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Fluid protection design guidance

Issue number: AO-2008-003-SI-10
Who it affects: Manufacturers and operators of transport category aircraft
Issue owner: US Federal Aviation Administration
Operation affected: Aviation: Other
Background: Investigation Report AO-2008-003
Date: 13 December 2010

Safety issue description

The United States Federal Aviation Administration regulations and associated guidance material did not fully address the potential harm to flight safety posed by liquid contamination of electrical system units in transport category aircraft.

 

Action taken by the ATSB

During the investigation, the ATSB discussed the background for this safety issue and the associated safety risk with the United States Federal Aviation Administration (US FAA). The potential for a reduction in the associated risk to as low as reasonably practicable by proactive US FAA safety action was highlighted.

The ATSB considers that the risk of ongoing or emerging design, operation and maintenance issues with the potential to result in liquid contamination of electrical system units in transport category aircraft could be significantly reduced over time by improved regulatory guidance and oversight. For example, existing designs and processes should be monitored for continuing effectiveness while consideration of alternative design principles may be applied to new aircraft designs.

 

Action taken by the US FAA

The US FAA did not provide comment in response to this safety issue.

Recommendation

Action organisation: US Federal Aviation Administration
Action number: AO-2008-003-SR-109
Date: 13 December 2010
Action status: Closed

The Australian Transport Safety Bureau recommends that the US FAA take safety action to address this safety issue.

Correspondence

Date received: 15 March 2011
Response from: US Federal Aviation Administration
Response text:

We are investigating the issue, considering the scope of the recommendation and planning the best course of action. We anticipate submitting a follow-on response updating our progress by 31 March 2012.

Date received: 01 May 2011
Response from: US Federal Aviation Administration
Action status: Closed - Accepted
Response text:

As Part of the Enhanced Airworthiness Program for Airplane Systems / Fuel Tank Safety (EAPAS/FTS) rulemaking and guidance project, the FAA developed and implemented several enhancements including aircraft wiring practice training for the engineers and inspectors and an internet-based comprehensive airplane job aid. The job aid includes information regarding installation, interfaces between wiring and equipment, and protections against foreseeable physical and functional faults.

The EAPAS/FTS rule contains new certification and operational requirements to mitigate aging issues in aircraft wiring and associated components for the current and future fleet of aircraft.

The EAPAS rule includes:

  • Requirements that Electrical Wire Interconnection System (EWIS) components be selected so that they are fit for purpose and appropriate for the environment in which they will be used, including requirements to address known characteristics of the wire such as arc tracking.
  • EWIS identification requirements for clear and consistent methods of wire component identification for such things as function, separation requirements, and design limitations.
  • Requirements that access be provided to allow inspection and replacement of EWIS components necessary to maintain the continued airworthiness, including the integrity of the required drip shields, of the airplane.
  • Enhanced EWIS maintenance and inspection requirements, including mandatory replacement of EWIS components as necessary.

The FAA issued accompanying Advisory Circulars (AC) with guidance on complying with the enhanced requirements. The AC provides guidance for developing an enhanced EWIS training program at operator and maintenance facilities which supplements internal FAA training. The objective of the comprehensive training program is to give operators, maintenance, and repair organizations a model for developing their own EWIS training program. The curriculum outlined in the AC ensures that proper processes, procedures, methods, techniques, and practices are used when performing maintenance, preventive maintenance, inspection, alteration, repair, and cleaning of EWIS.

We determined that current Title 14, Code of Federal Regulations (14 CFR) part 25 regulations, with the requirements added by the EAPAS/FTS rule, are adequate with respect to liquid contamination of electrical components. We are reviewing the related guidance material to determine if there are additional ways it might be improved to better address the issue.

ATSB response date: 10 May 2011
ATSB response:

The Australian Transport Safety Bureau (ATSB) is satisfied that there is sufficient airworthiness regulation and guidance relating to electronic wiring interconnection systems (EWIS), and notes that the advised safety action should further improve the level of risk regarding EWIS.

However, EWIS is not the subject of the ATSB recommendation, insofar as it is defined in FAR § 25.1701 and clarified by the final rule for the Enhanced Airworthiness Program for Airplane Systems/Fuel Tank Safety (EAPAS/FTS) as follows: ‘the EWIS certification and operational requirements in the final rule apply to wires that 'interconnect' airplane systems, as opposed to wiring located solely within the enclosure of a piece of avionics equipment, for example.’

The ATSB recommendation is intended to address the protection of non-EWIS electrical and electronic components, that is; avionics equipment such as line-replaceable units (LRUs). Such components are not required to be protected to a similar degree as EWIS; the only requirement being, that ‘critical environmental conditions must be considered’ [FAR § 25.1309 (e)]. The ATSB considers that, like EWIS, other parts of the avionics systems comprise off-the-shelf components which are uniquely installed and are subject to different environmental conditions in each aircraft type. For example, on some aircraft models the electronics bay is located directly underneath a galley, presenting an increased risk of water ingress. A component installed in that aircraft presents a completely different risk to the same component fitted in a different aircraft. The ATSB concern is that aircraft-specific environmental risks need to be considered separately for each LRU or avionics component.

In light of the above clarification, the ATSB asks that the FAA review and confirm its position that ‘current Title 14, Code of Federal Regulations (14 CFR) part 25 regulations, with the requirements added by the EAPAS/FTS rule, are adequate with respect to liquid contamination of electrical components.’ Specifically, whether it considers that the regulations and new rule adequately addresses the protection of non-EWIS electrical and electronic components.

Date received: 02 August 2011
Response from: US Federal Aviation Administration
Action status: Closed - Partially Accepted
Response text:

We understand that the ATSB’s concerns are not limited to Electrical Wire Interconnection System (EWIS) components but are directed at all electrical and avionics equipment in a broader sense.  The broader concerns are adequately addressed in our current regulations and policy governing both component-level qualification (Technical Standards Orders and guidance material such as DO-160, Environmental Conditions and Test Procedures for Airborne Equipment) and higher-level, aircraft installation requirements (such as 14 CFR 25.1309).

In highlighting the FAA’s more recent Enhanced Airworthiness Program for Airplane Systems / Fuel Tank Safety (EAPAS/FTS) rulemaking in our previous response, we pointed out that many of those improvements also inherently address non-EWIS components. Enhanced Zonal Analysis, periodic cleaning/inspection, and monitoring of the protective features such as drip shields, are some of the examples of the enhancements which have a positive impact on the conditions in which all electrical and electronic components are installed and operating.

In conclusion, our current component-level qualification standards and current 14 CFR Part 25 regulations, with the added requirements of the EAPAS/FTS rule, are adequate with respect to liquid contamination of electrical and electronic components.  We will continue to review related guidance material to determine if there are additional developments to further enhance the protection of installed electrical and electronic components.

We believe we have effectively addressed the ATSB’s concerns and consider our actions complete with regard to FAA Safety Recommendation 10.359.

ATSB response:

The ATSB considers that the FAA's actions will, when implemented as described, address the maintenance and operation aspects of the safety issue. However, the ATSB would like to further emphasise the design and modification aspects of the safety issue as these were potential problem areas identified by the investigation which could not be completely resolved by other means.

   
Current issue status: Safety action pending
Status justification:

The ATSB considers that the FAA's actions will, when implemented as described, address the maintenance and operation aspects of the safety issue. However, the ATSB would like to further emphasise the design and modification aspects of the safety issue as these were potential problem areas identified by the investigation which could not be completely resolved by other means.

 
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Last update 11 March 2014