Information relating to blocked/contaminated Jet A1 fuel filters


In February 2004, the CAA received notification that some aircraft were experiencing fuel filter bypass indications on medium-large transport aircraft using Jet A1 fuel. Subsequent inspections found that the filters contained a black sludge/film. Inspection of other aircraft found similar results to varying degrees.

Fuel supply companies have advised that an extra high level of vigilance is being maintained in the monitoring of the fuel supply installations, pipelines and tankers due to these reports. Caution is being maintained at all levels of the fuel supply chain with no issues of concern or contamination being found.

Consequently, a NOTAM was issued on 23 February 2004 advising aircraft operators to monitor Jet A-1 aircraft fuel filters with extra vigilance.

Note: The latest round of fuel filter blockages does not appear to be related to blockages reported in December 2003.

Fuel filter bypass warning operation

When a fuel filter starts to become blocked the differential pressure across the filter begins to increase. When the differential pressure exceeds a preset limit, a visual warning is provided indicating that the filter is becoming blocked. If the situation continues to the point where the differential pressure across the filter exceeds a higher preset level, fuel will be bypassed around the filter.

Generally, the filter is inspected and replaced immediately after a bypass warning indication. This means that although the warning may have activated, bypass may not have occurred. Even though a fuel filter may be in the bypass mode, fuel will still be supplied to the engine.

Extent of fuel filter blockage

To date:

Discolouration and contamination has been reported on a range of aircraft types from large-medium size air transport aircraft, to agricultural aircraft and helicopters.

Reports have been received from both the North and South Islands.

Fuel has been supplied to the affected aircraft from a variety of fuel supply locations, companies and supply sources.

Investigation results

The affected fuel filters and associated fuel tank samples have been forwarded for laboratory analysis. Initial indications are that the black material in the filters was largely inorganic in nature and consisted primarily of the elements sulphur, iron, sodium, and calcium.

Analysis of the fuel tank samples indicated that the microbiological (fungi and bacteria) contamination was very slight. One sample showed a 'very heavy' concentration of yeast.

A further fuel tank sample was independently tested and found to contain copper, sodium and iron.

Note: The December 2003 fuel filter blockages indicated that there was a high concentration of carbon in the fuel samples. The February 2004 samples do not have similar indications.


Analysis of the results is ongoing. To date (22 Mar 04) the source of the black sludge/film has not been determined. The relevant parties in the aviation industry are working constructively with the CAA to try and determine the potential sources of these issues.

Update 19 May 04

CAA/industry/fuel company/laboratory meeting

The CAA hosted a meeting of aviation industry, fuel company, and laboratory representatives on 6 May 2004 to review the available data and share information. Key action items from the meeting are:

  • The airlines and fuel companies remain vigilant in their checks of fuel filters and installations.
  • Routine quality control tests are being conducted by fuel companies. In addition, some interim testing for the presence of metals will be carried out.
  • A review of the fuel specification for fuel supplied from overseas is being carried out to determine whether there have been any changes to the additives etc.

It was noted that there have not been any more reports of filter blocking since the February 2004 reports.