Lulutai Airlines Saab 340B Runway Excursion Report Released
Tonga's Civil Aviation Office has released its report into a December 8, 2023, runway excursion incident at Fua'amotu International Airport (TBU), involving a Lulutai Airlines Saab 340B aircraft operating a flight from Fua'amotu to Lupepau'u Airport (VAV) in the Vava'u Islands.
During the flight, which had 35 passengers and three crew onboard, the two pilots identified an issue with the aircraft's main hydraulic system not indicating pressure to some systems, and no indication of hydraulic fluid in the tank, while on descent to Lupepau'u.
The pilots decided to return to Fua'amotu. They were able to lower the aircraft's landing gear with the auxiliary hydraulic system and landed safely.
However, the aircraft lost brake pressure during the taxi to the domestic terminal, exiting the apron and colliding with a disused refuelling bund, collapsing the right landing gear leg.
Whilst no one was hurt, the aircraft has yet to return to service, and it severely disrupted the day-to-day operations of the already operationally challenged government-owned airline.
Investigation focuses on tripped circuit breaker
During the subsequent investigation, it was discovered 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.
But it could not be determined when the circuit breaker tripped on the flight. In any case, this was not detected by the first officer while performing the initial hydraulic system troubleshooting. The report suggests this was likely due to an expectancy error.
The flight crew subsequently incorrectly identified 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 was operating normally until that time). Consequently, no hydraulic pressure was automatically provided to the hydraulic system.
The investigators determined that the crew did review sections of the hydraulic loss abnormal checklist during the emergency. However, they did not read all parts of the checklist. Furthermore, the flight crew did not understand the complexity of the Saab's hydraulic system.
"As a result, the flight crew had an incomplete appreciation of the limitations of the inoperative hydraulics system," the report noted.
Captain loses control of Saab 340B while taxiing
Having decided to head back to Fua'amotu, the flight crew decided to stop on the runway after landing in accordance with the abnormal checklist. The captain and first officer discussed the possibility of taxiing.
Having safely landed, with brake pressure still remaining and now towing tug in sight, the captain decided to continue taxiing to the terminal.
But when close to the terminal, the Saab lost wheel braking and directional control due to the hydraulic pressure loss, veering off the runway and colliding with the bund.
A bund is a fuel spill containment area used around and under fuel storage tanks to contain unanticipated fuel leaks.
The incident report concluded that using brakes to slow the aircraft during the taxi after landing, rather than reverse thrust, likely depleted the remaining hydraulic pressure in the brakes accumulators quickly, resulting in the loss of braking action.
During the investigation, it was discovered that the flight data recorder, while being supplied with power, was not recording flight data. The investigation also discovered that the underwater locator beacon and its mounting bracket had been removed at least five months before the accident.
The then 27-year-old aircraft is registered as A3-PUA (msn 408). Open source data from www.planespotters.net indicates Lulutai Airlines inherited it from defunct Real Tonga Airlines. However, it started its life flying for Mesaba Airlines in the United States, which then operated commuter feeder flights for larger US carriers.
There was no suggestion in the report that poor maintenance was a contributing factor. However, it did note ongoing troubleshooting activities for the avionics systems, including high-frequency radio receiving and transmitting issues and intermittent issues with hydraulic accumulators not holding pressure, and units were either recharged with nitrogen or replaced.
Contributing factors
Aside from depleting the hydraulic pressure during the landing, the investigation identified several other contributing factors to the incident, including:
The hydraulic indicators circuit breaker tripped. Investigators could not determine why, but it removed power to the cockpit indicators;
The first officer did not visually identify the tripped circuit breaker associated with the hydraulic system indicators while conducting the initial hydraulic system troubleshooting. The investigators say this was likely due to expectancy;
The pilots incorrectly determined the problem was a hydraulic leak, commencing the abnormal checklist for hydraulic fluid loss, which included turning off the hydraulic pump. As a result, no hydraulic pressure was automatically provided to the hydraulic system;
The pilots reviewed sections of the hydraulic loss abnormal checklist during the emergency. However, they did not read all parts of the checklist; and
After landing, when the captain realised that they had positive control of the aircraft on the ground and without a readily available aircraft tug, he continued to taxi to the terminal.
The report also said that because the flight data recorder was not recording, it limited the availability of "valuable evidence" to help the investigation. The investigation worked out how this happened (an engineer earlier troubleshooted an issue with the aircraft's communication system and inadvertently tripped a circuit breaker but did not record this happening, so it wasn't fixed).
The investigation also found that the underwater locator beacon and associated mounting bracket normally attached to the flight recorder had not been so for at least six months.
"This reduced the likelihood of locating the cockpit voice recorder in the event of an accident at sea," the report noted.
Lulutai Airlines puts corrective actions in place
In response to the findings, Lulutai Airlines has identified several corrective actions, including reviewing maintenance practices and integrating lessons learnt from the accident into their procedures.
On November 11, 2024, they provided the Civil Aviation Office with a remedial action plan. This was updated in January 2025.
"They considered their operational and maintenance procedures to be adequate," the report said. "However, they were planning to imbed lessons learnt into procedures and expanding maintenance procedures, including the CVR microphones and underwater locator beacon testing. They also intend to strengthen and foster a culture of continuous safety improvement."
In addition to the out-of-action Saab 340B, Lulutai's operational fleet includes a 19-passenger DHC-3-600 Twin Otter, a 12-passenger Harbin Y12, and a 34-passenger Saab340B damp-leased from New Zealand's Air Chathams.
Lulutai Airlines operates scheduled passenger flights from Fua'amotu to Eua (EUA), Ha'apai (HPA), and Lupepau'u. It is Tonga's only scheduled domestic passenger airline.
James Panuve from Tonga's Civil Aviation Office led the investigation, with assistance from the Australian Transport Safety Bureau.
You can read the full report here.
Photos: Tonga Civil Aviation Office