AC m-113 

DATE 10/22/81 

ADVISORY CIRCULAR 

DEPARTNEST OF TRASSPORTATION 

Federal Aviation Administration 

Washington, D.C. 

Subject : PILm! mFxAur10N!5ANL,mRDbRRS 1oDE 'I#KEIY IN FREVENTLVG 

AIRCRAFT REXIPRWATING ENGINE INDUCTIOI~ SYSTEM AND FUEL SXSTEM 

1cw%FRoFUmS 

1. EURFCJSE. This circular pmvides information pertaining to aircraft engine 

induction system icing and the use of fuel additives to reduce the hazards of 

aircraft cperation that may result from the presence of water and ice in aviation 

gasoline and aircraft fuel systems. 

2. CWCELLATIOK. This Advisory Circular omcels AC 60-9 and 20-92. 

3. RELATEDREADINGMATEEUAL. 

a. Advisory Circular AC 20-24A, 4/l/67, malification of Fuels, Lubricants, 

and Additives. 

b. Advisory Circular AC 20-29B, l/18/72, Use of Aircraft Fkel Anti-Icing 

Additives. 

c. Advisory Circular 20-73, 4/21/71, Aircraft Ice Protection. 

d. National Research Council of Canada, Mechanical Engineerirq report IH-536, 

Aircraft Carburetor Icing Studies, July 1570. 

e. Investigation of Icing Characteristics of Typical Light Airplane Erqine 

Induction Systems, NicA IN b. 1790, February 1949. 

f. Icing - Protection Requirements for Reciprocating Engine Induction 

Systems, IWZA Technical Report No. 982, June 1949. 

(J- 'Various Aircraft 0mers Handbooks, provided by the manufacturers. 

h. Carburetor Ice in General Aviation, NISE3 Special Report M-72-1. 

Initiated by: AWS-140 r 
AC 20-113 10/22/81 

4. B?EKGROUND/DISCUSSI~. Reciprocating engine icing conditions are a constant 

source of cxxcern in aircraft operations since they can result in loss of power 

and, if not eliminated, eventual engine malfunction or failure. The different 

types of icing conditions are characterized as air induction system icing and 

aircraft fuel system icing. Because of a substantial nurrber of aircraft accidents 

attributed to incidents involving such icing, it is imlprtant for a pilot to know 

the kinds of ice formation encountered, and the rrranner in tiich each is formed. 

5. INDUCTION SYSTEM ICING. Induction system icing may be dlaracterized as 

Impact Ice, 'I'txottle Ice, an3 Fuel Vaporization Ice. Any one, or a ambination of 

the three kinds of induction icing , can cause a serious loss of power by 

restricting the flow of the fuel/air mixture to the engine ard by interference 

with the proper fuel/air ratio. Because induction icing accidents can be 

prevented by the pilot in virtually all cases, improved pilot awareness, 

attention, and adherence to reccmmended m&ures &uld reduce accidents of this 

type* 

a. ImpactIce- rmpact ice is formed by misture-laden air at temperatures 

below freezing, striking and freezing on elements of the induction system which 

are at temperatures of 32" F. or below. Under these conditions, ice may build up 

on such merits as the air scoops, heat or alternate air valves, intake 

screens, and protrusions in the carburetor. Pilots should be particularly alert 

for such icing when flying in snow, sleet, rain , or clouds, expecially when they 

see ice forming on the windshield or leading edge of the wings. The ambient 

temperature at which impact ice can be expected to build nest rapidly is about 

25" F., when the supercooled rroisture in the air is still in a semiliquid state. 

This type of icing affects an engine with fuel injection, as well as carbureted 

f?rm ines . It is usually preferable im use r;lrhuretor heat or alternate air as an 

ice prevention means, rather than as a deicier, because fast forming ice tiich is 

not in-mediately recognized by the pilot my significantly lower the amount of heat 

available from the carburetor heating system. Additionally, to prevent power lass 

from impact ice, it nray be necessary to turn to carburetor heat or alternate air 

before the selector valve is frozen fast by the accumulation of ice around it. 

When icing conditions are present , it is wise to guard against a serious buildup 

before deicing capability is lost. The use of partial heat for ice prevention 

without some instrumentation to gauge its effect nray be Wrse than none at all 

under the circumstances. Impact icing is mlikely tier extremely cold 

conditions, because the relative humidity is usually low in cold air ax-d because 

such moisture as is present usually mists of ice crystals which pass through 

the air system harmlessly. The use of prtial heat *en the temperature is below 

32" F. may, for example, raise the mixture temperature up to the danger range, 

whereas, full carburetor heat mid bring it well above any danger of icing. 

b. Throttle Ice - Throttle ice is usually formed at or near a partially 

closed throttle, typical of an off-idle or cruise power setting. This occurs when 

water vapor in the air andenses and freezes because of the mfing restriction 

caused by the carburetor venturi and the throttle butterfly valve. The rate of 

ice accretion within and immediately downstream fran the carburetor venturi and 

throttle butterfly valve is a function of the amount of entrained rrpisture in the 

air. If this icing cxxdition is allowed to continue, the ice may build up until 

it effectively throttles the engine. Visible rroisture in the air is not necessary 

2 
10/22/81 AC 20-113 

for this type icing, sometimes making it difficult for the pilot ti believe unless 

he is fully aware of this icing effect. The effect of throttle icing is a ,urogressive decline in the power delivered by the engine. With a fixed pitch 

propeller this is evidenced by a loss in engine RP~I a& a 1~6s of altitude or 

airspeed unless the throttle is slowly advanced. With a constant spaed propeller, 

there will mxrnally be no darye in FPiQ but the same decrease in airplane 

performance will occur. A decrease in mnifold pressure or exhaust gas 

temperature will occur before any noticeable decrease in engine and airplane 

performance. If these indications are not noted by the pilot and no corrective 

action is taken, the decline in engine pwer will wobably continue progressively 

until it becomes necessary to retrim to'maintain altitude: and erqine roughness 

will occur probably followed w backfiring. Beyond this stage, insufficient 

power may be available to mintain flight; and clanplete stoppage may occur, 

especially if the throttle is noved abruptly. 

c. Fuel Vaporization Ice -This icing condition usually occurs in conjunction 

with throttle icing. It is roost prevelant with conventional float type 

carburetors, arrd to a lesser degree with pressure carburetors when the air/fuel 

mixture reaches a freezing temperature as a result of the cooling of the mixture 

during the expansion process that takes place between the carburetor ard engine 

manifold. This does r& present a problem on systems which inject fuel at a 

location beyond which the passages are kept warm lq engine heat. Thus the 

injection of fuel directly into each cylinder , or air heated by a supercharger, 

generally precludes such icing. Vaporization icing may occur at temperatures fran 

32" F. to as high as 100' F. with a relative humidity of 50 percent or above. 

Relative humidity relates the actual water vapor present to that which cculd be 

Dresent. Therefore. temgerature largely determines the maximum amount of water 

vapor air can hold. Since aviation weather reports normally include air 

temperature and dewpoint temperature , it is pssible to relate the temperature - 

dewpoint spread to relative humidity. As the spread beccnres less, relative 

humidity increases and becomes 100% when temperature and dewpoint arc? the same. 

In general, when the temperature-dewpoint spread reaches 20" F. or less, you have 

a relative humidity of 50% or higher and are in potential icing conditions. 

6. FUELSYSTEMICIIG. Ice formation in the aircraft fuel system results from the 

presence of water in the fuel system. This water may be Lndissolved or dissolved. 

One condition of undissolved water is entrained water tiich consists of minute 

water particles suspended in the fuel. This array occur as a result of mechanical 

agitation -of free water or conversion of dissolved water through temperature 

reduction. Entrained water will settle out in time under static conditions and 

may or may not be drained during normal servicing, depending on the rate at which 

it is converted to free water. In general, it is not likely that all entrained 

water can ever be separated from fuel mder field conditions. The settling rate 

depends on a series of factors including temperature, quiescence ard droplet size. 

a. The droplet size will vary depending upon the mechanics of formation. 

Usually, the particles are sD small as to be invisible to the naked eye, but in 

extreme cases, can cause slight haziness in the fuel. Water in mlution cannot.be 

removed except by deyhdration or by converting it through temperature reduction to 

entrained, then to free water. 

Par 5 3 ’ 
AC 20-113 10/22/a 

b. Another clonriition of undissolved water is free water tiich may be 

introduced as a result of refueling or the settliq of entrained water that 

collects at the bottom of a fuel tank. Free water is usually present in easily 

detecttile quantities at the bottom of the tank, separated by a continuous 

interface from the fuel above. Free water can be drained from a fuel tank through 

the sump drains which are provided for that prpose. Free water frozen on the 

bottom of reservoirs, such as the fuel tanks and fuel filter, may re&er water 

drains useless and can later melt releasing the water into the system tnereby 

causing engine malfunction or stoppage. If such a condition is detected, the 

aircraft may be placed in a warm hangar to reestablish proper draining of these 

reservoirs, an3 all sumps at-d drains should be activated and checked prior to any 

flying. Entrained water (i.e., Mter in solution with petroleum fuels) 

constitutes a relatively snail Fart of the total potential water in a particular 

system, the quantity dissolved being dependent on fuel tevrature ti tne 

existing pressure and the water volubility characteristics of the fuel. Entrained 

water will freeze in mid fuel and tend to stay in suspension longer since the 

specific gravity of ice is approximately the same as that of aviation gasoline. 

c. Water in suspension may freeze ard form ice crystals of sufficient size 

such that fuel screens, strainers, and filters may be blocked. .Scme of tnis water 

may be cooled further &en the fuel enters carburetor air passages anj causes 

carburetor metering mpnent icing, when atiitions are not otherwise conducive 

to this form of icing. 

7. PRlwEMrIoN FTuxFlm. 

a. Induction System Icing -'IQ prevent accidents due to irrluction system 

icing, the pilot should regularly use heat under mrditions known to be oorducive 

to atmospheric icing arrd be alert at all times for Mications of icing in the 

fuel system. The following precautions ti procedures will terd ti reduce the 

likelihood of induction system icing problems: 

(1) Periodically check the carburetor heat systems ard antrols for 

proper condition and -ration. 

(2) Start the engine with the carburetor heat control in the 03LD 

position to avoid possible damage to the system ar~!I a fire hazard because of a 

backfire while starting. 

(3) As a preflight item, check the carburewr heat effectiveness by 

noting the iower drop (when heat is applied) on runup. 

(4) When the relative humidity is above 50 percent and the temperature is 

below 70" F., apply carburetor heat briefly immediately before takeoff, 

particularly with float type carburetirs, 

accumulated during taxi ti runup. 

to remove any ice Wicn may have been 

Generally, the use of carburetor heat for 

taxiing is not remlwnded because of possible ingestion of foreign matter on some 

installations tiich have the unfiltered air admitted witn tne control in the HZC 

or ALTERMTE AIR msitions. 

k Par 6 
10/22/81 AC 20-113 

(5) Conduct takeoff without carburetor heat, unless extreme intake icing 

conditions are ,present. 

(6) Remain alert for itiications of induction system icing during takeoff 

and &i&-out, especially when the relative humidity is above 50 percent, or aen 

visible rrPisture is present in the atmosphere. 

(7) With instrumentation such as carburetor or mixture temperature 

gauges, partial heat should be used to keep tne intake temperature in a safe 

range. Without such instrumentation, full heat should be used intermittently as 

considered necessary 

(8) If induction system ice is suspected of causing a power lass, apply 

full heat or alternate air. Do not disturb the throttle until improvement is 

noted. Expect a further pwer loss momentarily ti then a rise in power as the 

ice is melted. 

(9) If the ice persists after a period with full heat, gradually advance 

the throttle to full power arrl climb at the mimun rate available to produce as 

much heat as possible. Leaning with the mixture control will generally increase 

the heat but should be used with caution as it may kill the engine under 

circumstances in which a restart is impossible. 

(10) Avoid clouds as much as possible. 

(11) As a last resort, and at the risk of catastrophic engine damage, a 

severely iced engine my sometimes be relieved by inducing backfiring with the 

mixture antrol. This is a critical socedure at best, should not be attempted 

with supercharged engines, and mst be done with the carburetor heat control in 

the CDLD position. 

(12) Heat should be applied for a short time to warm the itiuction system 

before beginning a prolonged descent with the engine throttled ard left on during 

the descent. Fewer lever advancement should be performed periodically during 

descent to assure that power recovery can be achieved. The pilot &uld be 

prepared to turn heat off after mer is regained to resume level flight or 

initiate a go-around from an abandoned approach. 

(13) _ The pilot should remember that irrluction system icing is possible, 

particularly with float type carburetors, with temperatures as high as 100' F. 

and the humidity as low as 50 percent. It is mre likely, bwever, with 

temperatures below 70' F. and the relative humidity above 80 percent. The 

likelihood of icing increases as the temperature decreases (dc~ to 32" F.) ti as 

the relative humidity increases. 

(14) General - When m carburetor air or mixture temperature instrumentation is available, the general practice with smaller engines should be to use 

full heat whenever carburetor heat is applied. With higher output engines, 

however, especially those with superchargers, discrimination in the use of heat 

Par 7 5 ' 
AC 20-113 10/22/81 

si~ould be exercised because of tne pssible engine merheating and detonation 

hazard involved. In the case of pressurized aircraft, use of alternate or heated 

carburetor air may require depressurization of tne passenger onlxrtment. H pilot 

of an airplane equipped with a carburetor air or mixture temperature gauge should 

make it a practice to regulate his carburetor neat by reference to this tiicator. 

In any airplane, the occessive use of heat during full power operations, such as 

takeoffs or emergency go-around3 , may result in serious reduction in the pzwer 

developed, as well as the hazard of engine damage. It should be noted that 

carburetor heat is rarely needed for brief high power qXrations. 

b. Fuel System Icing. The use of anti-icing additives for some piston-engine 

,powered aircraft has been approved as a means of preventing problems witn water 

ard ice in aviation gasoline. Some laboratory a~ flight testing irx3icated that 

the use of hexylene glycol, certain methanol derivatives ard ethylene glycol 

monanethyl ether (EGFZ) in small ancentrations inhibit fuel system icing. These 

tests indicate that the use of EGME at a maximum 0.15% by volume concentration 

substantially inhibits fuel system icing under rrPst qerating conditions. The 

concentration of additives in the fuel is critical. Marked deterioration in 

additive effectiveness wy result fran too little or too much additive. 

CAUTION: It snould be recognized that the anti-icing additive is in m 

way a substitute or replacement for carburetor heat. Strict adherence tp 

operating instructions involving the use of carburetor neat should be 

adhered to at all times den -rating under ampheric cwriitions 

clonducive to icing. 

t! . KNcLuSIcx~s. 

a. 'Ihe evidence is clear that carburetor icing arrl aviation gasoline fuel 

system icing problems are prevented with proper use of aircraft carburetor air 

heat and by good housekeeping to eliminate water from gasoline ard the aircraft 

fuel system. 

h. Fuel anti-icing additives have been found to have A beneficial effect on 

the prevention of fuel system icing men properly blended in the fuel systems of 

aircraft powered by reciprocating engines. 

C. Fuel anti-icing additives are not effective in preventing or reducing 

carburetor ice under all -rating anditions and are no substitute for tne 

necessity of carburetor heat or following prescribed flight mnual operating 

procedures. 

d. The effects an3 reccmnlendations described in this circular are general in 

nature arkd appropriate to most certificated airplanes. The pilot sr~ulo refer to 

all available operating instructions ancl placards pertaining to his airplane to 

determine whe-er any special consideration or procedures apply to its operation. 

Director of Airworthiness 

6 'Par 7