Mid air refuelling - a teacher needs help

[Guest]

Hi all,In my day job I teach Physics. As part of the course we mention the dangers of static charges, a particular example being the refuelling of airplanes and the need to connect the plane and the refuelling truck with a conducing wire to stop the build up of charge.Wee Alistair, great kid btw, asks, what happens in mid-air refuelling.Being an honest chap I say - actually I don't know but I'll try and find out.So I'm trying.Help, please. I'm sure some of you out there know the answer.My guess, the conducting wire is part of the refuelling rig. But my guesses are usually wrong :-(Thanks in advance,John

[Guest]

Bump,Please,The next lesson is Thursday :-eekThanks,John

[Guest BobL]

JohnI'm going to give this a try. First, the proper terminology is in-flight refueling or IFR. Second, the antistatic wire doesn't stop the build up of the charge. It prevents a spark discharge by grounding the charge. Electricity, static or dynamic, takes the path of least resistance. In the absence of the grounding wire, that path is through the air in the form of a spark. The living room light switch and St. Elmo's fire are two good examples of an airborne static discharge while a spark plug is a dynamic discharge. And a spark in the presence of fuel fumes can be catastophic. A grounding wire provides a path to ground without the spark and the fuel valves are not open at the time the wire is connected. No fumes...no kaboom.In-flight is the same thing. The nozzle contacts the UARRSI prior to fuel being present. Any spark is harmless. Only after a confirmed docking of the nozzle to the UARRSI has been accomlished are both nozzle valves and receiver valves opened permitting a flow of fuel. A secondary consideration are the static wires that most military aircraft have. IN theory, they are supposed to keep the aircraft charge neutral, but they aren't absolute. By confirming a docking first, the aircraft are then electrically bonded and the charge flows through the nozzle contact rather than through an airborne spark.And before anyone asks, commercial airliners are not permitted to in-flight refuel. Under various international treaties, the IFR capability renders that aircraft a military aircraft (you spell that T-A-R-G-E-T), and you don't want civilian passengers being shot at...I'm sure there are probably more engineering specific descriptions of this but it should suffice for a school classroom...Hope it helpsBobL

[Bruce Nicholson KMFR 20]

John,This is a very interesting question. I'll start by saying I don't have the definitive answer, but I do have some clues that may lead to the answer.Here is an article that discusses how static charge is built up in refueling hoses. http://www.globalair.com/discussions/Georg...icle~/msgID=237It also explains the static dissipation construction of fueling hoses.Next is a fluff piece on electro-static discharge rods built into the wings, stabilizers, and tails of modern aircraft.http://www.sri.com/about/timeline/electrostatic.htmlIt's not very complete in its detail, but it does refer to the dissipation of static charges during flight that cause sparks and explosions during refueling.I'd suggest emailing the Royal Air Force and submitting your question. They'd most certainly want to help out with an educational question, and the answer would possibly come from people that are the most familiar with the safety issues and remedies of air refueling.Regards,Bruce

[Guest]

John,This is a really good question, and I will do a little more research, but as I recall from my days as a KC135A pilot - The situation is much different in the air than if one of the aircraft is on the ground. Since neither plane is grounded, they build the same charge as they fly through the air together. Metal to metal contact is made before fuel flow is initiated, reducing the likelihood of a spark. I recall doing refueling in all kinds of weather, but we did avoid refueling if we were erxperiencing St Elmos fire (static discharge).Dale

[Guest]

Thanks for the input Dale,As I understand it the problem is that as the fuel is transferred friction can cause a static charge to build up between the refueller and the aircraft being refuelled. Therefore, as you say, metal to metal contact needs to be made. The question is how was that contact made? I just know that is the question the class will ask :-)Take the point about St Elmo's fire - I hadn't thought of that problem ;-)Thanks again for the help,All the best,John

[Guest]

Thanks Bob,Kind off confirms my thought that you connect the planes electrostatically before refuelling.All the best,John

[Guest]

Thanks Bruce,I knew this was the right place to ask :-)All the best,John

[Guest]

There are two types of in-flight refueling done by USAF. The first is a flying boom mounted under the tail of the tanker, which the boom operator manuevers in the slipstream by controlling the "ruddervators", much like ailerons or elevators on an aircraft. As the receiver aircraft moves into position, the boom operator postions the boom in line with the receiver's refueling receptacle, usually on the top of the aircraft, aft of the cockpit. The operator then extends the telescoping portion of the boom until it slides into the receptacle and "locks" in place, much like a quick disconnect fitting on an air hose. The boom operator and the tanker pilots get an indicator light showing the connection is made. The boom is metal, as are portions of the receptacle, thus giving metal to metal contact. Once the connection is complete the tanker copilot opens a valve and starts the refueling pumps.The second type is called probe and drogue refueling. In this method the tanker extends a flexible hose with quick disconnect fitting on the end and a "basket" around the receptacle which helps to funnel the probe into contact. In this method, the boom operator has no control and it is up to the receiver aircraft to manuever into position, placing his refueling probe into the drouge. Again, when contact is accomplished both receiver and tanker aircraft receive a connection made indication. I seem to recall that the hose was not just a rubber hose, but had steel reinforcements and probably a wire which provided the signal and grounding. The receiver's probe was metal.We sometimes had unexpected disconnects, which resulted in a short spray of fuel before flow was automatically stopped. On several occasions during the Viet Nam conflict, we refueled aircraft that were badly damaged and were leaking fuel from their tanks as we refueled them. I never heard of any of these resulting in a fire caused by static electricity. In the case of refueling B-52's, we were sometimes in contact for as much as 30 minutes.By the way, the first air to air refueling was done between two biplanes around 1926. The pictures show this to be a simple rubber hose, but they may have also attached a grounding wire. I work at the USAF Museum, so I will ask if anyone knows.Hope this helps,Dale

[Guest RichardL]

I believe the term "grounding" is a bit misunderstood.By making metal to metal contact, you are placing both aircraft at the same electricl potential. A spark happens when one aicraft or person is at a different electrical potential than another. That is, one would have a more positive or negative electrical charge; has nothing to do with the "ground".One aircraft could be "grounded" to the earth and still cause an electrical discharge by direct contact with another aircraft. This is why aircraft refueling happens in this sequence:-An aircraft is connected to earth with a cable.-The refueling vehicle is placed at the same electricl potential by connecting a cable to the SAME earth point.-Extra percautions can also be taken by conneting another cable directly from one vehicle to the other.Just because you're "earth grounded" doesn't mean you're at the same electrical potential. Electrical variations can exist (due to soil differences and corrosion) from one earth grounding point to the next. That is why it is best to connect both vehciles to the same earth point and having a secondary connection between vehicles. Richard

[RobertVA 0]

I'm not familiar with the USAF version of the probe and drogue hoses. I have assembled the hose for the version that was used by the Navy KA-6 tanker. Those hoses didn't have electrical lines in or attached to them. The drogue was attached to the hose fitting at a later stage of instalation, but the marker lights were powered by an air turbine on the drogue. The tanker was equiped with a powerfull device to cut the hose free if the reel had problems retracting the hose.Those were some tough hoses. The rubber was at least 1/4 inch thick and had a thick spiral steel reinforcing wire embeded in it. I had to use a specialy adapted heavy duty hydraulic jack (in acordance with engineering directives) to compress the fittings enough to get the threads started.The tanker crew could signal the receiving plane with lights mounted near the hose reel. The refueling probes I saw all had metal tips.

[Guest]

Thank you everyone for your help and for all the information. I has been much appreciated.All the best,John