It hasn’t been that long ago that self-serve aircraft fueling didn’t exist. Many currently involved in the industry got their start as relatively unskilled labor running fixed pumps, driving tanker trucks and climbing ladders to pump aviation fuel. Simple economics dictated the end of full-service fuel dispensing at many airports, replaced with self-serve facilities. The good news with self-fueling is it’s usually cheaper than full-service. But it doesn’t come without a price.
I recently attended a local runway safety meeting. In addition to discussing runway incursions, pilot deviations and other ATC safety concerns, the local FBO wanted to give a safety briefing on operation of the newly installed self-serve fueling facility. I have to admit, I was expecting a real snoozer. Seriously? A lesson on how to pump gas? Turns out it was something for which I’m glad I paid attention. Given the number of fuel-related GA accidents, choices and actions at the pump really matter.
Pumping fuel is serious
If gasoline were invented today, it would probably be illegal. It has 10 times the energy density of high explosives. It is both carcinogenic and toxic if ingested. Exposure to vapors can result in symptoms ranging from lightheadedness and mild confusion to agitation, hypomania or a psychosis-like state followed by headache, nausea, vomiting and potential collapse into a coma.
We take automobile gasoline and its cousins avgas, diesel and Jet A for granted because we have been around them most of our lives. Most of us pump our own car gas and it’s common to pump our own avgas as well. While the future of 100LL may be up in the air, the future of liquid hydrocarbons as transportation fuels is firmly ensconced in the physical chemistry of energy density.
According to the U.S. Energy Information Agency, “Few transportation fuels surpass the energy densities of gasoline and diesel.” Even liquid hydrogen has less hydrogen per gallon than gasoline. Other materials, like uranium, have orders of magnitudes greater energy density, but there are practical limits on the conversion of nuclear power into aircraft engine thrust.
Apart from energy density, what makes avgas, Jet A and mogas so convenient is the fact that they are liquids that can be easily transported, stored and transferred to our vehicles for use. Liquid hydrocarbon fuels flow by gravity or can be easily pumped. After passing through carburetors or injectors, they readily convert to a vapor that can be at least semi-stoichiometrically mixed with air and an ignition source in a way that makes an internal combustion or turbine engine sing.
The very attributes that make a great fuel are the same attributes that should be given some serious respect at the fuel pump. While we worry about sparks and spills, what should give us the most pause is vapor. That is what explodes.
Avoiding The Vapors
Avgas has the dangerous combination of a low flash point and a high vapor density. The flash point of a liquid is defined as the temperature above which the liquid produces ignitable vapors. The flash point of gasoline is -45 deg. F (-43 deg. C). Avgas is not that different from mogas, with a flash point of -040 deg. C, which is the crossover point for F and C temperature scales (-40 deg. C is also -40 F). In practical terms, if their temperature is above -40 deg. F or C, liquid gasoline and avgas are producing vapor that can ignite or explode.
High vapor density simply means that avgas fumes are heavier than air and tend to sink or form a blanket in containment vessels like gas tanks. Avgas has a lower and more uniform vapor pressure than automotive gasoline, so it remains in the liquid state despite the reduced atmospheric pressure at high altitude, thus preventing vapor lock. One of the noteworthy differences between avgas and mogas is the aviation fuel simply smells better.
Jet A, diesel and kerosene have a much lower flash point compared with 100LL. The flashpoint of Jet A/kerosene is 38 deg. C, or 100 deg. F, and the flash point of diesel fuel is 125 deg. F. The lower flash point means jet fuel also tends to have a lower corresponding flame spread. You can easily outwalk a spreading diesel or Jet A fire, which advances at roughly 100 feet per minute.
The flame spread of avgas, however, is 12 feet per second, which is a little faster than eight miles per hour. That is the average human running speed for someone in decent shape. It is considerably faster than a fat guy standing on a ladder with a grounding cable wrapped around his foot.
Another good-to-know physical attribute of a liquid fuel is its autoignition point. Avgas will spontaneously burst into flames at a temperature of 450 deg. C or 842 deg. F. That is pretty high, but still lower than exhaust gas temperatures, or the temperature of an overheated exhaust manifold or turbocharger. If you spill avgas on a surface at the autoignition temperature, prepare to run. The auto-ignition point for Jet A is much lower, between 210-245 deg. C or 410-473 deg. F. There are many engine components of aircraft and ground vehicles that operate at temperatures well above the autoignition point of Jet A.
Once burning, avgas, mogas, jet A and diesel will quickly arrive at temperatures capable of melting aluminum. Once a fuel fire is going, there is very little that can be done without special firefighting equipment.
There actually are a lot of small things to pay attention to before you start fueling. Scan the area around the pump to locate the emergency shut off valve and fire extinguisher. They should always be handy, and the time to locate them is before you start fueling. There is a good chance you may also see a spill kit or kitty litter supply nearby.
As you prepare to fuel, be aware of how you string the hose out to your aircraft; it creates a tripping hazard and you may not want it in the way of your best evacuation path. Also, check the windsock and be aware of which direction is upwind. In the event of a spill or fire, that will be the place you want to be. If you have passengers, that is where you want them staged while you are fueling.
Once you have familiarized yourself with the particular fueling setup, prep the aircraft by chocking it loosely. If chocks are too snug, adding something like 600 pounds of fuel may make them nearly impossible to remove. Chocking is a good idea because even though you may not see a gradient on the ramp, as you add fuel, the aircraft CG will shift in a way that could induce your airplane to roll. If the ramp surface has a noticeable dip or angle, fill the higher wing first and consider setting the fuel selector to draw from one tank only to avoid overfilling the low tank by crossfeeding it.
Do not stage equipment or ladders beneath aircraft surfaces. As you add weight, the aircraft will settle; you don’t want it settling onto an object that could damage the aircraft skin or—worse—a control surface. Some aircraft with outboard tip tanks are very sensitive to filling sequence. Some aircraft, like early Learjets, actually can tip over if fueled incorrectly.
Prepare the plane for accepting fuel. Turn off all electronic equipment in the aircraft. If the aircraft has been preheated, remove all preheating equipment from the area.
The two most common causes of fueling fires involving automobiles are running engines and cigarettes. In addition to probably being illegal during fueling operations, it is also just plain stupid. But Darwin had a point.
While it’s a fairly uncommon occurrence, a Petroleum Equipment Institute study identified an average of 10 fires per year at automobile refueling sites in the U.S. are related to static discharge. Given that we Americans pump gas 11-12 billion times a year, 10 incidents per year is a very low occurrence rate, so we don’t bother grounding cars.
Aircraft, on the other hand—particularly those flown at higher speeds and higher altitudes—can build up substantial static charges. Every FBO fueling operator I have talked to has a story about getting a nasty jolt from a static-soaked aircraft. While big iron is more susceptible, even lowly Pipers and Cessnas can carry a significant static charge.
Additionally, fluids like avgas can accumulate a static charge simply by flowing through lines. The faster it moves, the more charge it can build; conversely, slower fuel flows mean less electrical potential to worry about. Clothing and weather conditions can be another factor. Most of the autofueling static incidents happen during dry winter months. If it is cold and dry, you should probably consider trading your polyester leisure suit or nylon wind pants for a stylish Nomex flight suit.
Every avgas fueling station has what is commonly called a grounding cable used to attach to your aircraft. After using it to bond the aircraft to the fueling station, you should also touch the aircraft somewhere before removing the fuel cap. Once you and the aircraft have the same charge, leave the filler nozzle in contact with the aircraft throughout fueling operations.
If you are hand filling the aircraft from approved fuel containers, the same principle applies: Bond yourself to the airplane by touching it and remain bonded throughout fueling operations. See the sidebar above for additional details.
Careful with Vapors
When filling fuel tanks, the liquid gas you are adding will displace vapors are present in the seemingly empty tank. It is not empty, in fact. Instead, it is full of vapor. These vapors may actually be saturated, meaning they are mostly avgas with little to no oxygen present and, technically, may not be “explosive vapor” because of low oxygen availability. As the liquid in the tank increases, the blanket of fuel vapor is pushed out of the fuel filler, where it will come in contact with atmospheric oxygen. Now you have the right fuel-air mixture for an explosion.
This is not a good place for a cigarette, a lighter or a spark. The reason for all the ritual bonding with the aircraft is to avoid a spark at this precise location. Perhaps the most explosive situation you will encounter while fueling is the “empty” tank, because it has the highest proportional volume of explosive vapor.
As you fill the tanks with liquid, the displaced vapor will begin escaping from the filler hole and will invisibly flow along surfaces to the lowest nearby point as it dissipates in the atmosphere. If the wind is blowing at, say, 20 knots, vapors will dissipate quickly. If the wind is calm, you may notice a mirage-like shimmer of vapor variably affecting the index of refraction.
When on a ladder and opening fuel caps, be mindful that vapor exposure can cause dizziness or worse. If your nose is immediately above the cap, don’t inhale too deeply. Seriously; fumes in high concentration can be overwhelmingly dizzying, cause a person to black out, and can be fatal. Filling a high-wing aircraft from the top step of a ladder with a fuel nozzle in your hand is not a great place to be if you’re going to get dizzy from huffing avgas.
Vapors escaping as you fill the tank will flow downward, so there should be no open flames, sparks or engines running within 50 feet. You also should avoid filling aircraft in hangars or enclosed spaces where vapors can’t dissipate. Fuel vapor explosions have definitely been caused by inappropriate fueling indoors where vapors sink and flow to a low-lying ignition source like a gas water heater’s pilot light.
Operating a fuel lever is pretty basic stuff. Gas nozzles at a certified fueling station will be equipped with a deadman mechanism: If you let go of the handle while fueling, fuel flow will cease. Make sure it works and don’t be lazy or clever and block the deadman open. If you are fueling from something less-than-certified-looking, check the deadman and make sure it operates.
Also be aware that fuel flows from most aviation nozzles can be pretty sporty. Filling slowly will help keep you from overfilling. When it comes to the last little bit, not all tanks are created equal: I have found few FBOs who can fill the main tank on my Cub without overfilling. As it fills, there’s very little warning and, as soon as you see fuel near the neck, it’s time to stop. You need a fast trigger finger or you will have spillage.
Leave room for expansion, particularly in the summer. Avgas often is stored in underground tanks that can be quite cool. Avgas has an expansion rate of 0.00075/deg. F. Applying that formula, 50 gallons of avgas at 60 deg. F, as it might be in the morning or when coming from the underground tank, will occupy 51.5 gallons when it heats up to 100 deg. in your hangar later that day. If you topped off your tanks before putting it away, you may find 1.5 gallons of fuel on the hangar floor, spilled through the vents.
Finish the Job Thoughtfully
If you did everything well, you were spill-free. Unhook your aircraft, then rewind the hose and grounding gizmo carefully. Some of them are seriously spring-loaded or have aggressive retrieval mechanisms. Uncontrolled retrieval or a grinding drag of metal parts along concrete ramps can generate sparks. Not good if there is a puddle of fuel nearby. Even if there isn’t, banging around the equipment isn’t good form, and could mean it’s not available the next time you need it.
If you can’t find a proper holder or cap for the fuel nozzle that keeps it out of the elements, at least secure it so the nozzle is pointed down and won’t collect water or debris. If you wait 15 minutes, you can sump your tanks and drains to verify your fresh load of fuel has no water or other contaminants.
Perhaps the most important thing you do after fueling is verify your fuel caps. After fueling, inspect their O-rings’ condition: If they are weather-checked, cracked, brittle or missing, you have a problem. It’s also a really good idea to put them back on by securing them with the right amount of torque, making sure to align cap fins with the airflow. Failure to secure fuel caps has brought down a lot of planes and can even result in explosions (see the sidebar on page 18).
Avgas is an amazing energy carrier. Safe fueling will ensure that all the potential energy locked up in that precious, sweet-smelling 100LL unleashes its energy in the form of horsepower, not as an insurance claim.
Mike Hart is an instrument-rated commercial pilot who loves the smell of 100LL when it is not due to overfilling his 1954 Cessna 180 or 1946 J3 Cub.