Federal Aviotion 

Administration 

Advisory 

Circular 

Subject: Date: 12/10/85 AC No: 20-125 

WATER IN AVIATION FUELS Initiated by: AFS-340 Change: 

1 0 PURPOSE. This advisory circular (AC) alerts the aviation community to 

the potential hazards of water in aviation fuels. In addition, it outlines 

recommended procedures to prevent, detect, and eliminate water in the fuel 

systems of aircraft. 

2 a BACKGROUND. Water in fuel continues to contribute to aircraft incidents 

and accidents and, at times, fatal accidents. Aviation fuel can only serve 

it.s ultimate purpose if it is delivered to the aircraft engines(s) free from 

water. Care and attention are necessary to keep fuel dry (free of water) 

during transportation from the refinery, storage at airports, storage in 

refueling equipment, dispensing from refueling equipment into the aircraft; 

while in the tanks of an aircraft, and from the aircraft tanks to its 

engine(s). Accordingly, if all persons involved in fuel handling will 

accept their responsibility to keep fuel dry, water-in-fuel incidents and 

accidents can be prevented. 

3 0 SOURCE OF WATER IN AVIATION FUEL. 

a. Water can enter an airport fuel system through leaks in underground 

tanks, leaks in the seals of such items as dome covers, floating roofs, and 

hatches during rain or snow storms, when equipment is being washed, by 

marine or surface transport equipment delivering fuel to the airport, and by 

condensation and precipitation of dissolved water in fuel. 

b. Water can enter an aircraft fuel system through leaks in the vents, 

seals, or poorly fitting fuel caps on filler openings during rain or snow 

storms or when the aircraft is washed, from refueling system equipment, by 

condensation and precipitation (especially when an aircraft has partially 

filled tanks), and when refueling during rain or snow storms. 

4 l WATER. Water occurs in aviation fuels in two forms: Dissolved and 

free. 

a. Dissolved Water. All aviation fuels dissolve water in varying 

amounts depending upon the fuel composition and temperature. Dissolved 

water in fuel is similar to humidity in air. 

(1) Lowering fuel temperatures will cause dissolved water to come 

out of solution as free water somewhat like fog comes out of air. The 

creation of free water occurs at a rate of about one part per million per 

degree fahrenheit (1 ppm/deg. F). 
AC 20-125 12/10/85 

(2) Dissolved water is not a problem for aircraft operation as 

long as it remains in solution. Dissolved water cannot be removed by 

filtration but can become free water with temperature change. Once free, it 

can cause operating problems. 

b. Free Water. Any water in excess of that which will dissolve is called 

free water. Free water can appear either as water slugs (in bulk quantities) 

or as entrained water. 

(1) Water slugs are, as the name implies, a relatively large amount 

of water appearing .in one body or layer. A water slug may be a pint or less 

or may be measured in gallons depending on the capacity of a fuel tank. 

(2) Entrained water is suspended in tiny droplets in the fuel. 

Individual droplets may or may not be visible to the naked eye, but they can 

give the fuel a cloudy or hazy appearance depending upon their size and 

quantity. 

(3) When a water slug and fuel are violently agitated (for instance 

when passing through a pump), entrained water results. Entrained water will 

settle out in time depending upon the droplet size, specific gravity and 

viscosity of the fuel and currents within the tank. For this reason, a water 

haze may be seen in turbine fuel but the haze is seldom seen in aviation 

gasoline. Entrained water may also be formed by the lowering of the 

temperature of a fuel saturated with dissolved water. Furthermore, entrained 

water droplets can join together to form large drops or slugs of free water. 

(4) Aircraft engines will tolerate a small amount of free water (30 

ppm. is usually considered to be the maximum) if it is in a fine, uniformly 

dispersed state. The best way to minimize the amount of water entering a - 

system is through inspection and maintenance of equipment and by making 

certain that only clean and dry fuel is received into storage and delivered 

into an aircraft. 

5 l WATER DETECTION. Water can be detected in many ways. Free water, laying 

in the bottom of underground storage tanks can be discovered by the use of 

water indicator paste spread along the lower end of a gauge stick or tape bob 

(allow at least 30 seconds for the paste to react, as its reaction time can be 

slowed down by other contaminants). When the stick or tape bob is lowered to 

the bottom of the storage tank, the paste will change color in the presence of 

water. The highest point of the color change indicates the depth of water 

present. 

a. In above ground tanks and equipment, a sample can be drawn into a 

container and the free water actually observed. A small amount of liquid 

vegetable dye is sometimes helpful to outline the free water in a sample. It 

will mix with and color the water, but is insoluble in fuel. 

b Smaller amounts of entrained water can be detected by testing with a clean 

and dry clear glass bottle. If fuel is acceptably dry it will appear bright with 

a fluorescent appearance and will not be cloudy or hazy. The clear and dry bottle 

test is known as the "clean and bright" test. The fuel is clean when it is clear 

and is bright when it is dry. The container should be 1argGugh to provide for 

a test sample of 10 ounces or more. 

2 Par 4 
12/10/85 AC 20425 

co The white bucket test is useful in checking. for water in jet fuel. Obtain 

.an unchfpped, spotlessly clean, white porcelain, enameled, or stainless steel 

bucket (approximately 10 quart size). Drain about 4 or 5 inches of fuel from the 

sump to be tested into the bucket. With a clean mixing paddle, stir the fuel into 

a swirling "tornado-shaped" cone, remove paddle. As swirling stops, contaminants 

and water will gather under the vortex at the center of the bucket bottom. Add 

several drops of household red food dye. The dye will mix with any water in the 

bottom of the bucket. If no water is present, the dye will settle in the bottom 

of the bucket. 

d Water sensitive papers are also available that will change color in the 

prese;lce of water. . 

. 

e. Water detectors specially made for determining free or entrained water in 

jet fuels are available. When a chemical test is requested by a customer or local 

authority, the Hydro and Aqua-G10 II detector kits are recommended. The Hydro Kit 

chemical powder is sensitive to water concentrations down to 30 ppm. The Aqua-G10 

II Kit can detect levels of water as low as 1 ppm. 

6 lRESPONSIBILITY FOR DRY FUEL. The responsibility for maintaining dry fuel 

should be recognized as a joint responsibility of the aircraft manufacturer, 

maintenance personnel who work on an aircraft, each person who handles the fuel 

from the refinery to the aircraft, and the pilot who flys the aircraft. The 

greatest single danger of water in fuel results from human error that allows fuel 

contaminated with water to enter an aircraft fuel system or permits an aircraft to 

be operated before its fuel system is properly checked for water. The possibility 

of human error can never be eliminated, but it can be minimized through careful 

design and maintenance of airport fueling facilities and fuel equipment and by 

good operating procedures, inspections, checks, training, and recurrent training 

of fuel handling personnel and flight personnel. 

a. Manufacturers provide detailed information in the aircraft maintenance 

manuals related to an aircraft's fuel system. In addition, the manufacturers 

supply service information in the form of service letters, bulletins, notices, 

etc., when it is determined, through service experience, that its aircraft fuel 

system may be improved by some modification or that the service life of its 

product may be extended by some particular maintenance or repair. Additionally, 

appropriate Federal Aviation Administration (FAA) issued airworthiness directives . 

should be consulted to determine if required changes by some particular 

maintenance or repair are necessary. Such information should be reviewed by 

maintenance personnel, fuel servicing organizations, and especially the pilots of 

the aircraft to assure that all precautions and inspections to prevent or 

eliminate water in fuel are accomplished. 

b . Maintenance personnel should consult the manufacturers' maintenance 

manuals and service information, and airworthiness directives related to the 

aircraft fuel system for the latest requirements and information when doing 

maintenance on an aircraft fuel system. 

Par 5 
AC 20-125 12/10/85 

(1) Particular attention should be given to checks for water and to the 

removal of all water from fuel tank sump drains, fuel system line drains, 

gascolators, strainers, and filters where drain plugs/caps are provided to check 

and drain an aircraft system of water. Drain plugs/caps should be removed for 

checks and properly reinstalled. If quick drains are not installed in places 

required by an airworthiness directive, recommended by the manufacturer, or at 

points that can be checked frequently, such information should be brought to the 

attention of the owner/operator for corrective action. 

(2) For airplanes equipped with bladder type fuel cells, the general 

condition and security of each of the cells and the installation should be 

inspected to assure that the bladder has not deteriorated or loosened from its 

mountings. These conditions will cause wrinkles, wells, or depressions to form 

where water can accumulate, avoid drainage, and eventually, find its way to the 

engine(s) in flight. 

(3) Fuel tank filler openings and attachments should be checked for 

security, general condition, and sealing. Fuel tank caps should be checked for 

their general condition, plus security and proper sealing when installed. Fuel 

tank filler opening scupper drains should be checked for free flow to prevent both 

the accumulation of water in the tank and the entrance of water into the fuel 

system. 

C. Fuel handling personnel should have procedures to assure that clean and 

dry fuel is being delivered to the storage system. Daily, weekly, monthly, and i 

other calendar time checks should be made on a continuing basis to assure that the ' 

fuel in fixed storage and dispensing units and systems is free of water prior to 

delivery to an aircraft. Personnel dispensing fuel should be recurrently trained 

in fuel handling to assure that only clean and dry fuel is dispensed. 

d. Flight personnel. The owner/operator of an aircraft should be intimately 

familiar with the fuel system of the aircraft. This familiarity should include 

the knowledge of the specific requirements for the prevention, detection, and 

elimination of water in the aircraft fuel system. 

(1) The pilot-in-command has the final responsibility to determine that 

the aircraft is properly serviced. An important part of the preflight inspection 

is to drain aircraft fuel tank sumps, reservoirs, gascolators, filters, and other 

fuel system drains to assure that the fuel supply is free of water. A review of 

National Transportation Safety Board Briefs of Aircraft Accidents involving 114 

accidents due to fuel contaminaton with water occurring between January 7, 1980, 

and September 11, 1981, showed that the probable cause in 85 of those accidents 

was "Pilot-in-Command - Inadequate Preflight Preparation and/or Planning." Since 

water in fuel accounts for a major share of fuel quality accidents, pilots should 

make it a practice to include this check beginning with the next preflight 

inspection. 

(2) The pilot-in-command should also be present during the refueling 

operation to inspect a sample of the fuel from the dispensing unit prior to 

fueling the aircraft. 

4 Par 6 
12/10/85 x 201125 

7 

fuel 

WATER REMOVAL/PREVENTION. Water is best removed from jet fuel by passing the 

through approved filtration/separation equipment. A two-hour settling period 

and the use of floating suction to drain fuel from the top of a storage tank are 

also recommended to help prevent water from being introduced into a fuel system. 

Water will readily settle-out of aviation gasoline; therefore, filters/separators 

are not required for water removal, however, they are necessary for removal of 

solids. The best means to minimize the amount of water entering a fuel system is 

the inspection and proper maintenance of equipment and the training of ground and 

flight personnel. 

& Fixed Facilitiese A thorough knowledge of the construction, function, 

operation, and maintenance of each component of each piece of equipment of a fixed 

facility is recommended for all personnel responsible for its usage. A principal 

factor which minimizes the amount of water introduced into fixed storage is 

receiving only dry fuel. 

(1) All storage 'tank bottoms and filter/separator sumps should be checked 

for water. Accumulations should be removed before and after each receipt of fuel, 

after a heavy rainfall, snowfall, daily, or more often when in accordance with the 

fuel distributor's instructions. 

(2) Filter elements are required to be replaced whenever the differential 

pressure across the filter or filter/separator drops below the minimum specified 

by the manufacturer, when no pressure drop is indicated, or when the calendar time 

for change is due. 

(3) Only the proper maintenance and inspection of-the equipment in a 

fixed facility can assure the delivery of dry fuel to the refueling equipment, 

hydrant, and/or pit system. 

(4) Every possible precaution must be,taken to prevent contamination of 

tanks and piping by water. Daily handling procedures should be designed to reveal 

any malfunctions of equipment or other conditions which would indicate maintenance 

is necessary. 

(i) The major items of an operational nature shoul .d be checked and 

maintained on a minimum daily, weekly, monthly, and demand basis l 

(ii) Complete and accurate operating logs of all phases of the fuel 

handling system should be developed to fit the needs of each particular operation. 

Entries in those logs should not only include the daily, weekly, monthly, and 

demand checks, but should also include all information pertaining to fuel receipt, 

inventory, and delivery. 

(5) Refueling from drum storage or cans should be considered as an 

unsatisfactory operation and one to be avoided whenever possible. All containers 

of this type should be regarded with suspicion and the contents carefully 

inspected, identified, and checked for water and other contamination. 

(i) If it is necessary to use this type of storage, the drums or 

cans should, if practicable, be protected from the sun and weather. All drums 

should be stored off the ground and on their sides, with the bungs below the 

Par 7 5 
AC 20-125 l~2/10/85 

liquid level, and in such a manner that they are visible and accessible. 

Additionally, fuel in the drums or cans should be used according to the fueling 

delivery date - - oldest stock first. 

(ii) Only sound clean drums with good interiors should be used. 

Where fuel storage in drums has occurred for long periods, the use of the fuel is 

questionable unless it has been tested for quality. Bungs should always be 

screwed tightly into empty drums because an open bunghole allows hazardous vapors 

to escape from the drum after the drum has been emptied. 

(iii) When fueling from drums, it is advisable to use a S-micron 

filtered portable pumping unit, the best filtering equfpment available locally or, 

as a last resort, a chamois skin filter and filter funnel. 

(iv) Remember refueling from drums or cans is considered to be 

unsatisfactory. Extraordinary precautions are necessary to eliminate the hazards 

of water and other contaminants. 

b l Refueling Trucks. Tank bottom drains are provided to allow the truck 

operator to check for the presence of water. Avgas . fuelers are provided with a 

fuel filter capable of filtration of particles about 5 microns in size at rated 

flow of the fueler. The filters are equipped with a bottom drain. Turbine (jet) 

refuelers are provided with a filter/separator capable of removing free water. 

The filter is equipped with a manual bottom drain. Although the filter is not 

equipped with an automatic water drain, it is equipped with an automatic shutoff 

device to stop the flow of fuel in the event excessive amounts of water are 

removed from the fuel. 

(1) All tank and filter bottom drains should be checked for water. 

Accumulations should be removed after each reloading, after washing of the 

equipment, after a heavy rain or snow storm, and/or daily, or more often when in 

accordance with the fuel distributor% instructions. 

(2) Inspection and maintenance of truck filters and filters/separators 

are similar to those described for fixed facilities in paragraph 7(a)(2). Keeping 

refuelers filled with fuel will also help reduce the accumulation of water from 

condensation. 

co Aircraft Fuel Tanks. Aircraft fuel tanks are constructed with sumps to 

trap water. Since it is practically impossible to drain all water from the tanks 

through the fuel lines, the fuel tank sumps should be regularly drained in order 

to remove all water from the system. It may be necessary to gently rock the wings 

of some aircraft while draining the sumps in order to completely drain all the 

water. 

(1) On certain tailwheel type aircraft, raising the tail to level flight 

attitude may be necessary to get any water in a tank to flow to the gascolator or 

main fuel strainer so that it can be drained. 

(2) In aircraft containing crossfeed systems, the crossfeed system 

should be utilized, and fuel should be passed or pumped through the system and 

subsequently drained to make sure the crossfeed lines are free of water. If water 

that accumulates in the crossfeed'system is left undrained, it will flow through 

6 Par 7 
12/10/85 AC 2b-125 

the fuel lines to the engine(s) and may cause the engine(s) to lose power or stop 

operating. 

(3) On aircraft having fuel tanks located in each wing, 

the fuel tank selector valve to the "Both On" position may not a 

the system because fuel will take the path of least resistance. 

fuel selector valve should be positioned at each respective tank 

sump should be checked for water and drained of all water after 

tank selection. 

Pas itioning of 

.dequ ately drain 

In this case, the 

in turn, and the 

each individual 

(4) During preflight, a generous sample of gasoline (10 ounces or more) 

should be drained into a transparent container from each of the fuel sumps and 

from the main fuel strainer or gascolator. Visually check the fuel sample for 

water and by the clean and bright test. Remember bright is dry. For jet fuel, 

the white bucket or steel bucket test is appropriate to check fuel for water. 

Continue to drain fuel from the contaminated sump until certain the system is 

clear of all water. The use of quick-drain valves in the sumps and gascolators 

makes it easier to keep tanks free of significant quantities of water. 

(5) At postflight, completely fill the fuel tanks to provide an effective 

method against contamination from condensation. However, this procedure may be 

only practical on a few types of aircraft, since the type of aircraft, length of 

proposed flight, number of passengers, and weight and balance limitations dictate 

the amount of fuel to be added. 

(6) During routine maintenance, in addition to the preflight and 

postflight procedures, certain precautionary or routine maintenance should be 

performed on an aircraft including the inspection and cleaning of fuel tank outlet 

finger stainers, carburetor screens (filters), and flushing of the carburetor 

bowl. 

8 0 REPORTED PROBLEM AREAS AND RELATED INFORMATION. 

a. Aircraft Fuel Tank Cans. 

(1) Routine monitoring of the FAA service difficulty data bank revealed 

continuing failures of the vented 'fuel tank caps installed on aircraft. 

(2) Reported failure causes include warpage, cracks, and obstructed 

vents. Twenty failures were reported as warped, broken, and leaking caps. 

Aircraft have encountered problems with fuel siphoning from around the filler 

caps and problems with fuel cap vents. 

(3) A review of data in the FAA accident/incident data system revealed 

that some emergency landings were made in which the fuel storage system was listed 

as a causal factor. In many of those emergency landings, vented fuel. caps were' 

listed as a cause. The design of the vent opening allowed water to enter the fuel 

tanks, New types of vented fuel capswere designed for installation by the 

aircraft manufacturers to correct this problem. Information about these new 

design vented fuel caps can be found in the Aircraft Manufacturer's Service 

Bulletins. 

Par 7 
AC 20425 12/10/85 

b 0 Fuel Tank Drains. Many airplanes are equipped with fuel reservoirs 

(separate tanks) in addition to the wing fuel tanks. On these airplanes, the fuel 

reservoirs, as well as the wing fuel tanks, should be checked for contamination 

when water or other contaminants are found in the fuel system. Refer to the 

appropriate owner's manual for fuel reservoir location and proper draining 

procedures. Additionally, manufacturers' service information can be referenced 

for installation of fuel tank quick drain-kits. These kits are now available for 

airplanes built between 1946 and 1974. 

9 WATER CONTAMINATION. Normally, upon finding water contaminated fuel the 

procedures for removal of water outlined in this advisory circular should suffice. 

Should contamination persist or any doubt exists as to water contamination, the 

advice is to have the aircraft fuel system inspected by a qualified person. 

10 SUMMARY. The best insurance against a water-in-fuel problem, whether 

a&ion gasoline or jet fuel, is a responsible fuel handling chain. The 

information ahd procedures outlined in this advisory circular as well as goodinspection and housekeeping practices and constant vigilance should significantly 

reduce the possibility of an aircraft accident due to water-in-fuel contamination. 

William T. Brennan 

Acting Director of Flight Standards 

Par 8