What happened
On October 15, 2020, an ATR 42-500, registration HK5310, was performing a scheduled passenger flight from Cúcuta to Palonegro Airport in Lebrija, Colombia. After landing, the crew taxied toward parking position number 5. Following company standard operating procedures, the crew performed an ATPCS test and feathered the number one engine to reduce noise while approaching the gate.
As the aircraft neared the assigned position, the pilot attempted to apply the brakes following signals from a ground marshal. However, the brakes failed to respond. The pilot then instructed the co-pilot to apply the brakes, but this attempt was also unsuccessful. Due to the lack of braking capability and the limited space available, the left wing of the aircraft struck the structure of the boarding bridge at position 5.
The crew immediately shut down both engines and followed emergency procedures to secure the aircraft. All 21 occupants (3 crew and 18 passengers) evacuated the aircraft safely and were uninjured. The collision caused significant damage to the aircraft's left wing leading edge, the number one propeller, and the engine structure, as well as damage to the boarding bridge's structure and movement system.
The investigation
The GRIAA investigation focused on the electrical and hydraulic systems following the loss of braking function. Investigators examined the Generator Control Unit (GCU) with serial number 1693. The investigation revealed that a malfunction in the AC generator (GEN ACW) caused voltage fluctuations. Specifically, the frequency protection in the GCU failed to activate at the correct threshold, leading to an improper voltage output.
This electrical instability prevented the necessary energization of the hydraulic system components. Consequently, the aircraft experienced a sudden loss of pressure in the green hydraulic system, which is responsible for normal braking operations. The investigation also noted that the nature of this specific electrical failure did not trigger any cockpit alarms to alert the crew.
Findings
- The primary cause of the incident was the loss of effective hydraulic pressure in the braking system, triggered by the abnormal operation of the GEN ACW and the GCU SN 1693.
- Maintenance personnel had failed to properly address and track previous reports of electrical failures involving the same component.
- There was an absence of specific procedures in the operator's and manufacturer's manuals to prevent or manage similar braking failures during taxiing, particularly in confined ramp areas.
- The manufacturer had not implemented sufficient verification controls despite repeated reports of electrical system failures in ATR 42-500 aircraft.