“”How do you ground a composite aircraft during fueling?”” asks Russell Green.
“”How do you ground a composite aircraft during fueling?”” asks Russell Green.
When a non-conductive fuel like avgas is transferred from one vessel to another, it can build up an electrical charge. This electrical charge, or difference in electrical potential, needs to be dissipated or an arc can occur.
In a metal aircraft, an electrically conductive cable is connected from the refueling vessel to a point on the aircraft that has a conductive connection to the metal tank. This, plus an electrically conductive hose, will dissipate the electrical charge as it is generated by the fuel transfer.
If a composite aircraft has a metal tank, then you would need to determine or establish an electrically conductive path to a grounding point. This can be done with a volt-ohm meter. The problem Mr. Green has is that his tank also is composite. As fuel is pumped into his tank, there will be a difference in electrical potential between the fuel in the tank and the grounded fuel nozzle. When the fuel rises to a level near the tip of the nozzle, an arc can be generated. Fortunately, the atmosphere in the tank is almost 100% fuel vapor. Without an adequate supply of oxygen, the spark cannot cause a fire. However, if there is a lot of air movement in the tank or the fill nozzle is raised up and enough air is present to result in a combustible air fuel mixture, a fire could result.
Another concern is at very low ambient air temperatures, the amount of vapor given off by the fuel could be so low it could leave a combustible mixture in the vapor space of the tank.
The best safety procedure that I am aware of for this type of aircraft would be a long metal strip placed in the tank. The strip would need to be conductive (no paint or coating) and long enough to reach the bottom of the tank. During refueling, inspect the strip to ensure that it is clean. Then place it into the tank, being careful not to damage the surface of the tank. Once the strip is in the tank and in the existing fuel, connect the bonding cable to the strip and fuel your aircraft.
I cannot stress strongly enough the importance of cleanliness and caution when using this procedure. If you do not exercise care in completing the process correctly, you can be causing more risk than you are preventing.
The other part of this grounding/bonding process is the question of whether the aircraft needs to be grounded or just bonded to the refueling apparatus. In the past, the recommended procedure was to ensure that the aircraft had a good electrically conductive path to the refueling apparatus and to ground. This ensured that any electrical potential generated by the aircraft going through the air was dissipated prior to the refueling process.
However recent studies have shown that this risk is much less than the risk of a lightning strike during the refueling process. If lightning should strike and the aircraft is grounded, the aircraft would act as a lightning rod and be an electrically conductive path to ground. The data for this change was generated by studies done on airline operations. Although general aviation aircraft do not represent as large a lightning target, the principle, and thus the change, do apply to the refueling process for general aviation. So when refueling an aircraft, one needs a bonding cable to connect the aircraft to the refueling vessel. You should not connect the aircraft to a grounding point.
I also would hope that most general aviation pilots are smart enough or do not feel the need to refuel their aircraft in the middle of a lightning storm. I do miss the good old days when common sense was a common virtue.
Ben Visser is an aviation fuels and lubricants expert who spent 33 years with Shell Oil. He has been a private pilot since 1985. You can contact him at [email protected].
