REX SF34 at Flinders Island on Nov 4th 2022, rejected takeoff after veering left and unreliable airspeed

Last Update: May 17, 2023 / 16:06:27 GMT/Zulu time

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Photo of REX Regional Express VH-ZRC, SAAB 340 — REX Regional Express VH-ZRC, SAAB 340 (Photo credit: Alec BHX/KKC / Flickr / License: CC by-sa)

Incident Facts

Date of incident
Nov 4, 2022

Classification
Incident

Airline
REX Regional Express

Departure
Flinders Island, Australia

Destination
Burnie, Australia

Aircraft Registration
VH-ZRC

Aircraft Type
SAAB 340

ICAO Type Designator
SF34

A REX Regional Express Saab 340B, registration VH-ZRC performing a flight from Flinders Island,TA to Burnie,TA (Australia) with 25 passengers and 3 crew, was accelerating for takeoff from Flinders Island when the crew rejected takeoff after the aircraft veered to the left and the crew observed abnormal airspeed indications. The aircraft slowed and came to a stop with 4 landing gear tyres deflated.

Australia's TSB reported the aircraft sustained minor damage and opened a short investigation into the serious incident.

On May 17th 2023 the ATSB released their final report concluding the probable cause of the serious incident was:

Contributing factors

- The parking brake handle was likely not completely reset (seated in the panel) by the pilot, resulting in residual pressure remaining in the brake system.

- Residual pressure in the brake system resulted in a partial application of the brakes during taxi. This allowed heat to generate within the brake system resulting in a gradual increase in brake application.

- The significant and increasing drag associated with the partially applied brakes resulted in a flat spotted tyre and all of the main landing gear wheel fusible plugs activating during the take-off roll.

The ATSB analysed:

On 4 November 2022, a Saab Aircraft Co 340B operated by Pel-Air Aviation Pty Limited, taxied for runway 32 at Flinders Island Airport. During the take-off roll, the aircraft veered to the left and did not accelerate as expected. The crew rejected the take-off and subsequent inspection identified that one tyre was flat spotted and all the main landing gear tyres were deflated.

Subsequent post occurrence rectification and system testing was carried out. No defects or unusual wear was identified on the replaced components other than signs of overheating. The brake system and parking brake system, including handle operation, was tested with no faults identified. The aircraft was released for service with no further brake related faults reported.

Significant tyre marks, from all 4 main landing gear wheels, were observed on the runway after the occurrence. These tyre marks commenced early in the take-off roll and continued to the disabled aircraft. This evidence, as well as a review of the engine parameters from the flight data recorder and reports from the flight crew, identified that the occurrence was brake-related and not an issue with aircraft propulsion. The brake system, consisting of 2 separate hydraulic circuits, an inboard and an outboard circuit, share a common return line. For all 4 brakes to be applied and overheat the wheel assemblies, a fault would have had to occur in both systems simultaneously or in the single hydraulic return line.

The pilot conducted an immediate 180° left turn from the hardstand as the taxi commenced. This is evidence that the parking brake was released to the extent necessary for pressure to reduce below 900 psi and the PARK BRK ON annunciator to extinguish. The pilots observed that there were no cautions present prior to the take-off roll. The pilot also noted that the aircraft was able to be taxied with no abnormal handling or any extra engine power required however the pilot also reported taxiing slower than usual, to allow cabin crew to prepare the cabin, which may have masked any brake application.

If the parking brake handle is not completely seated by the pilot, the common return line from both brake hydraulic circuits will be partially obstructed within the parking brake valve. This will restrict hydraulic fluid from flowing out of the brake system, slowing down the rate at which pressure is relieved, resulting in a partial application or dragging of the brakes. During the taxi out, the partially applied brakes created heat due to friction resulting in further brake application to the restricted system as the temperature increased.

As the aircraft commenced the take-off roll, heat from the dragging brakes increased rapidly due to the acceleration of the wheel assemblies. At 46 knots the aircraft veered to the left, likely as a result of the left tyre locking and skidding. The pilot corrected towards the runway centreline and continued with the take-off. At approximately 80 knots the pilots observed that the aircraft’s acceleration rate had reduced and the take-off was rejected.

The pilot commented that, when they reduced power, and prior to manually applying the brakes, the aircraft slowed as though the brakes were applied. This was likely a combination of an application of the brakes and the tyres deflating as identified by the change in runway tyre marks. The tyre marks had changed from a solid tyre mark for the left outboard wheel to a set of parallel lines for each individual wheel. These parallel lines are a result of the weight of the aircraft acting only on the edges of the wheel rim through the tyre to the runway, indicative of a deflated tyre. This is evidence that that the core of the fusible plugs had melted due to a build-up of heat in the wheel assemblies, resulting in the tyres deflating during the latter stage of the take-off roll. The pilot commented that when they then applied the brakes, they felt pressure in the pedals and likened it to the parking brake being set.

Consideration of all the available evidence supports a conclusion that the push-pull parking brake handle was likely not completely seated in the console resulting in the parking brake valve partially restricting the hydraulic return line. This residual pressure allowed a partial application of the brakes, or an incomplete release of the brakes, at the commencement of the taxi, resulting in the brakes dragging. This resulted in the generation of heat in the brake system, a continual increase in brake application and the rejected take-off.