U.S. Department

of Transportation

Federal Aviation

Administration

 Advisory

 Circular

Subject: USE OF AIRCRAFT FUEL ANTI- Date: 18 Jan 72 AC No. 20-29B

ICING ADDITIVES

Initiated by : FS-140 Change:

 1. PURPOSE. This circular provides information on the use of

 anti-icing additives PFA-55MB and MIL-I-27686 as an

 acceptable means of compliance with the Federal Aviation

 Regulations that require assurance of continuous fuel flow

 under conditions where ice may occur in turbine aircraft

 fuel system.

 2. CANCELLATION. AC 20-29A, effective June 19, 1967, is

 cancelled.

 3. REFERENCE REGULATIONS. Part 25, Section 25.997(b), of the

 Federal Aviation Regulations.

 4. BACKGROUND.

 a. FAA - The Southern Region approved the use of a 0.10

 percent concentration (by volume) of PFA-55MB in all

 turbine aircraft fuels in December 1962. This approval

 was based upon compatibility tests of the additive in

 certificated aircraft and engines at a concentration of

 0.15 percent of the fuel by volume and did not include

 approval for the use of the additive in lieu of fuel

 heaters. FAA may approve the 0.15 percent concentration

 where compatibility with the aircraft and engine fuel

 systems has been demonstrated (see Advisory Circular

 AC 20-24A).

 b. Air Force - The United States Air Force, after extensive

 tests with PFA-55B in JP-4 fuel, concluded that the

 additive satisfactorily prevents the formation of ice in

 the fuel. Starting in April 1, 1962, this additive,

 under its Military Specification No. MIL-I-27686A to D

 was used in all JP-4 fuel produced in the United States.
 Since January 1971, anti-icing additive to Military

 Specification MIL-I-27686E has been used. The main

 change involved in this revision to the specification is

 deletion of the glycerin. Because of the possible

 effect on fuel tank coatings and seals, the use of the

 new MIL-I-27686E additive should be cleared for each

 aircraft as being compatible with the fuel system of the

 aircraft. This is discussed in paragraph 8 below. At

 the present time, with the exception of a few areas, the

 use of the additive in JP-4 fuel is worldwide.

 5. USE OF ADDITIVE IN KEROSENE. The characteristics of these

 additives, PFA-55MB and MIL-I-27686, in kerosene type fuel

 are similar to those in JP-4 fuel. Commercial aircraft

 kerosene, however, does not contain the additive and

 provisions for blending it into this type of fuel present a

 complication to its use. The additive is readily soluble in

 water but its solubility in the fuel is relatively low. It

 is, therefore, essential that the additive be uniformly

 blended into the fuel with proper equipment and procedures.

 Blending facilities and the additive are available at some

 airports, but cases will arise where it will be necessary to

 blend at the aircraft to assure icing protection. To provide

 for these cases, it may be necessary for the aircraft to

 carry both the additive and blending equipment.

 6. WATER IN FUEL.

 a. Presence of Water in Fuel - Although rigorous precautions

 are taken to ensure that fuel being pumped into an

 aircraft contains as little water as possible, an

 aircraft fuel containing no water is an impossibility.

 This is due primarily to the affinity that hydrocarbon

 fuels have for water. Even if fuels are prepared,

 handled, and used without even contacting liquid water,

 the fuels will contain water picked up from the air. The

 extent of such pickup is largely a function of the

 humidity of the air drawn into the fuel tank to equalize

 the pressure resulting from changes in tank temperature

 and pressure.

 b. Affinity of Fuel for Water - The affinity that fuel has

 for water varies with its composition and temperature.

 The saturation level for a jet fuel in parts per million

 by volume is approximately equivalent to the temperature

 in degrees F.; that is, a jet fuel at 50 degrees F. may

 contain approximately 50 parts per million of dissolved

 water. When the fuel is cooled, that water which is
 above the saturation level is rejected as discrete water

 in minute particles. Until this water can coalesce and

 migrate to the bottom of the tank, it will be carried in

 the fuel. At temperatures below the freezing point,

 these minute particles may be supercooled and will be

 deposited out only when they strike a solid obstruction

 and freeze.

 c. Effect of Gravity - The small differences in the specific

 gravity of water and jet fuel (1.0, 0.77, and 0.83,

 respectively, for water, JP-4 and kerosene) complicate

 the task of removing the discrete water dispersed in the

 fuel. While this water will in time settle out of the

 fuel, the particle size is so small that filters and

 water separators cannot be depended upon for its complete

 removal. Experience has shown that jet fuels may contain

 up to 70 parts per million of dispersed water.

 d. Condensation of Water in Flight - An aircraft after a

 long flight at high altitudes will have tank surfaces and

 fuel that are colder that the air that is drawn into the

 tank during descent. When moisture-laden air enters the

 tank space, condensation may occur in the tank. Due to

 the higher viscosity of cold fuel, this water will not

 settle out as readily and will be carried as dispersed

 water for a longer period of time. Under these

 conditions, the dispersed water in the fuel may reach 100

 parts per million.

 e. Microbia Control - Fuel tank corrosion has been a problem

 in jet aircraft due to the presence and growth of

 microbia at the water and fuel interface. Elimination of

 water and contaminants from the fuel and water reduces

 this corrosion, but complete elimination of water is not

 possible. Experience with fuel containing PFA-55MB and

 MIL-I-27686 has shown that when the concentration in the

 fuel is maintained above .05 percent by volume it is

 effective in eliminating many type of microbia growth and

 thus reduces tank corrosion.

 7. ADDITIVE CONCENTRATION. Data indicate that a .015 percent by

 volume concentration of the additive would prevent ice

 formation in fuel containing 100 parts of water per million

 down to fuel temperature of -40 degrees F. Additive

 concentrations exceeding .015 percent by volume are

 necessary, however, for the following reasons:

 a. The additive tends to separate out of the fuel during its
 transportation (often via long pipe lines) from refinery

 to aircraft, and during long-time storage in refueling

 tanks. A drop in additive concentration to one-third the

 original refinery value has been reported.

 b. Free water may have accumulated in the aircraft tank in

 sufficient quantity to leach further additive from the

 fuel.

 c. Techniques for measuring the additive concentration of

 fuels necessarily involve some accuracy tolerance.

 d. The FAA therefore considers that, to ensure continuous

 fuel flow under conditions where ice may occur in the

 fuel system, the minimum additive concentration for a

 loaded aircraft tank should be .035 percent by volume.

 Moreover, to ensure that this concentration is maintained

 in the loaded aircraft tank, it should be refueled with

 fuel containing an additive concentration of at least .06

 percent by volume.

 8. ACCEPTABLE MEANS OF COMPLIANCE. Fuel containing anti-icing

 additives PFA-55MB and MIL-I-27686 may be approved as an

 acceptable means of complying with Section 25.997(b), if such

 fuel has been demonstrated to be compatible with the engine

 and aircraft fuel system at an additive concentration of 0.15

 percent by volume using the procedures set forth in Advisory

 Circular AC 20-24A. Requests for such approval are

 considered by the Chief, Engineering and Manufacturing

 Branch, for the region in which the applicant is located.

 The approval, in each case, is subject to the following

 conditions:

 a. Placarding - The aircraft should be placarded near the

 fuel filler cover to show that fuel to be used must

 contain anti-icing additive PFA-55MB or MIL-I-27686

 within the minimum and maximum allowed concentration.

 b. Manuals - Federal Aviation Administration approved flight

 manuals should contain the following:

 (1) The minimum concentration of the additive in a

 loaded fuel tank should be at least .035 percent by

 volume.

 (2) The minimum concentration of the additive in the

 fuel with which the aircraft is to be refueled

 should be at least .06 percent by volume.
 (3) The maximum concentration of the additive that may

 be used in fuel is 0.15 percent by volume

 (4) The procedures that are to be used in blending the

 additive into the fuel.

 (5) A list of approved types of equipment that may be

 used for blending the additive into the fuel.

 (6) The procedures and approved equipment that may be

 used to determine the concentration of the additive

 in the fuel.

 (7) A list of trade names under which the additive may

 be obtained.

 (8) The minimum equipment that is required and approved

 for operations.

 c. Tests - The applicant should show by tests that his

 additive sampling and blending procedures are feasible.

 9. OTHER ADDITIVES. Fuel containing other anti-icing additives

 may be given similar approval after it has been conclusively

 demonstrated by test and service experience that:

 a. The additives have no adverse effect on the engine and

 aircraft.

 b. The additives provide satisfactory protection against the

 accumulation of ice that may clog fuel filters as

 required by Section 25.997(b).

 /s/ WILLIAM G. SHREVE, JR.