transformer Rectifier Unit 1 circuit breaker had tripped. In response, the R9 TR3 Disconnect Relay was replaced but not tested. On 10 January 2012, after a flight from Gdańsk to Warsaw, an automatic electrical system shutdown was noted, leading to replacement of the R3 Transfer Relay -1. Subsequent workshop tests on that relay revealed damage to contacts B1 and B2 (no continuity between contacts), rendering the relay unserviceable. No mechanical damage was found on its housing or terminals. Analysis of the Merlin database showed that after replacing the R3 Transfer Relay, the electrical system fault did not recur from 10 January 2012 to 19 March 2012. According to the B737 Operations Manual, capturing the glide slope triggers the R9 TR3 Disconnect Relay, disconnecting DC Bus 1 from DC Bus 2. This disconnection likely exposed the faulty Transfer Relay 1, causing the cascade of warnings seen in the described flights.
Investigation Findings
The investigation concluded that the multiple system warnings were caused by a malfunction of the electrical system, specifically a faulty R3 Transfer Relay -1. Workshop tests confirmed damage to its contacts. Earlier replacements of other electrical components (TRU 1, circuit breaker C308, R9 relay) had not resolved the issue, and the removed circuit breaker was found to be serviceable, further supporting the relay as the root cause.
Operator Actions
The operator did not issue preventive recommendations for flight crews but reminded maintenance technicians during periodic training to record Part Numbers and Serial Numbers of replaced components. The State Commission on Aircraft Accident Investigation did not issue any safety recommendations.