REX SF34 at Adelaide on Aug 20th 2019, wheel fracture
Last Update: August 26, 2020 / 17:45:15 GMT/Zulu time
Incident Facts
Date of incident
Aug 20, 2019
Classification
Incident
Airline
REX Regional Express
Flight number
ZL-4398
Departure
Adelaide, Australia
Destination
Port Lincoln, Australia
Aircraft Registration
VH-ZLX
Aircraft Type
SAAB 340
ICAO Type Designator
SF34
After the landing the crew observed the outboard left main tyre had deflated and detected the wheel damage.
The ATSB rated the occurrence an incident and opened a short investigation.
On Aug 26th 2020 the ATSB released their final report concluding the probable causes of the incident were:
- A fatigue crack initiated at the bead seat and led to the failure of the left outboard main landing gear wheel.
- This wheel design was susceptible to fatigue cracking in the bead seat region.
- It is probable that a fatigue crack was present at the time of the last non-destructive eddy current inspection but not detected.
- The operator’s wheel maintenance forms did not adequately convey the inspection requirements for wheels operated with flat tyres. Subsequently, when the flat tyre was detected and the wheel brought in for maintenance, inspections that may have detected the crack were not carried out.
The ATSB summarized the sequence of events:
Following the landing in Port Lincoln, the first officer conducted an external inspection of the aircraft and noticed that the left main outboard landing gear tyre was deflated.
The crew reported no issues with aircraft handling during the take-off, landing or taxi phases of the flight. Additionally, none of the crew or passengers advised of any vibration or unusual noises during the flight.
On closer inspection of the wheel, the first officer noted that a piece of the wheel rim was missing (Figure 1). Ground staff were notified and the Port Lincoln aerodrome reporting officer (ARO) was contacted to conduct a runway inspection. The ARO’s inspection did not reveal any foreign object debris or damage to the runway.
The following morning, the operator dispatched engineers to replace the wheel and brake assembly. They informed the flight crew that the missing piece of wheel had been located on the runway strip at Adelaide Airport.
The ATSB analysed:
Component failure
Failure of the left main outboard landing gear wheel from Saab 340B aircraft, VH-ZLX, was a result of the fracture and separation of a section of the inner wheel rim adjacent to the tyre bead seat. The cracking and fracture was typical of a progressive fatigue cracking mechanism, which had initiated on the internal bead seat transition radius. In conventional pneumatically pressured wheel designs, the internal bead seat radius is typically a region of high bending stresses. As such, it is pre-disposed to the initiation and growth of fatigue cracking. Operational stresses arising from tyre flexure during taxi and landing can further contribute to this failure mechanism.
The bead seat radius fatigue cracking was a known issue with wheels of the original ABSC P/N 5010488. To improve the reliability of these wheel assemblies, the wheel manufacturer introduced updated inspection methods and tyre change or cycle limits between overhauls. To further address the issue, a revised wheel design (P/N 5010488-1) with features that strengthened the bead seat region of the wheel aimed at preventing fatigue failures was introduced. The failed wheel from VH-ZLX was of the original design.
Wheel inspection and crack development
Each time the non-destructive testing was conducted at a tyre change, no cracks were identified and the wheel remained in service. Two scenarios were identified that could have accounted for the presence of the fatigue crack that initiated the failure:
- the initiation and growth of the crack occurred rapidly, with the crack developing in the time since the last non-destructive inspection was carried out, or
- a crack had initiated at the time of the last non-destructive inspection and was not detected.
The ATSB was not able to determine the crack growth rate that occurred in the wheel rim from the fracture surfaces. While rapid crack development could not be conclusively ruled out, for the following reasons the ATSB considered it probable that a crack had been present at the time of the last non-destructive inspection but had not been detected:
- The relatively large size of the fatigue area (86mm), compared to the relatively low number of flight cycles that had occurred (97) since the last eddy current inspection.
- The nominal interval between eddy current inspections was 300 flight cycles, however, this failure occurred well before the next inspection was due. The very small number of reported failures in this wheel type in last 10 years indicated that the inspection interval generally appears adequate to capture cracks before they progressed to failure in service.
- While it was possible that the wheel was operated for a short time with lower than recommended tyre pressure which could have influenced the crack growth rate, it was considered unlikely that it increased the growth rate enough to cause failure at one third of the nominal inspection interval.
Available forms
When the wheel was presented to the wheel bay following its removal from VH-ZXK, the personnel involved utilised the standard tyre change form to complete the inspection, noting on the form that this was not a standard tyre change, rather a ‘Repair’. Utilising the tyre change form, personnel signed for the ‘general wheel check and inspection admin’ task. This task did not include a visual inspection, as it was listed as a separate item on the form. When reinflated, the tyre retained pressure for the required 24 hours and so the tyre was not changed. As a result, a complete tyre change inspection, including non-destructive inspection, was not performed.
As the wheel had been operated with a flat tyre, even if for a short time, it should have undergone the additional inspections outlined in the ‘Special cases’ section of the CMM. However, the documentation that maintenance personnel used did not identify this as a requirement, nor did it direct them to the CMM for further guidance in non-standard cases.
The ‘Special cases’ section of the CMM required a detailed visual inspection and roundness check to be carried out. The ATSB was unable to determine whether either of these inspections could have identified the crack. However, they would have been opportunities to determine that something was out of place and prompt further investigations. Although not required as part of the ‘Special cases’ section, had the wheel been subject to non-destructive inspection, the crack would have almost certainly been detected.
Aircraft Registration Data
Incident Facts
Date of incident
Aug 20, 2019
Classification
Incident
Airline
REX Regional Express
Flight number
ZL-4398
Departure
Adelaide, Australia
Destination
Port Lincoln, Australia
Aircraft Registration
VH-ZLX
Aircraft Type
SAAB 340
ICAO Type Designator
SF34
This article is published under license from Avherald.com. © of text by Avherald.com.
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