05-01-1999 Ask Mike! Archive
Should I install the auto fuel STC?

Dear Mike:
I'm considering installing the auto fuel STC on my bird. What's your take on that?

Danny


Danny.

Auto fuel usage in aircraft poses some interesting questions.  Will it work?  Probably it will, provided the correct octane is used. (Remember, the octane rating system for auto fuel, or mogas, as it is called, is different than for aircraft fuel.) 

Is it FAA acceptable?  Following the approved usage procedures (i.e.: purchasing a Supplemental Type Certificate (STC), having your mechanic properly install the "kit", and abiding by the Flight manual supplement concerning it), certainly makes it sound safe.

What about insurance?  Shouldn't be a problem, provided you follow the approved usage procedures; but this might be something to look for in your policy.

All of this sound good, but let me tell you why I don't use it in my airplane.  First of all, it has a nasty smell to it.  Even things as simple as checking your fuel sumps or dipping for fuel level on preflight can be nauseating, then you put these tools back in the cabin with you.  Talk about accelerating air-sickness for passengers!  Secondly, the mogas is blended differently during different seasons.  This can cause vapor pressure problems if you don't fly for a few months; suddenly, it's hot, the fuel was "winter mix", and you're at altitude asking for a vapor lock. Third, aircraft engines were designed to use leaded fuels.  (Yes, most 80 AVGAS is now lead free; for that, just run an occasional load of 100LL. Used sparingly, you most likely won't have a problem.)  Fourth, few airports have a pump selling mogas.  Therefore, you have to haul it yourself.  Overwing fueling from jerry cans can lead to contamination of the fuel, and spillage.  It could run down the wing, enter the cabin area, and contribute to the cabin aroma.  There could be problems with static electricity, as most often this type of operation happens at the tie down or hangar areas, and no grounding provision is made.  Fifth, the STCs calls out a certain type of fuel, under a specification called ASTM.  I'll bet if you asked at the local BP, Conoco, or other gas station, the attendant would just give you a blank stare if you asked what the ASTM number was for their product.  Sixth (and finally), the cost factor.  My airplane consumes 6 gallons per hour.  With all taxes, I pay about $2.30 per gallon of avgas at my home airport, which translates to $13.80 per hour for fuel expenses. Last week, I bought gas for my car.  I paid $1.67 per gallon, so I'd only save $3.78 per hour direct operating costs.  Obviously, if my bird consumed more fuel or auto fuel prices go down again, that figure would grow, but I have to ask myself, is that $3.78 worth the hassles in one through five, above?  Not to me.

Referencing the new grade of 100, my understanding is the only steps taken toward this is the establishment of the ASTM specification number.  They are still working on all the actual specs., it sound as if this won't be to market anytime in the next ten years.  

Gear Green,
Mike