Read: The DOT/FAA/CT-94/74 Aircraft Lightning Design Doc

DOT/FAA/CT-94/74 Aircraft Fuel System Lightning Protection Design and Qualification Test Procedures Development

This report from Keith Crouch of Lightning Technologies is the foundational document for lightning testing aircraft fuel system components and structures.
Key information in this document include:
  1. The design, material list, and operation of the Standard Voltage Spark Ignition Source (SVSIS) and Controlled Energy Ignition Voltage Spark Source that is used in SAE ARP5416A and RTCA/DO-160G Section 23. 
  2. The development and documentation for the use of the hydrogen, oxygen, and argon combustible fuel mixture that would ignite when exposed to a 200 microjoule spark.
  3. The use of P3200 black and white 35mm film to detection 200 microjoule ignition source.
Each of these accomplishments, by themselves, had an immeasurable impact on improving aircraft fuel system safety.  With the hydrogen mixture, ignition sources could be safely detected without the use of highly explosive propane/air mixtures.  The voltage spark source produced a consistent and repeatable 200 microjoule spark which then allowed the multiple test labs across the world to verify the accuracy of their ignition detection equipment (cameras, fuel mixtures, light detection devices, etc.). 

Polaroid film was the spark source detection method described in SAE AE4L-78 (Blue Book) but it had many issues - in particular, the film had random bits of contamination that looked like small ignition sources.  35mm film was a much more consistent medium that produced repeatable results. 

The beautiful feature of using the hydrogen mixture is that it doesn't create visible light when it burns.  This allows cameras to be used with the hydrogen mixture to detect ignition sources, even when the hydrogen mixture is ignited.  Propane and other combustible gases produce significant amounts of visible light which "white-out" film images and make ignition source detection impossible.

The DOT/FAA/CT-94/74 document is available for download here: DOT/FAA/CT-94/74

DOT/FAA/CT-94/74 document

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