In the Aug. 10 issue of GANews there was a letter to the editor in response to my article on unleaded avgas and the insanity of developing a marketable product (Unleaded avgas: You’d have to be insane to try to develop it, June 6 issue). The letter stated “”What’s to develop? Leave out the lead and 100LL automatically becomes 95UL. Since virtually the entire GA fleet is already technically capable of running on 93-octane car gas, why would that be insane?””
In the Aug. 10 issue of GANews there was a letter to the editor in response to my article on unleaded avgas and the insanity of developing a marketable product (Unleaded avgas: You’d have to be insane to try to develop it, June 6 issue). The letter stated “”What’s to develop? Leave out the lead and 100LL automatically becomes 95UL. Since virtually the entire GA fleet is already technically capable of running on 93-octane car gas, why would that be insane?””
I really do not know where to start here. First off, 100LL is actually 100/130LL. When you take the base product without the lead, the lean rating ? the 100 part ? would probably be in the 93 to 94 range. It would take additional refining to ensure that it would consistently meet a 95 limit. Then we have the 130 ? or rich ? rating requirements, which seems to be the more important of the two octane specs. This spec has very poor repeatability on current fuels, and when you test an unleaded fuel it gets a lot worse.
The really big problem is when people test non-alkylate fuels: The results are basically worthless. If the industry took out the 130 part of the spec and lowered the lean rating to even 95, no one ? and I mean no one ? could tell you which aircraft engines would knock and under what conditions.
It is very critical to understand that the industry cannot just assume that all future products will be chemically like the current product, only without the lead. The ASTM specification lists all of the performance requirements for a product. If, in the future, some company found it could meet the spec with some non-conventional fuel, that company could market it even though the fuel may not meet the actual requirements of aircraft in the field.
The letter also seems to make the most common mistake when discussing octane: It assumes that all octane numbers are the same. A 93 octane car fuel is not just two numbers less than a 95 lean rating fuel. In most cases 93 octane car gas would have about an 88 or 89 lean rating, or a difference of six to seven numbers.
The letter goes on to state that virtually the entire GA fleet is already technically capable of running on 93-octane car gas. The actual figure is that about 80% of the fleet can run on 80/87 fuel ? but not all of them are STC’d for auto fuel. The problem is that the other 20% that cannot run on 80/87 rated fuels consume about 80% of the avgas sold in the U.S. If the oil companies did market an unleaded 95 lean rating fuel, they would have to build a second fuel delivery system for a very few gallons. The cost for this would be very high.
The letter closes with “”the definition of insanity is that 100LL still exists in 2007.”” No, the definition of insanity is trying to solve a complex issue by ignoring technical realities and not even considering today’s world of high liability requirements. As the great philosopher Pogo once said “”we have been to the mountain and we have seen the enemy, and the enemy is us!””
P.S.: Thanks for the letter. I appreciate the feedback.
Ben Visser is an aviation fuels and lubricants expert who spent 33 years with Shell Oil. He has been a private pilot since 1985. You can contact him at [email protected].
