I understand that 100% Isopropyl Alcohol (IPA) is used as a Fuel System Icing Inhibitor (FSII) additive in aviation gasoline systems—including 87 through 100LL fuel types—for cold weather operations. I know that IPA is, or can be, included as part of AvGas formulation. However, I am referring to operational use of IPA as an additive when fueling the aircraft. Pure IPA is also known as anhydrous isopropanol.

  • What documentation is available to support such use?

  • What is a proper treatment ratio for using IPA as an FSII (or what is the concentration limit)?


3 Answers 3


FAA Advisory Circular AC No: 23.1521-1B deals with use of IPA for Part 23 aircraft. It states:

ASTM D 910, Standard Specification for Aviation Gasolines, allows the use of isopropyl alcohol conforming to the requirements of ASTM D 4171, specifications for Fuel System Icing Inhibitor, as a fuel system icing inhibitor. Accordingly, isopropyl alcohol conforming to ASTM D 4171 may be used in concentrations up to 1 percent by volume, to benefit safety, as an icing inhibitor in automobile gasoline.

However, it is left to the aircraft manufacturers (according to ASTM 910 too) to determine whether or not to use IPA as an additive. For example, in a letter regarding Lycoming engines in certain of their aircraft, Cessna says,

Isopropyl alcohol in amounts not to exceed 1% by volume can be added only to aviation fuel (not automotive fuel) to prevent ice formation in fuel lines and tanks. Although approved for use in Lycoming engines, do not use isopropyl alcohol in the aircraft fuel systems unless approved by the aircraft manufacturer.

Note that Cessna allows the addition of IPA only to aviation fuel for the applicable aircraft. In UK, the MoD Defense Standard 91-90 again leaves the matter to the individual manufacturers, saying,

The concentration of Isopropyl Alcohol shall be recommended by the aircraft manufacturer at the time of delivery to the purchaser and typically not exceeding 1% v/v.

Though almost all these cap the IPA at 1%, the best way is to refer to the individual aircraft's appropriate manual as it would have the necessary details. But generally, I think that manufacturers (of large aircraft, atleast) prefer not using IPA these days due to environmental concerns.

  • $\begingroup$ I wonder what the effect of a Peterson Auto Fuel STC is on Cessna's note regarding automotive fuel. $\endgroup$
    – J W
    Commented Jan 15, 2016 at 19:41

It seems as though there was an accident involving a PA-23-180 a while back that lead to this advisory. You can see the full report here (interestingly this was published by the NTSB not the FAA).

Require that Piper, Beech, and other airplane manufacturers who have not already done so issue service and operating information regarding the use of fuel additives in piston-powered airplanes for cold weather operation and incorporate this information in the pilot operating handbooks of all newly manufactured airplanes. (Class 11, Priority Action) (A-85-79)

In terms of limits it looks like it might be in the POH and should be in the AFM for the aircraft or in a service bulletin like this one issued for MD helicopters.

The AFM for this 172S (page 4-23) notes the use of IPA and redirects to section 8 (page 8-17) which says a 1% by volume quantity should be used. It also offers some steps on how to add it to the fuel and a nice chart on volume of additive vs volume of fuel.


I fly a PA-30 Twin Comanche regularly,and,since the POH does not discuss use of isopropyl alcohol or Prist,have contacted Piper repeatedly to seek approval.No response has come from Piper.AOPA and Int’l Comanche Society are equally silent on this topic.I even requested that Air Safety Foundation look into it. I consider this issue of great importance, as the PA-30 fuel system lacks individual drains for main & aux tanks, putting it at high risk for ice formation in its fuel system.This system, along with its suboptimal procedure of draining the fuel bowl via the selector valve was the subject of criticism by NTSB in reviewing a Piper Apache fatal accident blamed on icing and obstruction of the fuel bowl.This led to a Safety Recommendation, apparently ignored by Piper.Beech & Cessna both had statements of approval in POH for use of IPA or Prist for my previous A36 & C310 aircraft. Why is everyone so mute on this subject? Piper ought to be held accountable as irresponsible if preventable fuel system icing causes a future accident.

  • $\begingroup$ The answer by Dave indicates that Piper should rectify this, if they haven't, I'd suggest posting a new question about how to address that issue as it will get you the best exposure and most advice. $\endgroup$
    – zymhan
    Commented Aug 29, 2019 at 16:29

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