The specifications for UL 102 are described in this Wikipedia article. The fuel appears to exceed the specifications of 100LL in octane number and energy per unit volume although the fuel weighs 7 pounds per gallon instead of the 6 pounds per gallon for 100LL. Is the additional weight the reason the FAA rejected this fuel as an 100LL alternative?

  • $\begingroup$ Technically, denser fuel is advantageous: in a burning mixture, the masses matter, not volumes. So you can take more fuel in the same tanks. (Weight/balance must be done by mass anyway, so those will be ensured). Whether it is compatible with existing injection and carburettor systems is another matter, and I guess this is the stumbling block... $\endgroup$
    – Zeus
    Aug 8, 2019 at 0:38

2 Answers 2


The problem is basically bureaucratic delays, helped along by easing of EPA timetables on the banning of leaded fuel, taking some of the pressure off, which has caused the developer of UL102, Swift Fuels, to suspend its 2nd Phase testing until the machine gets moving.

  • 1
    $\begingroup$ @ John K: I found a press release from Swift Fuels that says that Swift intentionally exited the PAFI program and sought out a different route for certification so I guess the fuel was not actually rejected by PAFI. I did hear that PAFI was a bureaucratic nightmare and many manufactures refused to participate. I had assumed the fuel was rejected because PAFI announced they rejected all submitted fuels but it look like UL102 is no longer part of PAFI. $\endgroup$
    – DLH
    Aug 8, 2019 at 19:26

AOPA article on unleaded fuel from Oct 2018.


The FAA has re-evaluated its Piston Aviation Fuels Initiative schedule, reaffirming its commitment to develop a replacement for leaded avgas but delaying expected testing completion from December 2018 to mid-2020.

The new timeline focuses on testing of an unleaded fuel being developed by Shell—now the lone participant in PAFI following the elimination of Swift Fuels’ candidate fuel.

A more recent article, July 2019


A fuel under evaluation as a replacement for leaded avgas for the 170,000 aircraft in the piston-powered general aviation fleet will need “additional refinement,” and three new fuels have undergone preliminary testing in 2019, the FAA said in an update on its Piston Aviation Fuels Initiative.

According to the June 2019 program update, “PAFI's focus during the first 6 months of 2019 has included testing at the William J. Hughes Technical Center of an optimized Shell fuel and screening testing of 3 fuels not previously part of the PAFI program. The scope of PAFI has continued to evolve with the preliminary evaluation of 3 other fuels representing PAFI's commitment to research and evaluate all candidate unleaded fuels. Test results with the optimized Shell fuel were not successful with testing indicating additional refinements are required.”

AOPA's article appears to be an excerpt from this FAA article which has more details


  • $\begingroup$ My plane's a homebuilt so I just use mogas with no need for an STC. I run some 100LL through it from time to time and also put some 100LL when it sits for the winter. Modern mogas is amazingly clean. I got a quart or so spilled on me when a friend dropped a gas can from the upper wing of his biplane and once it evaporated there was no residual odor on my shirt. Even my wife couldn't tell it had gasoline on it. $\endgroup$
    – John K
    Aug 8, 2019 at 20:24
  • $\begingroup$ 100LL is the same re; evaporation. How high have you flown with Mogas? (which is no longer offered in many places, like my home field). What is the compression ratio on your cylinders? My engine is O360-A1F6D, 360 cubic inch, 180HP engine 8.5:1 compression ratio cylinders needs 100/100LL min per this Lycoming document: Operator’s Manual Lycoming O-360, HO-360, IO-360, AIO-360, HIO-360 & TIO-360 Series Approved by FAA 8th Edition Part No. 60297-12 October 2005 lycoming.com/sites/default/files/… $\endgroup$
    – CrossRoads
    Aug 9, 2019 at 4:18
  • $\begingroup$ Up to 9000 ft.. My O-290 is 7.5:1. In Canada the STC/STA has a 6000 ft restriction if you put it on a certified a/c, but not in the US. The comp ratio's not that big a deal. Believe it or not the STA in Can is available for various supercharged radials like the 985. My glider club runs mogas in its O-540 Pawnee and 160HP O-320 Citabria. Since your engine can scavenge the lead on 100LL and there are limitations to mogas and if you can afford the plane itself you can afford the gas, you are better off with 100LL anyway. $\endgroup$
    – John K
    Aug 9, 2019 at 13:32

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