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Recommendation issued to: CASA

Recommendation details
Output No: R20050013
Date issued: 23 December 2005
Safety action status:
Background: Why this Recommendation was developed

Output text

Safety Recommendation

The Australian Transport Safety Bureau recommends that the Civil Aviation Safety Authority alert operators and review the continuing airworthiness of all Australian registered Fairchild Industries SA227 model aircraft, or other aircraft model types using fuel immersed capacitance-type fuel sensors (probes), with specific regard to possible high impedance wire chafing within the fuel tank.

Initial response
Date issued: 23 February 2006
Response from: CASA
Action status: Monitor
Response text:

Following an investigation into the occurrence, and a review of the ATSB recommendation, CASA supplied the results of that investigation and review to the ATSB. A summary of those findings is included below:

Interim Decision

Following the preliminary review of data available the decision was made not to ground the Australian fleet on the following basis:

  • No Previous history of  defects/incidents over a significant time.  
  • The FQIS [fuel quantity indicating  system] is a low power system, current limited [0.2 amperes according to the  manufacturer, which was equal to 3.2 watts] to minimise the possibility of a  spark of sufficient energy to ignite the fuel being allowed to form in the  event of a short circuit.  
  • The abrasion and arcing had  occurred at some time, the evidence indicated that it occurred at very low  power but there was no indication whether it was an old event, recent or  ongoing.  
  • The down time for operators would  be significant in that all ten probes would have to be removed (inspection in  situ is not possible). Reinstallation would require resealing (and associated  cure time) and possible recalibration of the fuel system. Estimated total down  time would be 48 hours.

Conclusion

Considering the age of the aircraft and the uncertain maintenance regime that existed for the majority of its operational life causal factors that may have contributed to this defect are numerous, as an example:

  • Orientation of the probe in the  fuel tank and he subsequent susceptibility to wire deflection due to fuel  surge during refuelling or aircraft manoeuvring.  
  • Deflection of wiring during the  aircraft incident.  
  • Operation of the aircraft in a  high vibration condition eg out of balance propeller, extended ground  taxi.  
  • Resonant airframe  vibration.

There is no indication as to whether this is an old defect, recent failure or a continuing issue. No evidence exists that would indicate a fleet trend. From the data available, I agree with the finding of the TC [type certificate] holder in that this is an isolated incident.   Whilst the potential for fire is always present when an ignition source and a volatile fuel are brought together, there is minimal risk that the arcing that occurred could cause a fire in the aircraft considering the low power and the high flash point of the Jet A1 fuel in use.

Recommendation

An Airworthiness Bulletin is to be issued recommending greater scrutiny of the FQIS, particularly the fuel quantity probes, during scheduled and unscheduled maintenance. Operators will be encouraged to report any anomalies found in an attempt to identify possible trends. This would include verification of the FQIS reading against physical quantity of fuel present.

Further correspondence
Date issued: 05 July 2006
Response from: CASA
Response status: Closed - Partially Accepted
Response text:

As a result of further communications with the Civil Aviation Safety Authority (CASA) on the issue following notification of another fuel probe from a different aircraft displaying the same anomaly, CASA conducted a review to further examine the issue. It provided the Bureau with an extensive report detailing the review. The conclusion of the report stated:

 

'Following the review CASA maintains that:

  1. CASA's original findings are  valid.
  2. The risk of fuel tank explosions  due to electrical shorting of the fuel probes in the SA227 is  negligible.
  3. The Metroliner SA227 is not  covered by SFAR [Special Federal Aviation Regulation] 88 or recent amendments  to FAR [Federal Aviation Regulation] 25 issued by the FAA [US Federal Aviation  Administration].
  4. The SDR [Service Difficulty  Report] records on this defect needs to be reviewed in light of conflicting  anecdotal evidence to the contrary.'

The review also included proposed actions to revise Airworthiness Bulletin 28-1 to better represent fuel related hazards and to develop and publish additional educational material on the matter.

ATSB response:

Although the Civil Aviation Safety Authority considers the risk of fuel tank explosions due to electrical shorting as negligible, the Bureau considers the possible introduction of an ignition source into a fuel tank by way of electrical discharge or shorting, even though of low energy, as undesirable.

 
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Last update 03 April 2012