What happened
On July 20, 1997, a Boeing 737-50 and registration EI-CDT, operating a scheduled flight from Dublin to Copenhagen, experienced a series of significant electrical malfunctions during its approach to runway 22L. While completing the landing checklist, the crew attempted to arm the speed brakes, which triggered a sequence of instrument failures. The landing gear indicator lights went dark, and the crew found the gear selector stuck in the down position.
As the flight continued, the crew noted the loss of several critical indications, including N1, fuel flow, and the standby horizon, alongside various system warnings. During a go-around, the crew attempted to troubleshoot the electrical issues by disconnecting generators, which briefly restored normal indications. However, after landing successfully, the electrical faults recurred while the aircraft was taxiing. Shortly thereafter, smoke was detected in both the cockpit and the cabin. The pilot subsequently ordered an emergency evacuation on a taxiway, and all 66 persons on board exited via the aircraft slides without injury.
The investigation
The investigation focused on the electrical system, specifically the Battery Bus and its power sources. Investigators determined that the Battery Bus Relay (R1) had failed, preventing the Battery Bus from receiving current from either the battery charger or Transformer Rectifier 3. This loss of power affected 56 essential systems, leading to the observed instrument failures.
Physical examination of the R1 relay revealed that internal arcing had caused the contacts to weld, generating enough heat to melt and bend a contact blade at a 90-degree angle. The investigation also found that the smoke in the cabin was a secondary effect: the loss of the battery bus caused the pack cooling turbofans to stop, leading to the overheating of the Air Cycle Machine (ACM). This overheating caused oil and dirt within the ACM and adjacent ducts to produce smoke.
Findings
- The primary cause of the electrical failure was a faulty R1 relay that failed to supply the Battery Bus with current.
- The relay failure was caused by electrical arcing and welding of the contacts, which resulted in extreme heat and physical damage to the component.
- The smoke in the cabin was caused by the overheating of the Air Cycle Machine due to the failure of the cooling turbofans.
- The flight crew lacked the necessary information to identify the specific nature of the failure, as the aircraft's manuals and checklists did not include procedures for a Battery Bus failure or the specific symptoms observed.
- The crew was unaware that switching the standby power to the battery position could restore the necessary indications.
Safety action
Following the investigation, the airline issued new flight crew instructions. Additionally, the investigation led to recommendations for the Civil Aviation Administration to reevaluate the performance of the R1 relay in Boeing 737 series aircraft and to ensure crews have access to procedures for restoring power during such failures. In response, Boeing issued a service letter recommending a preferred replacement relay with improved in-rush current tolerance and a technical bulletin providing guidance on identifying and managing Battery Bus failures.