Only a limited number of additives are permitted in aviation fuels, and for each fuel grade, the type and concentration are closely controlled by the appropriate fuel specification. Addi- tives may be included for a number of reasons, but, in every case, the specification defines the requirements as follows:
Mandatory—must be present between minimum and maxi- mum limits.
Optional—may be added by fuel manufacturer’s choice up to a maximum limit.
Permitted—may be added only by agreement of user/pur- chaser within specified limits.
Not allowed—additives not listed in specifications cannot be added to aviation fuels.
As part of this process, the fuel manufacturer, blender, or handling agent is required to declare the type of additive and its concentration in the fuel. This documentation should accompany the fuel throughout its movement to the airport.
In the case of aviation gasolines, there is little variation in the types and concentrations of additives normally present
in each standard grade, but considerable variations occur in the additive content of jet fuels, depending on the coun- try of origin and whether they are for civil or military use.
Table 10 summarizes the most usual additive content of avi- ation fuels on a worldwide basis (except for Russian grades). Many exceptions occur, and reference to the speci- fication is recommended.
Fig. 1—Standard form for reporting inspection data on aviation turbine fuels. (Source: ASTM D1655-02.)
Additive Types
Additives may be included in aviation fuels for various rea- sons. While their general purpose is to improve certain aspects of fuel performance, they usually achieve the desired effect by suppressing some undesirable fuel behavior, such as corrosion, icing, oxidation, detonation, etc. Additive effec- tiveness is due to their chemical nature and the resulting interaction with fuel constituents, usually on the trace level.
During additive approval, it is important to establish not only that the additive achieves the desired results and is fully compatible with all materials likely to be contacted, but also to ensure that it does not react in other ways to produce adverse side effects (possibly by interfering with the actions of other additives). Individual aircraft and engine manufac- turers, generally called OEMs, normally carry out the approval testing of aviation additives. Their results and conclusions appear in company documents and are then approved by appropriate governmental certifying agencies. Once this proc- ess is completed, international specification groups can review this approval for adoption into specifications. Although addi- tives for civil fuels are listed in industry specifications follow- ing consensus decision, additive listing in ASTM specifications does not constitute ASTM approval, because only the equip- ment manufacturer has the legal authority for additive approval. However, it is up to ASTM to ensure that approvals have been obtained from all pertinent manufacturers before the additive is listed. For military fuels, additive approvals rest with the military authority and are often designed to satisfy specific military considerations. In some cases, military experi- ence is cited as a reason for approving civil use of an additive.
However, civil approval still has to go through the formal process outlined earlier.
To rationalize the expensive approval procedure for avi- ation fuel additives, ASTM Practice for Evaluating the Com- patibility of Additives with Aviation Turbine Fuels and Aircraft System Materials (D4054) was created. It was used in conjunction with ASTM Guidelines for Additive Approval
(Research Report D02-1125), and Compatibility Testing with Fuel System Materials (Research Report D02-1137), the pro- cedure offered the possibility of testing by a single manufac- turer with the results acceptable to others. At the time of this writing (2009), ASTM D4054 has been completely rewritten and was approved at the December 2009 ASTM meeting in Anaheim, California, as a replacement for the old specification. The new specification will retain the same ASTM D4054 designation with the following new title:
Standard Practice for Qualification and Approval of New Aviation Turbine Fuels and Fuel Additives and, Research Report D02-1137 and Research Report D02-1137 will be withdrawn. The subject of this ballot is a complete rewrite of ASTM D4054. The standard practice provides a frame- work for the qualification and approval of new fuels and new fuel additives for use in commercial and military avia- tion gas turbine engines. The practice was developed as a guide by the aviation gas-turbine engine and airframe OEMs with FAA and ASTM International member support. The intent of this standard practice is to streamline the approval process. The objective is to permit a new fuel or additive to be evaluated and transitioned into field use in a cost-effective and timely manner. Its purpose is to guide the sponsor of a new fuel or new fuel additive through a clearly defined approval process that includes the prerequisite testing and required interactions with the engine and airframe manu- facturers, the FAA, EASA, and listing bodies. This standard practice provides a basis for calculating the volume of addi- tive or fuel required for assessment, insight into the cost associated with taking a new fuel or new fuel additive through the approval process, and a clear path forward for introducing a new technology for the benefit of the avia- tion community.
The following paragraphs describe the aviation fuel addi- tives in current use. Table 10 lists the additive types and an indication whether the additives are optional, mandatory, or allowed with specific limitations. No attempt is made to list
TABLE 10—Summary of Additive Requirements for U.S. and British Aviation Fuels
Additive Aviation Gasoline Civil Jet Fuels Military Jet Fuels
Tetraethyl-lead OptionalA Not allowed Not allowed
Color dyes Mandatory Not allowed Not allowed
Antioxidant Optional OptionalB OptionalB
Metal deactivator Not allowedC Optional OptionalD
Corrosion inhibitor/lubricity improver Optional Optional Mandatory Fuel system icing inhibitor (FSII) OptionalE OptionalD Mandatory
Conductivity improver OptionalF Optional Mandatory
Leak detector Not allowedD OptionalD PermittedD
Note.For detailed additive requirements and limitations, refer to individual specification.
AMandatory for ASTM D910 and Def Stan 91-90, max unintentional limit of 0.013 g of lead/L allowed in ASTM D6227.
BMandatory for hydroprocessed fuels in British, major U.S. military, and international civil Jet A-1 fuel.
COptional in ASTM D6227.
DBy customer agreement only.
EUser option but, if required, normally added by aircraft operator.
FMandatory in Canada.
the various chemical and trade names of all approved materi- als, as these will be found in the individual specifications.
TETRAETHYL-LEAD (TEL)
TEL is used widely to improve the antiknock characteristics of aviation gasoline. An adverse side effect of this material is the deposition of solid lead compounds on engine parts, leading to spark plugs fouling and corrosion of cylinders, valves, etc. To alleviate this potential problem, a scavenging chemical—ethylene dibromide—is always mixed with the TEL.
Ethylene dibromide largely converts the lead oxides into vol- atile lead bromides, which are expelled with the exhaust gases. As a compromise between economic considerations and the avoidance of side effects, the maximum level of TEL is carefully controlled in specifications by using tests for Lead in Gasoline (ASTM D5059 or D3341). TEL is not permitted in jet fuels, because lead compounds, even in trace amounts, could damage turbine blades and other hot engine parts.
COLOR DYES
Dyes are required in all leaded fuels as a toxicity warning.
They are also used in aviation gasoline to identify the differ- ent grades. The required colors are achieved by the addition of up to three special anthraquinone-based and azo dyes (blue, yellow, and red). The amounts permitted are controlled between closely specified limits to obtain the desired colors.
The Test Method for Color of Dyed Aviation Gasolines (D2392) is used to determine minimum required color levels, while maximum color is controlled by dye concentration.
In general, dyes are not permitted in jet fuels, except in special circumstances.
ANTIOXIDANTS (GUM INHIBITORS)
Antioxidant additive is normally added to aviation gasoline to prevent the formation of gum and precipitation of lead compounds. The additive type and concentration are con- trolled closely by specifications.
Jet fuels are inherently more stable than aviation gaso- line. Antioxidants are optional, but not mandatory in all cases. To combat the problem of peroxide formation men- tioned earlier, some specifications require the addition of oxidation inhibitors to all hydrogen-treated fuels. Antioxidant use in all hydrogen-treated fuels is probably unnecessary, but it is easier to add the antioxidant to all such fuels than to establish which fuels need the additive and which fuels do not. A maximum concentration of 24.0 mg/L applies for all jet fuels, with a minimum of 17.2 mg/L when the additive is mandatory.
Antioxidants are defined by composition. A wide range of antioxidants is approved with some variations of chemical types among specifications. Hindered phenols predominate among various specifications.
METAL DEACTIVATOR
One approved metal deactivator (N,N0-disalicylidene-1,2-propane diamine) is permitted in jet fuels but not in aviation gasoline.
The purpose of the additive is to passivate certain dissolved met- als, which degrade the storage stability or thermal stability of the fuel by catalytic action. Copper is the worst of these materi- als and is sometimes picked up during distribution from the refinery to the airport. Copper-containing heating coils in some marine tankers have been identified as one copper source. If
thermal stability has been degraded by such copper pickup, it can sometimes be restored by doping the fuel with metal deacti- vator additive (MDA).
On initial manufacture of fuel at the refinery, MDA content is limited to 2.0 mg/L, not including the weight of solvent. Higher initial concentrations are permitted in cir- cumstances when copper contamination is suspected to occur during distribution. Cumulative concentration of MDA after re-treating the fuel shall not exceed 5.7 mg/L.
CORROSION INHIBITORS/LUBRICITY IMPROVERS The corrosion inhibitors/lubricity improvers (CI/LIs) are used, when specifically authorized, in jet engine fuels, for the prevention of corrosion in fuel handling, transportation, and storage equipment. Certain of the inhibitors are also used in automotive gasoline, diesel fuel, and related petroleum products. CI/LI additives are used to protect fuel handling infrastructure from corrosion, but that purpose is usually secondary in aviation fuels. Their primary role is to improve the lubricating properties of the fuel. Lubricity additives may be blended into Jet A-1 per Def Stan 91-91 without prior cus- tomer notification to correct a lubricity problem, but use of these additives in Jet A/Jet A-1 per ASTM D1655 is by agree- ment of the purchaser. Aircraft and engine fuel system com- ponents and fuel control units rely on the fuel to lubricate their moving parts. The effectiveness of a jet fuel as a bound- ary lubricant in such equipment is referred to as its lubricity.
Differences in fuel system components design and materials result in varying degrees of equipment sensitivity to fuel lubricity. Similarly, jet fuels vary in their level of lubricity.
In-service problems experienced have ranged in severity from reductions in flow to unexpected mechanical failure leading to in-flight engine shutdown. The chemical and phys- ical properties of jet fuel cause it to be a relatively poor lubricating material under high temperature and high load conditions. Severe hydroprocessing removes trace compo- nents resulting in fuels tend, tend to have lower lubricity than other fuels, such as straight-run, wet-treated, or mildly hydrogen-treated fuels. Certain additives, for example, corro- sion inhibitors, can improve the lubricity and are widely used in military fuels. They have been used occasionally in civil jet fuel to overcome aircraft problems but only as a temporary remedy while improvements to the fuel system components or changes to fuel were achieved. Because of their polar nature, these additives can have adverse effects on filtration systems qualified to API 1581 Third Edition or older. The API 1581 Fifth Edition has extensive qualification tests using a full package of additive including a CI/LI and has much improved fuel/water separation characteristics with surfactant-laden fuel. Some modern aircraft fuel sys- tems components have been designed to operate on low lubricity fuel. Other aircraft may have fuel system compo- nents that are sensitive to fuel lubricity. In these cases, the manufacturer can advise precautionary measures, such as use of an approved lubricity additive to enhance the lubric- ity of a particular fuel. Problems are most likely to occur when aircraft operations are confined to a single refinery source where fuel is severely hydroprocessed and where there is no co-mingling with fuels from other sources during distribution between refinery and aircraft. ASTM D5001 (BOCLE) is a test for assessing fuel lubricity and is used for in-service troubleshooting, for lubricity additive evaluation, and in the monitoring of low lubricity test fluid during
endurance testing of equipment. However, because the BOCLE may not accurately model all types of wear that cause in-service problems, other methods may be developed to better simulate the type of wear most commonly found in the field.
Both U.S. and British military specifications require the additives on a mandatory basis. U.S. and British military authorities publish specifications for corrosion inhibitors/
lubricity agents, the U.S. specification being MIL-PRF-25017, while the British specification is Def Stan 68/251. Approved additives for each specification are in Qualified Products Lists (QPL), the U.S. list being QPL 25017 and the British list, QPL 68/251. Additives on these lists are approved as individual proprietary materials, and the QPLs show relative effective minimum and maximum allowable concentrations for each additive. As required by MIL-PRF-25017, the Rela- tive Effective Concentration (REC) was determined by the Rusting Test Method; the Minimum Effective Concentration was determined by either the BOCLE or 1.53 REC; and the Maximum Allowable Concentration was determined by the lowest of the following: 54 g of inhibitor/m3 of fuel, 43 REC, microseparometer rating, or the change in electrical conductivity with fuels containing static dissipater additive.
There is currently an international effort to create a single list of approved additives, and ASTM is expected to adopt this coordinated listing into civil jet fuel specifications.
FUEL SYSTEM ICING INHIBITORS (ANTI-ICING ADDITIVE)
A fuel system icing inhibitor (FSII) was developed originally to overcome fuel system icing problems in USAF aircraft.
Most commercial aircraft and many British military aircraft heat the fuel ahead of the main engine filter to prevent the formation of ice by water precipitated from fuel in flight. To maximize aircraft performance, many U.S. military aircraft do not have such heaters, and FSII is required to prevent icing problems. FSII is designed to lower the freezing point of water to such a level that no ice formation occurs.
FSII is now a mandatory requirement in most military fuels, especially those covered by NATO standards. The origi- nal FSII was ethylene glycol monomethyl ether (EGME), known also as methyl cellosolve, methyl oxitol, and 2-methox- yethanol by various manufacturers. When this additive was added to jet fuel for naval aircraft (JP-5/Avcat), it was some- times difficult to meet the minimum 60C flash point, due to the low flash point of EGME (about 40C). Consequently, a new type of FSII was introduced into military fuels consisting of diethylene glycol monomethyl ether (diEGME) with a higher flash point (about 65C) and lower health and safety risks. However, both glycols suffer from poor solubility in jet fuel that has to be overcome by thorough mixing, and they also have a high partition coefficient that causes ready addi- tive extraction by free water. Additive concentration is required to be between 0.10 and 0.15%by volume. Following the introduction of diEGME, approval of EGME as an icing inhibitor was rescinded due to environmental concerns.
Shortly after introducing FSII to combat icing problems, the USAF experienced a great reduction in the number of microbiological contamination problems in both aircraft tanks and ground storage systems. Studies confirmed that this improvement was due to the biocidal nature of the addi- tive. It is now generally accepted that EGME and diEGME are effective biostats if used continually in fuel.
With minor exceptions, commercial aircraft heat the fuel ahead of the engine filter and have no requirement for FSII.
A few turbine-powered helicopters and corporate aircraft do not have fuel heaters, and most operators make their own arrangements for additive injection into their fuel. In tropical areas, some civil aircraft operators require fuel with FSII for its biocidal properties. In these cases, local arrangements tend to be made to inject the additive at the airport.
Although primarily a jet fuel additive, EGME or diEGME is sometimes used as an anti-icer in aviation gasoline for fuel- injected engines. However, for such aircraft, it is more com- mon to use isopropyl alcohol (IPA). The Specification for Fuel System Icing Inhibitors (D4171) defines the properties of all these materials. Concentration limits for the additives are given in the pertinent fuel specification. In addition, the Test Method for Measurement of Fuel System Icing Inhibi- tors (Ether type) in Aviation Fuels (D5006) provides a field method for measuring the concentration of FSII.
It has been observed that when isopropyl alcohol is added to Grade 100 AVGAS, the antiknock rating of the fuel may be significantly reduced. Typical performance number reductions with the addition of 1 volume percent of IPA have been about 0.5 PN for the lean rating and 3.0 to 3.5 PN on the rich rating. Nonetheless, there have been no field reports of engine distress resulting from these effects. Speci- fication for Aviation Gasoline (D910) contains cautionary statements and gives further details on the phenomenon in Appendix XI.
STATIC DISSIPATOR ADDITIVE (CONDUCTIVITY IMPROVER ADDITIVE)
Static charges can build up during movement of fuel and can lead to high-energy spark discharges. Static dissipator additives (SDAs) are designed to prevent this hazard by increasing the electrical conductivity of the fuel, which, in turn, promotes a rapid relaxation of any static charge.
Almost all jet fuel specifications permit the optional use of SDA, but many make it mandatory. SDA is now mandatory in U.S. military grades of JP-8 and JP-4, as well as in Def Stan 91-91 and 91-97. International Jet A-l specifications also contain the requirement. Only U.S. domestic jet fuel leaves the additive as optional, and most such fuel does not contain the additive.
In Canada and the United States, SDA is optional in avi- ation gasoline because the hazards of static discharges are particularly severe under very low ambient conditions.
The only static dissipator additive currently available for use in aviation fuels is Innospec’s Stadis¤450 additive. Its composi- tion is proprietary. The additive is used at very low dosage levels, being limited to 3 mg/L at the time of fuel manufacture and a cumulative total of 5 mg/L after re-treatment. Additive concen- tration is not measured in the field; instead, additive presence is checked by conductivity measurements by D2624 to ensure that fuel electrical conductivity is within specification limits.
LEAK DETECTOR
Leaks in underground portions of fuel systems have long presented detection problems, particularly where such leaks were small but allowed fuel to accumulate underground.
Recent regulations have made periodic system leak checks mandatory. One way of conducting such checks is by the use of a leak detection additive. The only such additive approved for aviation fuel depends on a unique composition (sulfur