Fuel system icing and thoughts on ISO-HEET vs. Isopropyl Alcohol

[SpamPilot]

I previously reported the one encounter I have had to date with fuel system icing, causing the engine to sputter, briefly, due to fuel starvation, despite having sumped the tanks and found no water in them.  Since then I have remained vigilant about checking my fuel for water and using 99.9% isopropyl alcohol prophylactically, particularly during winter months.  This weekend I found water in my fuel and thought I should share my experience.

When removing my fuel tank caps, I am careful to dump any water that has accumulated under the cap lever away from the tank opening.  I do see occasional drops or blobs of water on the tank flap under the cap.  By some magic of surface tension, water does not seep past the flap and instead collects on top of it, allowing me to remove it with a paper towel.  Fuel drains right past the flap, so the flap acts as a sort of fuel/bulk water separator.  I use silicone gaskets for the fuel cap.  I have never seen any more than a drop of water at most in my sample cup when I sump the tanks, and even that's rare.

This weekend I drained several cups of fuel and all I saw was this:

 

 

The fuel was an opaque milky blue.  Adding a small amount of isopropyl alcohol turned it clear, confirming the presence of entrained water.  This was a surprise, as I had added about 12 ounces of 99.9% isopropanol to each tank the previous evening before getting a fillup from the FBO fuel truck.  I reported it to the line guy, who affirmed he had personally checked the fuel in the truck for water and found none (my suspicion is that he only checked for bulk water; if there is a water absorber on the truck to pull suspended/dissolved water out of the fuel, it probably needs to be changed) .

Unfortunately, I was not as prepared as I should have been for the trip and had just used the last of the isopropanol I normally carry in my airplane.  I get 99.9% "medical-grade" isopropanol off of Amazon (e.g. https://www.amazon.com/gp/product/B08HX5VCYJ).  It's currently $10/quart, which I consider a good value.

Local drugstores carried only 91% isopropanol, which I elect not to use since I assume the remaining 9% is water and that seems like too much.  I found this discussion on MooneySpace where people mentioned using ISO-HEET (red bottle) successfully.  I ran out to an auto parts store and bought every bottle on the shelf.

Here are my thoughts and observations about ISO-HEET:

- In the discussion mentioned above, neilpilot stated that ISO-HEET is "100% alcohol" and kortopates stated that ISO-HEET "is 99& alcohol and 1% proprietary".  Neither cited a reference, so I was cautious in interpreting those claims.  I found two MSDS for ISO-HEET, and they have potentially conflicting information:

https://www.in.gov/fssa/thehub/files/heet-antifreeze-iso-gas-line.pdf, dated 2010, supports kortopates' claim that it is 99% isopropyl alcohol and 1% "proprietary".

https://media.napaonline.com/is/content/GenuinePartsCompany/136832608pdf?$PDF$, dated 2019, says something different.  It says that ISO-HEET is >=75% isopropyl alcohol and 0.075-0.15% ethylbenzene.  Ethylbenzene is apparently a normal component of aviation fuel, so perhaps it's nothing to worry about, especially in such a low concentration.  What worries me is the up to ~25% that isn't mentioned.  An MSDS "must list the chemical name of all hazardous ingredients", so apparently up to ~25% of ISO-HEET could be non-hazardous chemicals.  Not sure what that could be other than (mostly) water...  Not knowing what it is doesn't give me any comfort.

- neilpilot stated, without reference, that "One bottle [of ISO-HEET] is the perfect size for one tank".  Even assuming that ISO-HEET is 99% isopropanol, my previous experiences using isopropanol suggested that a mere 12 oz bottle of it might not be enough.  So here for everyone's benefit is the result of my original research.  You see from the image above that I had a full tank (40 gallons) of milky blue-white 100LL (at an ambient temperature of 45F) to which about 12 ounces of 99.9% "medical grade" isopropanol had already been added, and it obviously wasn't enough to clear up the fuel.  First, I added a small amount of ISO-HEET to the sample shown in the picture and it turned clear, which I interpreted to mean that the entrained water was now successfully dissolved into the fuel, so the ISO-HEET was "doing its job".  Then I added not one but three 12 oz bottles of ISO-HEET to the 40 gallon tank and sloshed it around for several minutes by pushing on the wing.  I then sampled the fuel and it was still milky blue-white.  Ultimately I had to add eight bottles of ISO-HEET and slosh it around before the fuel cleared up.  That's 3 quarts, and that's on top of the ~0.375 quarts of 99.9% isopropanol I had already added.

Note that Mooney SIM20-64 permits up to 3% isopropyl alcohol by volume for the M20K 231 (and 1% for other models, based on the respective engine manufacturer's recommendations).  3% by volume is 4.8 quarts in a 40 gallon tank.

I flew the airplane home and the engine ran fine.

- I want to say something about the difference between the (certified) airplane world and the automotive world.  ISO-HEET isn't made for the airplane world.  I'm sure its manufacturer would completely disown any claim to be a suitable fuel system de-icer for aircraft.  More to the point, there is no legal requirement, beyond stating certain broad ranges on an MSDS, to confirm that the product is guaranteed to have a specific composition or achieve a specific result.  There is no legal requirement that it even be the same thing from year to year or production lot to production lot.  The bottle itself doesn't make any specific claims about the composition of what's inside it, only that it "helps prevent fuel line freeze-ups" (this is a meaningless marketing claim, not a performance claim) and that it does contain at least some isopropyl alcohol.  When I was pouring out all those ISO-HEET bottles into my tank, I noticed that some of the product was colorless water-clear and some was almost urine-yellow.  I take that to mean that different ISO-HEET mixtures could have significant compositional differences and still be sold as ISO-HEET.  That's not comforting when airborne and your life is on the line, but it's perfectly acceptable for the automotive world (and typical of so many aftermarket automotive chemical products that are marketed based on little more than snake oil claims).  As such, you should definitely not interpret my successful flight home as an indication of ISO-HEET's suitability for this application.

- Somewhere I saw a comment that, instead of ISO-HEET, one pilot advocated using E85.  I considered E85 briefly until I looked up its composition.  E85 can have as little as 51% alcohol and still meet the ASTM-5798 spec.  E85 octane rating is determined by a much different method than 100LL, and again it's an automotive product, not an aviation product, so not subject to the same rules.  Whether or not it works, I suspect both manufacturers and the FAA would argue you are crossing the line between using a recognized fuel system additive and fueling your airplane with an unapproved fuel.

- I'm going to stick with 99.9% "medical grade" isopropanol.  It's not an aviation product and isn't sold through a distribution network that can guarantee traceability, conformance with standards, and accountability for problems (like aviation fuel is).  But it is a recognized fuel system additive for this purpose, it's a nearly pure chemical, and the product I buy is from an established supplier (assuming it's not counterfeit).  It's also marketed under a very specific compositional claim that would expose the supplier to a certain level of legal jeopardy if they intentionally or unintentionally violated that claim.

 

One more thing to mention for completeness:  Both tanks had milky blue-white fuel, but one tank required only about a quart of ISO-HEET to clear up.  That was the tank that got less fuel from the FBO truck.  The aircraft did sit through rain, but it sits through rain all the time at my home airport year-round and I have never had entrained water as a result.

Edited December 29, 2024 by SpamPilot