Recommendation

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.

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.