Earthing an aircraft when refuelling

[steve181]

I’d love to know exactly what goes on here. I’ve got my CPL & was only told this is done to prevent a static spark from igniting the fuel/aircraft etc, I never learnt in depth what goes on here.

Say we pull our Cessna 172 up next to an AVGAS pump after a long flight on a hot day. First question; after stepping out of the aircraft would the aircraft still have static energy from flying through the air or would it have dissipated before pulling up to the pumps?

We’ve attached the anti-static line (earth) to a wheel nut for instance, before even picking up the fuel nozzle (& until my first question is answered) assuming the aircraft is statically charged what has gone on at this point? For example has the static electricity moved from the aircraft & been earthed via the anti-static line?

So now the fuel is pumping away into the fuel tank. If there is a static build up is it caused by the movement of the fuel through the hose? If there was a static discharge would it come from the tip of the fuel nozzle or not necessarily?

In theory if the aircraft is earthed is there no way a static discharge will occur? With an earthed aircraft does it mean there is now nowhere for static electricity to flow/discharge because the aircraft, pump & terra firma are on the same “wavelength”?


Thnaks in advance.

[Farmer 1]

If there is a static build up is it caused by the movement of the fuel through the hose?

Not quite. If I recall correctly, it is caused by the flow of fuel through the neck of the refuelling tube of the aircraft. This is why the refuelling hose should be connected directly to the aircraft.

[eliptic]

We’ve attached the anti-static line (earth) to a wheel nut for instance

That don´t sounds to be a correct way of "grounding" to me, maybe i am wrong but there are not a very good connection to the A/C if you consider the axle bearing, joints etc etc..

correct me if i am wrong

[piggybank]

In my opinion. I would not trust that the static has dissipated before you reach the pumps. You only get it wrong once when you are dealing with avgas on a hot day. I would not trust earthing to a wheel nut either, there is usually something, a plate say, sticking out near the filler or a proper plug socket to earth to. From memory the tyres have a specific conductance and the surface used to be 6 megohms resistance. I bet thats not true for cast concrete, so use the supplied earth cable. Thats my two pennys worth

[SNS3Guppy]

The primary consideration isn't "earthing" or grounding, but bonding. A ground puts the electrical potential to ground (or "earth"), but a bond equalizes potential between two sources. When one fuels, one is concerned about bonding between the fueling source and the fueling receptacle.

In an automotive application this takes place thorugh the fuel nozzle and the hose itself...which carries conductive material. In an aircraft application, one bonds the airplane to the fuel source with an "earthing" or grounding wire. Often the fuel truck is also grounded, meaning a bond between the truck and the ramp (apron) surface.

Bonding and grounding does not take place through aircraft or fuel truck tires, which act as insulators.

Regardless of whether the aircraft carries a charge from flight, other charges are available, from improperly bonded or grounded strobe power packs to other electrical charges on board the aircraft. Additionally, any electrical dissimiliarity or potential between the truck and the aircraft is worthy of consideration...regardless of whether the aircraft carries any kind of charge. The fact that fuel is flowing, and that the fuel truck has an operative electrical source (battery, generator, etc) means that while the airplane may not have a charge, the truck may, and whatever state exists between the two must be equalized by bonding before the fuel can be pumped.

Sometimes individuals ground the airplane to the ramp, and fail to bond with the fuel truck or fuel source. This is an incorrect thing to do, and may increase the danger...because now the charge in the truck has a place to go to the ground....from the fuel nozzle, where the fuel is vaporizing, to the aircraft fuel inlet...and through the aircraft to ground (earth).

The most important consideration is bonding the source and fuel recipient. After that, earthing or grounding is a wise choice from either the aircraft itself, or through the truck.

[steve181]

"The primary consideration isn't "earthing" or grounding, but bonding."

Ohh ok so that explains a lot. So in simple terms if we look at my Cessna 172 scenario your not so much earthing the plane to the pump (stationary pump with the flying I do) via the anti-static line, your enabling the aircraft & the pumps to become one object in an electrical sense instead of two seperate entities?

So when they "become one" with the line in place the static energy can't run from one object to another because the energy both objects have (aircraft & pumps) is "equalized" so to speak?

Thanks for the replies by the way.

[Bus429]

Or, put another way, ensure the aircraft, pump and nozzle are at the same potential.

[SNS3Guppy]

So in simple terms if we look at my Cessna 172 scenario your not so much earthing the plane to the pump (stationary pump with the flying I do) via the anti-static line, your enabling the aircraft & the pumps to become one object in an electrical sense instead of two seperate entities?

So when they "become one" with the line in place the static energy can't run from one object to another because the energy both objects have (aircraft & pumps) is "equalized" so to speak?

Correct. It's all about reducing potential and eliminating a spark.

[eliptic]

so this means you don´t have to ground the A/C itself to earth, just to the truck?

"""This is referenced in FAA Order 8400.10 Vol 3, Chapter 15 Manuals, Procedures, and Checklists

“NOTE Grounding of the aircraft and/or fuel truck is no longer recommended because it does not prevent sparks at the fuel source, and the grounding cable may not be sufficient to discharge the electrical current.”

FAA Order 4040.9 Appendix 14 states:

“14. FUEL AND OIL SERVICING. The PIC or designated crewmember shall normally supervise fuel, oil, or other ramp services when these services are accomplished by other than FAA or qualified maintenance personnel. During fueling operations, the following safety precautions shall be observed:

a. The aircraft will be bonded by the use of a cable to the fuel truck or pit, and to the fuel nozzle.
b. Engine switches and nonessential electrical equipment shall be in the "off" position, or as dictated in the aircraft manual. Operation of an auxiliary power unit, if available while refueling, shall be governed by instructions contained in the appropriate airplane flight manual (AFM) or rotorcraft flight manual (RFM).
c. Smoking is prohibited aboard and within 100 feet of the aircraft.
d. No active radar transmitters shall be within 100 feet of the aircraft. Weather radar on the aircraft shall be turned off.
e. The correct grade of fuel has been put into the aircraft fuel tanks and the quantity received verified.
f. Fuel shall be checked for presence of water in accordance with the aircraft flight manual, pilot's operating handbook, or manufacturers' recommendations. “

Just for general bonding information the FAA recommends the following in AC 150/5210-15 Airport Rescue And Firefighting Station which calls out FAA-STD-019, Lightning Protection, Grounding, Bonding and Shielding Requirements for Facilities.

[con-pilot]

Correct. It's all about reducing potential and eliminating a spark.

The odds of a spark being produced while an aircraft is being refuled, is in truth, very remote, however, even with such low odds the chance of a catastrophic accident is still high enough that you cannot take the chance of not grounding/bonding any aircraft before starting the refueling.

When I was 16 years old I was standing in front of the Base Operations Building and hangar at Tinker AFB waiting for my father to land. Back then there were eight T-33s that were assigned to base parked just to the northeast side of the hangar. Suddenly there was a bright flash from just around the corner of the hangar immediately followed by sound of a very large explosion. Black smoke started bellowing into the sky and I could see flames shooting up into the sky mixed in with the black smoke.

Being young and stupid I immediately ran toward the source of the explosion along with a group of officers and senior NCOs, who should have known better. As we rounded the corner of the hangar I saw the remains of one T-33, what was left of a refueling truck and two other T-33s that were burning. I stopped right in my tracks fixated on the scene that I was observing. A Master Sergent grabbed me by my shoulder, turned me around and started yelling for everyone else to go back a take cover on the other side of the hangar; he didn't have to tell me twice. Shortly after we all got back to in front of the hangar the second T-33 exploded.

To make a long story a little shorter the CFR Equipment, they were just called Crash Trucks back then, were able to prevent the third T-33 from exploding and extinguish the fire.

The cause of the explosion was that a spark was produced, neither the fuel truck or the T-33 had been grounded. Two T-33s were destroyed when they exploded, one T-33 had to scrapped due to fire damage and a fourth T-33 had to be scrapped due to damage suffered by debris from the T-33s that had exploded. The airman doing the refueling was killed, the fuel truck driver and the crew chief were also killed.

So, all that damage and the deaths of three people were all caused because someone was too lazy, or was in too big of a hurry to ground the aircraft.

Contributing factors were that is was a hot day, calm wind, over-wing refueling and that the fuel was JP-4.

[west lakes]

The (electricity industry) term for this is "Equipotential Bonding"

[rahulras1993]

Well as far as I have learned at school , we learned that aircraft tires are made radial , with some kind of conductance so as to transfer charges accumulated during flight to dissipate on the ground while landing, well I guess this is just a basic answer.

[Munnyspinner]

I’ve got my CPL & was only told this is done to prevent a static spark from igniting the fuel/aircraft etc, I never learnt in depth what goes on here.

Steve 181, Did you not do the PPL before getting your CPL. I am very suprised that you have completed a commercial licence without covering this in more detail. Do they not teach you anything these days?

Thankfully, you have received some good answers which you will be able to relay to any students you ever come across in the future.

It is the same principle that the HT engineers use when working on hot wires. They get themselves ( via a Faraday cage/suit) up to the same charge as the wire they are working on - the electricity then simply flows around them.

[NutLoose]

One example that always has been a reminder to me, was an RAF Pembroke being defuelled in Germany, the aircraft was not earthed to the defuelling drum and the open fuel dropping over a height of about 2ft 6 caused a spark that resulted in the loss of the aircraft

[Karl Bamforth]

Yes I still think of the Pembroke when discussing this subject.

Essentially the fuel drum was under the aircraft and the fuel was exiting the tank via a defuel point that is approx 2 inch across. When Avgas falls through 3 ft of air it can pick up enough static energy to ignite itself on contact. This did just that. A 2 inch column of fuel falling into a drum that was on fire and the only shut off cock was immediatly above the flames prevented attempts to extinguish it.

The aircraft was a write off within a few short mins.

[Krystal n chips]

It's probably long since been disposed of, but a long time ago ( so to speak ) as a happy little RAF apprentice I watched a Flt Safety film about bonding and refuelling a Whirlwind.....based on a true event as I recall.....which was destroyed during the process...due to not being bonded ( if you can ever get hold of the film, it's worth watching.....simple lesson of fact to be precise as to what can and does go wrong ).......a few years later, sitting in a Jet Ranger for one of those "see NY from the air" trips....watched "with interest and sphincter fluctuations " from inside the beast as said chopper is running refuelled and the bonding pin is dangling in the breeze during the process....ho hum !