Created Friday, Dec 8th 2023 08:57Z, last updated Tuesday, Sep 2nd 2025 15:25Z
A Lulutai Saab 340B, registration A3-PUA performing flight L8-14 from Nuku'alofa to Vava'u (Tonga) with 35 people on board, was on approach to Vava'u when the crew decided to return to Nuku'alofa due to the failure of a hydraulic system and problems with the landing gear. On landing back on Nuku'alofa's Fua'amoto Airport's runway 29 at about 14:00L (01:00Z) the aircraft collided with a concrete structure off the runway causing the collapse of the right hand landing gear. There were no injuries, the aircraft sustained substantial damage.
Passengers reported while on landing approach to Vava'u the aircraft suddenly changed direction and turned away from Vava'u again, the captain subsequently announced there had been a technical problem and they were returning to Nuku'alofa. Cabin crew told the passengers there was a hydraulic problem. When the aircraft landed back in Nuku'alofa there was a massive boom and the aircraft went sideways, seats broke, then the aircraft suddenly stopped as if it had hit something. The aircraft was evacuated.
In a press release the airline reported the aircraft had safely landed back and was taxiing to the terminal when upon turning the aircraft experienced a technical issue, turned and hit a cement block on the side of the apron with the right hand wing becoming lodged on the cement block.
On Jan 24th 2024 the Swedish Haverikommission reported the aircraft experienced hydraulic problems on approach and worked the related checklists. After an eventless landing at Nuku'alofa the aircraft vacated the runway and was taxiing towards the apron when the aircraft lost its braking capability and collided with a concrete foundation at low speed. There were no injuries, the aircraft sustained substantial damage. The occurrence was rated an accident, the Swedish Accident Investigation nominated an accredited representative to join the investigation.
Tonga's Civil Aviation Office (TCAO) released their final report via the Swedish SHK concluding the probable causes of the accident were:
Contributing factors
- While taxiing to the terminal after landing, the aircraft lost wheel braking and directional control, resulting in the aircraft veering off the apron, impacting a disused refuelling installation bund.
- For reasons that could not be determined, between engines' start and descent, the circuit breaker providing power to the main hydraulic system pressure and quantity indicators, and inboard brakes hydraulic system pressure indicator, had tripped, removing power to those indicators.
- Likely due to expectancy, the first officer did not visually identify the tripped circuit breaker associated with the hydraulic system indicators while performing the initial hydraulic system troubleshooting.
- The crew misidentified that there was a hydraulic leak, and commenced 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.
- 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. As a result, the flight crew had an incomplete appreciation of the limitations of the inoperative hydraulics system.
- During the return to Fua'amotu, the flight crew made the decision to stop on the runway after landing in accordance with the abnormal checklist. During the approach, and after landing, the crew discussed the possibility of taxiing, and after the captain recognised that they had positive control of the aircraft on the ground and without a readily available aircraft tug, continued to taxi to the terminal.
Other factors that increased risk
- Although power was available to the flight data recorder, it did not record flight data for about 5 months prior to, and including the accident. This limited valuable evidence for this investigation to enhance flight safety.
- A fault within the flight data acquisition unit resulted in the electrical system circuit breaker for the unit being tripped, removing power from the unit. This resulted in flight data not being sent to or recorded by the flight data recorder. The fault identified by the tripped circuit breaker, went unrectified as it was misdiagnosed as an issue with the aircraft's high frequency radio system.
- While troubleshooting an issue with the aircraft's communication system, a maintenance engineer inadvertently secured the tripped circuit breaker for the data acquisition unit in the tripped position. However, no maintenance action had been recorded, limiting the opportunity to recognise that the flight data recorder was not recording.
- Although the cockpit voice recorder was functioning correctly, the underwater locator beacon and associated mounting bracket had been removed from the cockpit voice recorder at least 5 months prior to the accident. This reduced the likelihood of locating the cockpit voice recorder in the event of an accident at sea, limiting valuable evidence for enhancing flight safety.
Other findings
- Two maintenance defects identified during the investigation were not recorded in the aircraft's technical logbook, or appropriately actioned when identified. The investigation considered that it may be a broader issue, however there was considerable evidence to indicate that defect rectification was regularly being conducted and recorded correctly.
- While the chief executive officer was seated on the flight deck in an observation seat, there was no evidence to indicate that they influenced the crew's decision making during the accident flight.
The TCAO analysed the hydraulic failure:
In normal operation, the braking system was reliant on the electric hydraulic pump providing pressure to the inboard and outboard brake accumulators. As the hydraulic pump was turned off as part of the hydraulic fluid loss abnormal checklist, the only way to provide pressure to both wheel brake accumulators was by manually hand pumping the auxiliary hydraulic system, when individually selected to each brake system. The auxiliary system was utilised during the flight for completing the lowering of the landing gear, however was not used at any stage to maintain pressure in either brake accumulator, or just the outboard brakes accumulator for which they had an indication.
The captain confirmed that brakes were working after landing and continued to taxi to the domestic apron using residual brake pressure stored by the inboard and outboard brake accumulators. Both crewmembers confirmed that they didn’t check the hydraulic pressure indication for the outboard brakes during taxi, therefore not identifying the depleting pressure. The abnormal checklist noted that a large number of pumps, and continuous pumping of the auxiliary pump would have been required to maintain normal brake pressure. It could not be determined that if during the taxi, had the crew identified the depleting brake pressure, that sufficient hydraulic pressure could have been restored through use of the auxiliary hydraulic pump to avert a complete loss of braking.
Using the 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. With limited effective steering control from the rudder at low speed and no wheel brakes available, directional control was lost and the aircraft exited the taxiway, impacting the disused refuelling installation bund.
Hydraulic indicators circuit breaker tripped
During the descent, the crew discovered that the indicators for the main hydraulic fluid quantity, main accumulator pressure, and the accumulator pressure for the inboard brakes, were not displaying. The aircraft had previously completed 2 flights on the morning of the accident, with no hydraulic indication issues identified by the flight crew.
Had a hydraulic system indication issue been present, it is likely that this would have been identified during the ‘before engine start’ and ‘landing’ normal checklists. It is also likely that a tripped circuit breaker would have been identified and rectified as a result of the ‘before engine start’ checklist on the previous flights and in preparation for the accident flight.
Verification of the main hydraulic fluid quantity, main accumulator pressure, and the operation of the inboard brakes accumulator indicators was confirmed during the postaccident aircraft systems examination by resetting the ‘HYDRAULIC – PR IND / QTY IND’ circuit breaker, bringing the indicators back on-line when power was applied.
Therefore, the circuit breaker must have tripped during the flight, resulting in power not being applied to the indicators. However, extended power application to the electrical system during testing did not trip the circuit breaker. At the conclusion of the investigation examination, the reason why the ‘HYDRAULIC – PR IND / QTY IND’ circuit breaker tripped was unable to be determined.
The aircraft seen in its final position:
Map (Graphics: AVH/Google Earth):
