By Katalina Siasau
A tripped circuit breaker was highlighted as a primary issue in the Lulutai Airline’s Saab 340B accident, which led to a loss of power to the hydraulic quantity indicator, the main hydraulic accumulator pressure, and the inboard wheel brakes accumulator pressure indicators.
The final report was released on 26 June, following a media briefing with the Chief Investigator, James Panuve and Minister for Civil Aviation, Deputy PM Hon. Dr Taniela Fusimalohi.
The report investigated the accident on the runway apron on 8 December 2022. Upon landing, the aircraft lost brake pressure while taxiing to the domestic terminal. It exited the taxiway, struck a disused refuelling bund, and the right landing gear collapsed. All three crew members and 35 passengers evacuated without injury.
The key findings of the investigation includes:
- A tripped circuit breaker cut power to the hydraulic quantity, main hydraulic accumulator pressure, and inboard wheel brakes accumulator pressure indicators.
- The flight data recorder had power but was not recording data.
- The underwater locator beacon and its mounting bracket had been removed five months before the accident.
- Two maintenance defects were not recorded in the aircraft's technical logbook, or appropriately actioned when identified: the missing underwater locator beacon and the collared data acquisition unit circuit breaker.
(A tripped circuit breaker is a safety mechanism in an electrical system that automatically shuts off the flow of electricity to a circuit when it detects an overload or short circuit. This prevents potential hazards like overheating wires, electrical fires, or damage to equipment.)
Right landing gear collapsed
On December 8, 2023, a Lulutai Airlines SAAB 340B (A3-PUA) experienced a hydraulic system issue while on descent into Lupepau'u Airport, Vava'u, during a scheduled passenger service from Fua’amotu International Airport.
The flight crew noted no pressure to some main hydraulic systems and no fluid in the tank. They decided to return to Fua’amotu, where a longer runway and maintenance facilities were available. The landing gear was successfully lowered using the auxiliary hydraulic system. Upon landing, the aircraft lost brake pressure while taxiing to the domestic terminal. It exited the taxiway, struck a disused refuelling bund, and the right landing gear collapsed.
Tripped circuit breaker
The report stated that the circuit breaker powering the main hydraulic system pressure and quantity indicators, as well as the inboard brakes hydraulic system pressure indicator, tripped at some point between engine start and descent, which resulted in a loss of power to these indicators.
During the descent, the crew noted that the indicators for main hydraulic fluid quantity, main accumulator pressure, and the inboard brakes accumulator pressure were not displaying. The post-accident examination of the aircraft systems confirmed this.
Resetting the 'HYDRAULIC – PR IND / QTY IND' circuit breaker restored power and brought the indicators back online, which confirms that the circuit breaker must have tripped during the flight, causing the indicators to lose power. However, investigators noted that extended power application to the electrical system during subsequent testing did not cause the circuit breaker to trip again.
Ultimately, the investigation was unable to determine why the 'HYDRAULIC – PR IND / QTY IND' circuit breaker tripped.
It also noted that the first officer did not visually identify the tripped circuit breaker associated with the hydraulic system indicators during initial troubleshooting, likely due to expectancy.
Panuve further explained saying, “The pilot's daily view of the circuit breakers in the cockpit would be considered normal. His assumption, therefore, is that everything will consistently appear normal, leading him to perceive it as such.”
Misidentified hydraulic leak
Investigators found that the crew incorrectly identified a hydraulic leak, leading them to initiate the abnormal checklist for hydraulic fluid loss, which included shutting off the hydraulic pump. As a result, no hydraulic pressure was automatically provided to the hydraulic system.
(Figure 2) (Figure 3)
The report, highlights several key issues which includes incomplete checklist review, troubleshooting focus, informal checklist handling, and lack of system knowledge.
“While the crew reviewed sections of the hydraulic loss abnormal checklist during the emergency, they did not read all parts of the checklist.
"Further, the flight crew only had a basic understanding of the aircraft's hydraulic system."
While the captain correctly observed that the hydraulic light was not illuminated and tested the warning panel lights, the first officer was convinced of a complete loss of hydraulic fluid.
"However, the first officer became overly focussed on the gauge readings, convinced they had lost all hydraulic fluid.”
Meanwhile, the flight crew's approach to the abnormal checklist "was quite informal and frequently interrupted by discussions" rather than being a comprehensive "read and do" procedure.
"The crew also did not extend the landing gear in conjunction with the hydraulic fluid loss checklist, although they did successfully extend it.”
Lack of system knowledge
Despite ground school training for the SAAB 340B, the flight crew could not describe the aircraft's hydraulic system in detail during the investigation.
"Also, they were not able to recall the functionality of the auxiliary system apart from its ability to lower the landing gear, and what additional services it was able to provide."
The report concludes that a more thorough understanding of the hydraulic system and a complete review of the abnormal checklist would have provided the crew with a better understanding of the auxiliary hydraulic system's limitations, including the risk of losing wheel braking action.
Flight recorded data loss
The investigators after successfully downloading the flight data recorder (FDR) found that it had not recorded any data for about five months prior to the incident.
“A successful download of the FDR fitted to this aircraft would have provided multiple flight data parameters for multiple flights, however it had not been recording flight data for about 5 months."
This was due to a fault in the flight data acquisition unit (FDAU) which caused the circuit breaker to trip, preventing flight parameter data from being sent to the FDR.
"Therefore any evidence that may have assisted further with this investigation for future safety enhancement was lost.”
Investigators also found that this tripped circuit breaker was misdiagnosed as an issue with the aircraft's high-frequency radio system, leaving the fault unrectified.
Underwater locator beacon removed
While the cockpit voice recorder (CVR) was functioning correctly, its associated Underwater Locator Beacon (ULB) and mounting bracket had been removed at least six months before the accident, the investigation found.
The absence of the ULB significantly hinders the ability to locate and recover aircraft wreckage and recorders.
“Without these devices fitted to either the CVR and/or FDR, there is limited ability for the wreckage and recorders to be located and recovered. As the majority of the flying conducted with A3-PUA was over water, this enhanced the importance of the ULB’s installation and proper function.”
Furthermore, the report stated that without the ULB fitment, post-accident location and recovery in the event of an accident at sea would be impaired as well as precluding the ability to extract vital data from the CVR, losing the potential to identify safety issues.
Two maintenance defects
The investigation also identified two maintenance defects that were not recorded in the aircraft's technical log or Minimum Equipment List (MEL): the missing ULB and the tripped "DATA AQUIS PWR" circuit breaker due to the flight data acquisition unit (FDAU) fault. The report noted that while these undocumented defects were discovered during the investigation, there was no evidence to suggest this was a regular occurrence, as other defects were regularly identified and rectified.
The report noted that while these undocumented defects were discovered during the investigation, there was no evidence to suggest this was a regular occurrence, as other defects were regularly identified and rectified.
The investigators stated that Lulutai Airlines has identified several key observations and potential corrective actions that could be drawn from the available information provided in the preliminary report for this investigation. These included reviewing maintenance practices around hydraulic and CVR systems, and integrating lessons learnt from the accident into their procedures.
“Short way to summarise was there was no recommendation from the investigation to the Airline, because the Airline has provided remedial action to counter the issues that were raised here," said Panuve.
Saab 340B
The SAAB 340B is a low-wing, pressurised regional commuter aircraft, fitted with 2 General Electric (GE) CT7-9B turboprop engines.
A3-PUA, serial number 408, was manufactured in Sweden in 1996 and first registered in Tonga in April 2016. It was first registered with Lulutai Airlines Limited in September 2020.
At the time of the occurrence, the airframe had accumulated 39,094.5 hours total time in service.
The preliminary and final investigations were supported by financial assistance from the Australian Government represented by the Department of Foreign Affairs & Trade of Australia (DFAT).
Lulutai Airlines Ltd. is owned by the Tonga Government and the Retirement Fund Board. The Chairman of the airline's Board is the Prime Minister, Hon. Dr 'Aisake Eke.
