|No Breakdown Lanes in the Sky
Safety Concerns Dominate the Specification of Aviation Fuels
by Mike Farmery
Balancing the demand for development and change in aviation fuels specifications with the overriding need for safety is a big challenge for Subcommittee D02.J of Committee D02 on Petroleum Products and Lubricants. Here is a brief review of the current issues.
If you want to know what is going on in the world of aviation fuels, ASTM Subcommittee D02.J on Aviation Fuels is a pretty good place to start. It represents one of the most important international meeting points for all those interested in aviation fuels. J, as its known, provides the campfire around which sit airframers, engine manufacturers, fuel suppliers, filter manufacturers, military, airlines, private flyers, other specification writers, petroleum associations, U.S. Federal Aviation Administration representativesin fact just about everybody who has an interest in aviation fuel.
With such a wide group of stakeholders, many of whom are both real enthusiasts as well as dedicated professionals, there is inevitably a lot of talk around that campfire. However, the sentiment expressed by Oscar WildeThere is only one thing in the world worse than being talked about, and that is not being talked abouthas a real resonance here. All views need to be heard and, when it comes to the technical and specification issues relating to aviation fuels, Subcommittee J is where you will hear them. Once the technical case has been made the specification moves on. It may seem slow at times but aviation is a conservative industry; if given the choice, most passengers would prefer scheduled flights to test flights.
Subcommittee J looks after the jet fuel specifications D 1655 (Jet A and Jet A-1), D 6615 (Jet B), D 910 (Aviation Gasolines 80, 100LL and 100), and D 6227 (Aviation Gasoline 82 Unleaded). It is split into five groups. Progress is made via discussion, balloting, voting, more discussion, balloting, voting, and so on. The technical work that supports the debate is usually conducted either by a members organization or by the Coordinating Research Council (CRC).
All this said, what is actually happening in J? Here is a brief walk through the talk of Js five groups.
J.01TURBINE FUEL SPECIFICATION
Group J.01 has overall responsibility for the jet fuel specifications. Traditionally, these fuel specifications have been performance-based rather than compositionally based and have assumed conventional manufacture from crude oil. However, a range of issues from how to deal with components made from non-traditional sources by new processes (so-called synthetic fuels) to assuring that fuels meeting the specifications really are fit for purpose, are suggesting the need for change. There is discussion about including a material specification that sets limits for properties that are assumed but not normally tested, such as dielectric constant and bulk modulus. More compositional requirements may also be introduced as old-fashioned test methods are replaced. J.01 is leading this whole debate.
There is also increasing discussion about how to deal with additives, particularly their approval by equipment manufacturers and users, and how to control their addition and documentation in the distribution system. Other topics are the rationalization of the Jet B specification in the light of the Canadian military switching from wide cut fuel to kerosene, and the whole issue of harmonizing Western and Russian fuel specifications.
(Any discussion of J.01s activities could not be complete without reference to long standing chairman Kurt Strauss who has just stepped down. Kurt, a J legend in his own lifetime, has made a great contribution and will be a hard act to follow; we are fortunate that J has some good folk waiting in the wings.)
Aviation gasoline has Group J.02 all to itself. Undoubtedly the big issue continues to be the quest for an unleaded replacement for aviation gasoline. Progress has been mixed. On the positive side, a low octane unleaded aviation gasoline (82UL) was championed by Cessna and now sits on the books as D 6227. The high octane side (100/130) has been much more challenging. Tetra-ethyl lead really is a great octane booster. Achieving the same performance without lead is proving a difficult trick and there is even evidence that it is not so much the octane that the aero piston engines like but rather the lead itself.
With the aviation gasoline powered aircraft fleet, there is also a big legacy issue that effectively slows progress. Any unleaded replacement for aviation gasoline 100 will have to meet the needs of the existing fleet, much of which is pretty old. The race has more to run but the stakes are getting higher. TEL manufacturers are dropping out and environmental pressures on lead are certainly not going away.
Solutions may come from left field. For example, aero diesel engines that operate on Jet A are becoming retrofit options, and electronic timing control and engine management may deliver equivalent performance from lower octane fuels. The old 91/98 grade has been resurrected to test this out.
Embracing and responding to the rapid developments in the very small aircraft sector is another real challenge for this group. The use of automotive gasoline in aviation is not the best solution but demonstrates a response to a need that the industry, and this group in particular, must meet.
J.08COMBUSTION CHARACTERISTICS AND THERMAL STABILITY
Given that the ultimate purpose for jet fuel is combustion, there are relatively few specific parameters controlling combustion performance in the specification. Moreover, the test methods concerned are relatively old and crude (smoke point is a good example). The challenge for Group 8 is all about modernizing the test methods and introducing more meaningful parameters that better define fit for purpose. In this way both fuel availability and engine performance can be optimized simultaneously. Promoting the use of high pressure liquid chromatography for aromatics determination is a good illustration. This need for better test methods is becoming increasingly apparent; it is driven by the development of new combustion systems to meet the ever more demanding performance and emission requirements for engines and airplanes.
Combustion is not the only part of the specification that has to face up to non-ideal test methods for important performance parameters. Whilst standing the industry in good stead for many years, the jet fuel thermal oxidation tester (JFTOT) does have shortcomings. With the increasing pressure being put on fuel thermal stability from engine manufacturers seeking better and better performance and the growing interest in thermal stability additives, the need for more quantitative methods is acute. The elipsometric tube analyser and high Reynolds number thermal stability tester are two new developments and both may have roles to play. Work at Southwest Research Institute to quantify the effects of red dye contamination in Jet A has focused on thermal stability and results should help to link test methods to performance in more realistic situations such as nozzle fouling.
J.09ADDITIVE RELATED PROPERTIES AND ELECTRICAL PROPERTIES
Additives represent a real challenge for the specification. Traditionally, performance-improving additives have been a slow-moving line for aviation fuels, typically requiring seven to 10 years for adoption. Unclear, lengthy, and complex approval regimes have created a mindset that such additives may be good for automotive fuels but not aviation. However, we may be witnessing a change. The success of the USAF JP-8 +100 additive has raised awareness of the potential for additive solutions for aviation fuels. For example, there is an overwhelming business case for the approval of drag reducing additives in jet fuel to de-bottleneck supply pipelines to many major airports. While this approval process is currently in full swing, those leading the charge have had to constantly break new ground. The industry desperately needs a flow chart for how to approve a new additive and Group 9 is busy with this undertaking. This is not an easy job. Approval of additives carries the same legacy issues that an unleaded aviation gasoline has to deal with. Whilst screening tests can highlight big problems, there is the question of continual use in old equipment that cannot be ignored if additives are to be approved in the specification. Moreover, impacts on fuel handling and distribution systems must also be considered.
With airlines wanting to do more of the longer, higher, colder type of flights, cold flow properties of jet fuel are coming under the spotlight. Wax modifying additives similar to those used in diesel fuel could either improve performance or allow a wider cut of the barrel, thereby improving availability.
Another big issue that Group 9 has been considering recently is should static dissipator additive be required in Jet A in the United States? This question is especially important, given that it is used extensively in Jet A-1 outside of the United States. The Coordinating Research Council is currently looking at this issue on behalf of Subcommittee J and plans to report this year. Independent of whether the case for inclusion is made or not, the operation of the U.S. pipeline system makes dosing at refinery difficult to envisage. In fact, the widespread use of clay filters makes doping anywhere other than terminals or the airport very difficult to implement.
Significant levels of bulk or dispersed water in aviation fuels are a real showstopper. The presence of even trace concentrations of surface-active agents can hinder water and dirt separation by settling or filter coalescence. The search for a test to predict and control the water separation characteristics of jet fuel has become something of a holy grail. The current micro-separometer (MSEP) test is acknowledged to be flawed, especially as it is overly sensitive to the presence of the static dissipator additive used to improve conductivity (which itself does not normally cause water separation problems). Recent work by the U.S. Navy, soon to be published, may shed light on how to improve this test, but there is a strong body of thought that says trying to mimic coalescence, as the MSEP does, is no longer the best approach, especially now that most fueling vehicles use water absorbent filters instead of filter water separators (coalescers).
There is no doubt that the specification needs to control the level of surfactants but maybe it is those surfactants themselves, which promote emulsification of water and hinder settling, that need to be limited.
If all this was not enough, Group J.10 must come to a common view on the value of the old water reaction test, decide whether an off-the-shelf filtration time test is a good means for controlling dirt levels in fuels and revise the ASTM Manual 5 on Quality Control for Aviation Fuels.
So what are the common threads that summarize all this activity? One of the key challenges for Subcommittee J is the ongoing development of fuel specifications in a direction that is responsive to the needs of all the key stakeholders. This will require improved understanding of the relationship between composition and performance and, moreover, new test methods that accurately predict fuel performance in the new generation of aircraft and engines. The specifications will have to become more transparent and robust to components from non-traditional sources and additive approvals must become simpler and cheaper without jeopardizing flight safety. All this is needed in an industry that is focusing increasingly on costs.
Where will the resources come from? With the traditional big contributing companies and organizations asking themselves why they should finance all the activity, it may be time for funding of these initiatives via industry levies where all beneficiaries pay their contribution via ASTM or the CRC. For example, the recent cross-industry funding of the red dye program via the CRC could become a model for future big initiatives.
Whichever way, there is certainly enough to keep J talking for a long time to come. //
Copyright 2002, ASTM