Pilot Courses of Instruction

Aviation Fuels Usage and Historical Notes

Steve Sconfienza, Ph.D.

Airline Transport Pilot

Flight Instructor: Airplane Single and Multiengine; Instrument Airplane

cell: 518.366.3957

e-mail: docsteve@localnet.com

Fuel Usage and Historical Notes

The FAA and the EPA are working on an alternative to leaded fuels, with a phase-out of 100LL being on an 11 year profile from 2012. Various plans exist for unleaded 80/87, 94 and 100. Other alternatives include diesel and turbine engines for light aircraft. Nothing is certain in years into the program.

Notes on aviation fuels with respect to light general aviation aircraft

In the mid-1970s, the fuel producers decided to reduce the variety of fuels produced: as late as the early 1975 there was still wide available three grades: 80/85 (tinted red), 100/115 (tinted green), and 115/145 (tinted purple) -- kerosene (JP1 and on up) is clear. [BTW, 90 octane fuel, tinted blue, had disappeared back in the 60s: note the number of letters in the tint: red (3), blue (4), green (5), and purple (6), in ascending order, the grade of the fuel: there was also a 70 octane at some point, for engines that could run on cheap vodka like the old J3s with 45 hp engines.]

After the ancient 70 octane and later 90 octane, the first fuel to widely disappear was 115/145 (which killed the civilian use of the Lockheed Constellation and the military use of the Grumman Albatross). In the late 1970s, the producers decided that the miniscule amount of 80 octane being produced was more trouble (and cost) than it was worth, so that was discontinued, along with 100 octane, to be jointly replaced by 100LL, low lead fuel with octane for 100 but not so much lead as to clog the low-compression engines designed for 80 octane. It works fine for the 100 octane crowd, but 100LL still has about four times the lead as 80 octane did, so for the low compression engines, IT DID NOT WORK WELL!

By the way, specifically, the lead amounts are as follows. Note that 100LL has over four times the amount of lead (in milliliters/liter) as 80 had.

Table One
Tetraethyl lead concentration

(mL TEL /L)

Source: Chevron Corportion: "Aviation Gasoline Specifications and Test Methods" (page from the Internet Archive)

The venerable Cessna 150's engine was a four cylinder, 100 horsepower, Continental 0-200. This engine was designed to run on 80/85 octane fuel, and could not clear high lead concentraions. To these engines the new 100LL still had far too high a lead concentration, so the 152 was supposed to be more tolerant of the relatively "High Lead" 100LL "Low Lead" fuel. Still, this is a small engine that does not clear the lead out of the cylinders very well, resulting in . . .


Oh, and it can cause valves to stick, too.

What to Do

Pulling the plugs after every one or two flights is not an option. An effective lead scavenger is tricresyl phosphate -- TCP. When the plugs do foul, they typically clear by themselves during run-up (or after what people sometimes refer to as a "hard run-up": running-up very briefly at a higher-than-specified RPM). Fundamentally, this is not the greatest thing in the world to do (engine cooling under high RPMs in an un-moving aircraft sitting on the ground is the issue), but it generally works and works quickly (as opposed to a hung valve, which also runs rough, but is a problem no matter which magneto you check and does not clear).

One scenario for solving the loading problem might be the following procedure:

You should always use your own best judgment; however, when it is a single magneto that is rough, and there is no reason to believe that the problem is not with the plugs (i.e., a magneto failure), there is nothing unsafe or inherently dangerous in attempting to clear the fouled plugs during a briefly extended run-up; if the plugs clear at that point, there is likely not a reason to believe that the aircraft is not safe for flight. So, in summation,

  1. The plug rotation procedure should solve most of the problem.
  2. Try to not idle below 1200 rpm
  3. Take offs are generally allowable if smooth engine operation can be obtained and the rpm's are between 2280 and 2380 rpm.
  4. In most instances, there is nothing to be gained by leaning the mixture below 3000 feet.

By the way, the FAA has published the following concerning "legacy" engines:

Source: FAA General Aviation News, July-August 1987

Other Reasons for Fouled Plugs

Besides the tetraethyl lead problem, the next most common causes of foulded plugs are (1) carbon build-up from running the engine excessively rich and (2) oil seepage from worn rings. Running excessively rich simply means that excessive fuel, thus excessive hydro-carbons, are entering the cylinders. The amount of oxygen available for combustion is insufficient to successfully burn all of the fuel, and some of it winds up on the plugs. Worn rings may allow oil to seep into the combustion chamber, and again it winds up on the plugs.

A Note on Lead and Lead Additives

Lead has been used since ancient times, with recorded use from ancient Rome, ancient Greece, and ancient China. It is used in building construction and plumbing; lead-acid batteries; bullets and shot; SCUBA, fishing, and other weights; as part of solders, pewters, and fusible alloys; and as a radiation shield. It has found uses in paint and in type-setting. Of course, it has been essential in the development of high-octane fuels. Over six millennia it has proven to be a foundation of civilization.

Lead is poisonous to animals, including humans: it damages the nervous system and causes brain disorders. Lead is a neurotoxin that accumulates both in soft tissues and the bones. Excessive lead also causes blood disorders in mammals. Any person using lead or exposed to lead is being poisoned, neurologically damaged. Its removal from the environment is absolutely essential to the survival of -- as far as can be determined -- all life on the plant. There is no valid reason to use this chemical element.

Other Fuel and Fueling Facts

Avgas fuelling nozzles for overwing dispensing are generally painted red (overwing as opposed to underwing attachments), and to help prevent the dispensing of jet fuel into a piston engine aircraft the nozzle of an Avgas fueller is limited to a maximum diameter of 49 mm (in the U.S., 40 mm elsewhere). The diameter of a piston aircraft's Avgas tank filler neck is limited to a maximum of 60 mm diameter. Nozzles for Jet A-1 are larger than 60 mm and thus cannot be placed into a piston aircraft's fuel tank.

Jet Fuel Trivia: "Aviation Turbine Fuel"

Aviation turbine fuels are used for powering jet and turbo-prop engined aircraft and are not to be confused with "Avgas." Outside former communist areas, there are currently two main grades of turbine fuel in use in civil commercial aviation: Jet A-1 and Jet A, both of which are kerosine-based fuels. There is another grade of jet fuel, Jet B, which is a "wide cut" kerosine (a blend of gasoline and kerosine), but it is rarely used except in very cold climates, the gasoline being an addative to keep the kerosine from becoming too thickened from the cold to flow properly. Military turbine fuel exists in a plethora of variants, numbered one through eight (with a few skipped spots).

Civil Jet Fuels

Military Jet Fuels

Aviation Fuel Additives

Aviation fuel additives are compounds added to the fuel in very small quantities, usually measurable only in parts per million, to provide special or improved qualities. The quantity to be added and approval for its use in various grades of fuel is strictly controlled by the appropriate specifications.

A few additives in common use are the following:

  1. Anti-knock additives reduce the tendency of gasoline to detonate. Tetraethyl lead (TEL) is the only approved anti-knock additive for aviation use and has been used in motor and aviation gasolines since the early 1930s.

  2. Scavengers to remove tetraethyl lead, which may be persistent in low-compression engines, include tricresyl phosphate, TCP. During combustion TCP converts tetraethyl lead to lead phosphate, which will not foul plugs or cause stuck valves.

  3. Anti-oxidants prevent the formation of gum deposits on fuel system components caused by oxidation of the fuel in storage and also inhibit the formation of peroxide compounds in certain jet fuels.

  4. Static dissipator additives reduce the hazardous effects of static electricity generated by movement of fuel through modern high flow-rate fuel transfer systems. Static dissipator additives do not reduce the need for "bonding" to ensure electrical continuity between metal components (e.g. aircraft and fuelling equipment) nor do they influence hazards from lightning strikes.

  5. Corrosion inhibitors protect ferrous metals in fuel handling systems, such as pipelines and fuel storage tanks, from corrosion. Some corrosion inhibitors also improve the lubricating properties (lubricity) of certain jet fuels.

  6. Fuel System Icing Inhibitors (anti-icing additives) reduce the freezing point of water precipitated from jet fuels due to cooling at high altitudes and prevent the formation of ice crystals that restrict the flow of fuel to the engine. This type of additive (such as Prist) does not affect the freezing point of the fuel itself. Anti-icing additives can also provide some protection against microbiological growth in jet fuel.

  7. Metal de-activators suppress the catalytic effect which some metals, particularly copper, have on fuel oxidation.

  8. Biocide additives are sometimes used to combat microbiological growths in jet fuel, often by direct addition to aircraft tanks; as indicated above some anti-icing additives appear to possess biocidal properties.

  9. Thermal Stability Improver additives are sometimes used in military JP-8 fuel, to produce a grade referred to as JP-8+100, to inhibit deposit formation in the high temperature areas of the aircraft fuel system.

Power Boosting Fluids

It used to be commonplace for large piston engines to require special fluids to increase their take-off power. Similar injection systems are also incorporated in some turbo-jet and turbo-prop engines. Water or a mixture of water and alcohol will be injected in the air entering the compressor of the engine: the evaporation of this injected liquid extracts heat from the air (heat from the air fuels the change of state, liquid to gas, of the water or water/alcohol mixture), and the result is of a raise in the density of the inlet gasses and thereby a higher compression ratio. The increased compression ratio appears throughout the engine and ultimately increases the exhaust-gas velocity. (It has also been noted that the water will be absorbing some of the heat produced during compression: after combustion begins, the droplets are rapidly converted into steam with a high quantity of heat, promoting rapid expansion upon exhaust.) This effect can be obtained by using water alone, but it is usual to inject a mixture of methanol and water to produce a greater degree of evaporative cooling, reduce the risk of the water freezing, and to provide additional fuel energy.

For piston engines, methanol/water mixtures are used and these may have one percent of a corrosion inhibiting oil added. The injection system may be used to compensate for the power lost when operating under high temperature and/or high altitude conditions (i.e. with low air densities) or to obtain increased take-off power under normal atmospheric conditions, by permitting higher boost pressure for a short period.

Both water alone and methanol/water mixtures are used in gas turbine engines, principally to restore the take-off power (or thrust) lost when operating under low air density conditions. Use of a corrosion inhibitor in power boost fluids supplied for these engines is not permitted.

The methanol and water used must be of very high quality to avoid formation of engine deposits. The water must be either demineralised or distilled and the only adulterant permitted in the methanol is up to 0.5 per cent of pyridine if required by local regulations as a de-naturant. In the past there were several different grades of water/methanol mixtures, e.g. 45/55/0 for turbine engines, 50/50/0 for piston engines (this was also available with one percent corrosion inhibiting oil and was designated 50/50/1) and 60/40/0. There is no longer a lot of demand for this stuff, but there are still companies selling water/methanol units.


Chevron Corportion:
"Aviation Gasoline Specifications and Test Methods" (page from the Internet Archive)
CSGNetwork and Computer Support Group
"Aviation Fuel - AvGas Information Aviation Gasoline"
"Aviation Jet Fuel Information"
Everything 2
Alcor Aviation, Inc.
"TCP Fuel Treatment"
NASA/National Advisory Committee for Aeronautics
"Analysis of Thrust Augmentation of Turbojet Engines by Water Injection at Compressor Inlet"