History of the Flight
On 8 December 2023, a Lulutai Airlines Limited SAAB 340B, registered A3-PUA, was conducting a scheduled regional passenger service from Fua’amotu International Airport, Tongatapu, to Lupepau’u Airport, Vava’u, and return. The outbound flights were uneventful, and the aircraft landed back at Fua’amotu at about 1130 Tonga Standard Time. After arrival, the same flight crew prepared the aircraft for another scheduled domestic air service to Lupepau’u, operating as Tonga 14. On board were 3 crew—a captain, first officer, and flight attendant—and 35 passengers. The company’s chief executive officer (CEO) requested and was permitted to travel in the observation seat on the flight deck to observe the operation. The flight crew conducted appropriate preparatory checklists and departed Fua’amotu at about 1226 with the first officer as pilot flying.
In-Flight Hydraulic Issue
At about 1306, approximately 35 nautical miles from Lupepau’u Airport, the flight crew commenced descent from 11,000 ft. While verifying the main hydraulic system quantity as part of the landing checklist, the first officer identified that the main hydraulic system quantity was indicating empty. The crew commenced troubleshooting and selected the hydraulic pump switch to override, but there was no change in the instrument indication. They subsequently misidentified the indication issue as a hydraulic leak and commenced the abnormal checklist for hydraulic fluid loss, which included turning off the hydraulic pump. The hydraulic system had been operating normally until that time. The crew reviewed sections of the hydraulic loss abnormal checklist during the emergency but did not read all parts of the checklist. The flight crew had only a basic understanding of the aircraft's hydraulic system, leading to an incomplete appreciation of the limitations of the inoperative hydraulics system.
Return and Landing
The crew elected to return to Fua’amotu and were able to lower the aircraft’s landing gear with the auxiliary hydraulic system. They subsequently performed a successful landing. After landing, the crew discussed the possibility of taxiing to the terminal. The captain recognized that brake pressure remained after landing, and without a readily available aircraft tug, continued to taxi to the terminal.
Taxi and Ground Collision
During taxi, the aircraft lost brake pressure and directional control due to the depletion of hydraulic pressure. The aircraft veered off the apron, exiting the taxiway, and impacted a disused refuelling bund, collapsing the right landing gear leg. The 3 crew and 35 passengers were able to evacuate the aircraft without injury.
Investigation Findings
The investigation found that a tripped circuit breaker had removed power from the hydraulic quantity indicator, as well as the main hydraulic accumulator pressure and inboard wheel brakes accumulator pressure indicators. It could not be determined when the circuit breaker tripped during the flight; however, this was not detected by the first officer while performing initial hydraulic system troubleshooting, most likely due to an expectancy error. The flight crew subsequently misidentified the indication issue as a hydraulic leak and commenced the abnormal checklist for hydraulic fluid loss, which included turning off the hydraulic pump. The crew did not read all parts of the checklist and had only a basic understanding of the hydraulic system, resulting in an incomplete appreciation of the limitations of the inoperative hydraulics system. During the return, the crew made the decision to stop on the runway after landing in accordance with the abnormal checklist. After landing, the captain recognized remaining brake pressure and, without an available tug, continued taxiing until hydraulic pressure depletion caused loss of braking and directional control, leading to the excursion and collision.
Other Issues Found
The investigation also identified that the flight data recorder was not recording flight data due to a flight data acquisition unit having an internal fault that caused its circuit breaker to trip. This fault had been misdiagnosed as an issue with the aircraft's high frequency radio system and was isolated with a locking collar to prevent resetting, likely 8 days before the accident. Examination of the cockpit voice recorder revealed that the underwater locator beacon and its mounting bracket had been removed at least 5 months before the accident. Additionally, the missing underwater locator beacon and the collared data acquisition unit circuit breaker were not recorded in the aircraft’s technical log or appropriately actioned when identified. The investigation noted considerable evidence that defect rectification was regularly conducted and recorded correctly. While the CEO was seated on the flight deck in an observation seat, there was no evidence that they influenced the crew's decision making during the accident flight.