Government airworthiness certification requirements stress fuel system safety because this system has been responsible for most lightning-related aircraft accidents.
Elements of the fuel system are typically spread throughout much of an aircraft and occupy a significant amount of its volume They include the fuel tanks themselves, associated vent and transfer plumbing, and electrical controls and fuel quantity instrumentation.
Careful attention must be paid to all of these components if adequate protection is to be obtained. The main objective of fuel system lightning protection is to keep ignition of fuel from destroying the aircraft 'during a strike. This goal is quite challenging because thousands of amperes of current must be transferred through the airframe when the aircraft is struck by lightning and a tiny spark of less than one ampere may release sufficient energy inside a fuel tank to ignite the fuel vapor and initiate an explosion.
There are 4 methods of preventing aircraft fuel hazards from lightning:
- Containment - The expansion due to the explosion is contained without structural issue.
- Inerting - Introducing nitrogen into the fuel tank.
- Foaming - Fuel permeable foam like are cars
- Eliminating Ignition Sources
CiES Magnetic Field Fuel Quantity Design embraces #4. CiES simply eliminates the wiring and the wiring hazard from the fuel volume. The wiring and fuel sensor element are kept completely out of the fuel volume, if the sender is mounted to the tank surface.
This methodology eliminates any electrical ignition source from the fuel volume.
While this issue has not proven to be an issue with some fuels namely AVGAS - lightning induced ignition of fuel tanks containing gasoline is not an unknown phenomenon as demonstrated below in Ohio
So while lightning indirect and direct effects are addressed for the wiring. Induced effects become paramount for good design.
Arcing and sparking within the fuel vapor space of a fuel tank is one of the primary concerns of the fuel quantity system designer. The distinction between arcing and sparking is worth reviewing. Arcing is the result of current through the interface between two conducting materials which are making limited electrical contact. Such a condition may exist at the interface of two moving metal components like the CIES rotor and pivot or between the arm and the internal tank surface as shown.
A spark, on the other hand, is an electrical die-charge resulting from a difference of potential across an air gap or along a dielectric material. This condition may exist, for example, in the CiES fuel sender as the metallic components are anodized and teflon impregnated for corrosion protection. Current through the tank may result in difference of potential between these components.
By insuring conductivity is maintained throughout the fuel quantity sender this hazard is mitigated. CiES utilizes clever details to maintain corrosion protection but also to insure induced charges dissipate harmlessly to aircraft grounding structure