Document
AC 20-113 DATE 10/22/81
ADVISORY CIRCULAR
DEPARnlEXT Ot' TRAN'SPORTATION' Federal Aviation Administration Washington, D.C.
Subject: PII..01' .RU:X::ACJI'IOt..s ANlJ :mcx::1':0lr.n.ES 'ID BE 'J)).RE!t,,i IN ffiEVENTL\JG AIRCRAFT RECIPRCX:ATING ~INE INIXJC'.rICX~ SYSTEM .AND FUEL &'YSTEM ICING ffiOBI.E.MS 1. PURFCSE. 'Ibis circular provides information i;ertaining to aircraft engine
ioouction system icing aoo the use of fuel a::iditives to reduce the hazards of
aircraft cperation that may result from the presence of water aoo ice in aviation
gasoline arx:] aircraft fuel systems.
2. CANCEUATICN. '!his Advisory Circular cancels AC 60-9 aoo 20-92.
3. RELATED READING MATERIAL.
a. Advisory Circular AC 20-24A, 4/1/67, QJalification of Fuels, ll.lbricants, and Additives.
b. Advisory Circular AC 20-29~, 1/18/72, Use of Aircraft Fuel Anti-Icing Additives.
c. Advisory Circular 20-73, 4/21/71, Aircraft Ice Protection.
d. National Research Council of Canada, Mechanical Engineeri03 report IR-536, Aircraft Carburetor Icing Studies, July 1970.
e. Investigation of Icing Characteristics of Typical Light Airplane Engine Irduction Systems, ~.CA 'IN ~. 1790, February 1949.
f. !cir¥:, - Protection Requirements for Reciprocating Engine Induction Systems, N\CA Technical Report No. 982, June 1949.
g. Various Aircraft Owners Hardl::xx>ks, provided by the manufacturers.
h. Carburetor Ice in General Aviation, Nl'SB Si;ecial Report AA..C.::-72-1.
Initiated f>v: AWS-140 10/22/81 AC 20-113 4. FACKGROUND/bISCUSSION. Reciprocating engine icing oonditions are a oonstant source of concern in aircraft q:,erations since they can result in lass of p:>wer and, if oot eliminated, eventual engine malfunction or failure. The different types of icing oonditions are dlaracterized as air irrluction system icing and aircraft fuel system icing. Because of a substantial nunt>er of aircraft accidents attributed to incidents involving such icing, it is imp:.,rtant for a pilot to know the kinds of ice formation encountered, and the manner in W"iich each is formed.
5. INDUCTION SYSTEM ICING. Induction system icing may be d1aracterized as Impact Ice, 'lbrottle Ice, and Fuel Vaporization Ice. Any one, or a a::rnbination of the three kinds of ioouction icing, can cause a serious lass of p:>wer by restricting the flow of the fuel/air mixture to the engine a.rrl by interference with the proper fuel/air ratio. Because irrluction icing accidents can be prevented by the pilot in virtually all cases, improved pilot awareness, attention, arrl adherence to recanrreooed procedures srould reduce accidents of this type.
a. Impact Ice - Im~ct ice is formed by noisture-laden air at temperatures
below freezing, striking aoo freezing oo elements of the ioouction system W1ich
are at t~ratures of 32° F. or below. Under these con:Htions, ice nay ruild up on sudl canponents as the air scoops, heat or alternate air valves, intake
screens, am protrusions in the carburetor. Pilots srould be ~rticularly alert
for such icing when flying in snow, sleet, rain, or clouds, expecially W'len they see ice forming on the wiooshield or leading ooge of the wings. The ambient temperature at which iJnE:act ice can be expected to ruild nost rapidly is alx>ut 25 ° F. , when the supercooled noisture in the air is still in a semiliguid state.
This type of icing affects an engine with fuel injection, as W:!ll as carbureted eno ines. It is usually preferable to u..c::e carburetor heat or alternate air as an ice prevention means, rather than as a de icier, because fast formin;J ice "4"1ich is not inrnediately recognized by the pilot may significantly !<:Mer the anount of heat available from the carburetor heating system. Additionally, to prevent p:,wer lass from impact ice, it nay 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 p;irtial heat for ice prevention without some instrumentation to gauge its effect may be worse than rone at all under the circumstances. Impact icing is mlikely under extremely cold
conditions, because the relative humidity is usually !CM in cold air am because
such noisture as is present usually consists of ice crystals which pass through the air system harmlessly. The use of p:lrtial heat W1en the t~rature is below 32° F. may, for example, raise the mixture temperature up to the danger range, whereas, full carburetor heat would 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 p:>wer setting. This occurs W1en
water vapor in the air coooenses am freezes because of the cooling restriction
caused by the carburetor venturi aoo the throttle butterfly valve. The rate of
ice accretion within aoo imnediately downstream fran the carburetor venturi am
throttle butterfly valve is a function of the arrount of entrained 1TOisture in the air. If this icing corx:li tion is allowed to continue, the ice may build up until it effectively throttles the engine. Visible noisture in the air is oot necessary Par 4 AC 2.0-113 10/22/81 for this type icing, oomE:!times making it difficult for the pilot to believe unless he is fully aware of this icing effect. The effect of throttle icing is a pro gressive decline in the i;::ower delivered by the engine. With a fixed pitch propeller this is evidenced by a loss in engine RP~1 arrl a lass of altitude or airspeed LD1less the throttle is slowly a::Jvanced. With a constant sr,eed propeller, there will oormally t:e oo change in RPN but the same decrease in airplane performance will occur. A decrease in manifold pressure or exhaust gas temperature will occur before any ooticeable decrease in engine arrl airplane performance. If these indications are rot noted by the pilot arrl no corrective action is taken, the decline in engine p::>wer will J:XObably continue progressively until it becomes necessary to retrim to maintain altitude: am engine roughness will occur probably followed by back£ iring. Beyond this stage, insufficient power may be available to maintain flight; arrl canplete stoppage may occur, especially if the throttle is rroved abruptly.
c. Fuel Vaporization Ice - This icing oorxlition usually occurs in oonjunction with throttle icing. It is rrost prevelant with conventional float type carburetors, arrl to a lesser degree with pressure carburetors "*1.en the air/fuel mixture reaches a freezing temperature as a result of the a:x:>ling of the mixture during the expansion pt"Ocess tnat takes place t:etween the carburetor arrl engine manifold. This does rot present a problem on systems \llhich inject fuel at a location beyorxl "*1.ich the passages are kept warm by erg ine teat. Thus the injection of fuel directly into each cylirxler, or air heated by a supercharger, generally precludes such icing. VaEX)rization 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 could be oresent. Therefore. temperature laroely determines the maximum amount of water vapor air can told. Since aviation weather reports normally inclooe air
temperature aoo dewpoint tem{::erature, it is i;::ossible to relate the tem{::erature
dewpoint spread to relative humidity. As the spread beccmes less, relative humidity increases and t:ecomes 100% when temperature and dewpoint are the same.
In general, when the temperature-dewpoint spread reaches 20 ° F. or less, you have a relative humidity of 50% or higher arrl are in p:>tential icing oonditions.
6. FUEL SYSTEM ICING. Ice formation in the aircraft fuel system results from the presence of water l.n the fuel system. 'Ihis water may be undissolved or dissolved.
One oorrlition of undissolved water is entrained water W'l.ich consists of minute water particles susperrled in the fuel. 'Ihis may oc01r as a result of mechanical agitation -of free water or oonversion of dissolved water through temperature reduction. Entrained water will settle out in time under static oorrlitions and may or may not be drained during oormal servicing, de{::erxling on the rate at which it is oonverted to free water. In general, it is rot likely that all entrained water can ever be separated frcm fuel Lllder field rorrlitions. The settling rate deperrls on a series of factors including temperature, quiescence and droplet size.
a. The droplet size will vary deperxliOj upon the medlanics of formation.
Usually, the p:irticles are oo small as to l:e invisible to the naked e:1e, rut in extreme cases, can cause slight haziness in the fuel. Water in solution cannot be rerroved except by deyhdration or by converting it through tem:[:erature reduction to entrained, then to free water.
Par 5 AC 20-11.3 10/22/81 b. Another ooooition of undissolved water is free water which may be introduced as a result of refueling or the settling of entrained water that oollects at the bottom of a fuel tank. Free water is usually present in easily detectable quantities at the bottom of the tank, separated by a continuous interface fron the fuel above. Free water can be drained fron a fuel tank through the sump drains which are provided for that purpose. Free water frozen on the bottom of reservoirs, such as the fuel tanks arrl fuel filter, may reooer water drains useless arrl can later melt releasing the water into the system t:11ereby causing engine malfunction or stowage. If such a corxiition is detected, the aircraft may be placed in a warm hangar to reestablish p:-oper draining of tt1ese reservoirs, arrl all sumps arrl drains s.t:ould be activated arrl d'lecked prior to any flying. Entrained water (i.e., water in S'.:>lution with Fetroleum fuels) constitutes a relatively snall :i;:art of the total I,X>tential water in a p:3.rticular system, the quantity dissolved being dependent on fuel temperature arrl the existing pressure arrl the water s::::>lubility characteristics of the fuel. Entrained water will freeze in rol<'.t fuel and tend to stay in suspension lorY3er since the specific gravity of ice is approximately the same as that of aviation gasoline.
c. Water in suspension ,my freeze and form ice crystals of sufficient size such that fuel screens, strainers, arrl filters may be blocked. sane of tnis water may be cooled further when the fuel enters carburetor air faSSages arrl causes carburetor rretering canp:ment icing, when corr.litions are oot otherwise C'Ol"Xlucive to this form of icing.
7. mEVENI'IOO PRCX:EDURES.
a. Induction System Icing - '.I'o pt:event accidents due to irrluction system 1c1ng, the pilot should regularly use heat under oonditions known to be oorrlucive to atmospheric icing arrl be alert at all times for irrlications of icing in the fuel system. The following precautions and procedures will terrl to reduce the likelihood of ioouction system icing problems: (1) Periodically d'leck the carburetor heat systems arrl oontrols for proper oondition arrl q:,eration.
( 2) Start the erg ine with the carburetor heat control in the CDW y;:osition to avoid p:;,ssible damage to the system an::! a fire hetzard because of a backfire \\bile starting.
( 3) As a preflight item, check the carburetor heat effectiveness by noting the J;X)'t>ler drop (when heat is applied) on runup.
( 4) wnen the relative humidity is above 50 percent arrl the tem~rature ic; below 70 ° F., apply carburetor heat briefly immediately before takeoff, particularly with float type carburetors, to remove any ice whicn rray have been accumulated during taxi arrl runup. Generally, the use of carburetor heat for taxiing is oot recommerrled because of p::>ssible irY3estion of foreign rratter on s::>rne installations which have the unfiltered air a:lmitted witn tne control in the HO!'
or ALTERNI\TE AIR I,X>Sitions.
Par 6 AC 20-113 10/22/81 ( 5) Conduct takeoff without carburetor heat, unless extreme intake ici03 conditions are present.
(6) Remain alert for irrlications of irrluction system icing during takeoff and clinb-out, especially when the relative humidity is above 50 percent, or \o.hen visible roisture is present in the atmosphere.
( 7) With instrumentation such as carburetor or mixture temperature gauges, pa.rtial heat sl'x>uld te used to keep tne intake temperature in a safe range. Witoout such instrumentation, full heat stx:>uld be used intermittently as considered necessary ( 8) If irrluction system ice is suspected of causing a p:,wer loss, apply full heat or alternate air. I):, oot disturb the throttle Ll'ltil improvement is noted. Expect a further p:>wer loss rromentarily arrl then a rise in i:ower as the ice is irelted.
(9) If the ice persists after a period with full heat, grc:K:iually c:K:ivance the throttle to full p:,wer arrl climb at the naximllll rate available to produce as much heat as p:,ssible. leaning with the mixture control will generally increase the heat b.Jt sooulrl te used with caution as it may kill the eo;ine under circumstances in w:1ich a restart is imp:,ssible.
( 10) Avoid clouds as nuch as p:,ssible.
(11) As a last resort, arrl at the risk of catastrophic e03ine damage, a severely iced engine may oometimes te relieved by irrlucing backfiring with the mixture control. This is a critical J;Xocedure at best, should oot te attempted with supercharged en:1ines, arrl rust te done with the carburetor heat control in the CDLD 'fX>S i tion.
( 12) Heat should l:e applied for a sl'x>rt ti.rte to warm the irrluction 5/stern before beginning a prolonged descent with the ergine throttled arrl left on durirg the descent. :Ebwer lever advancement sh:>uld te :E):!rformed periodically during descent to assure that .c;:ower recovery can l:e achieved. The pilot should be prepared to turn heat off after fewer is regained to resurne level flight or initiate a go-arourrl from an abarrloned approach.
( 13) · The pilot sh:>uld remember that irrluction system 1c1ng is El)SSible, particularly with float type carburetors, with temperatures as high as 100° F.
and the humidity as low as 50 percent. It is rore likely, oowever, with temperatures below 70 ° F. arrl the relative humidity above 80 percent. The
likel il'x:>od of icing increases as the temperature decreases ( down to 32 ° F. ) ana as
the relative humidity increases.
( 14) General - When oo carburetor air or mixture temperature instrumen tation is available, the general practice with smaller engines should te to use full heat whenever carburetor heat is applied. With higher ootput e03ines, however, especially those with superchargers, discr~nination in the use of heat Par 7 AC a:}-113 10/22/81 should be exercised tecause of tne i;:ossible engine OV'erheating arrl detonation hazard involved. In the c.ase of i'.)ressurized aircraft, use of alternate or heated carburetor air may require depressurization of tne p:1ssenger cnnpartment. A pilot of an airplane equipped wi tt1 a carburetor air or mixture temperature gauge sh:Juld make it a practice to regulate his carburetor neat by reference to this in:iicator.
In any airplane, the excessive use of heat during full p::,wer operations, such as takeoffs or anergency go-arounds, may result in serious reduction in the p:>wer develof,ed, as well as the hazard of engine damage. It stould be noted that carburetor heat is rarely needed for brief high p,wer q:ierations.
b. Fuel System Icing. The use of anti-icing crlditives for EDl11e piston-engine p'.)wered aircraft has been approved as a rreans of preventing problen1S witn water arrl ice in aviation ga&)line. Some laboratory an::i flight testing imicated that the use of hexylene glycol, certain methanol derivatives arrl ethylene glycol monanethyl et.her (EGME) in small concentrations inhibit fuel systam icing. These tests irrlicate that the use of EGME at a maximum 0.15% by volume ooncentration subztantially inhibits fuel system icing under nost Of)erating corrlitions. The concentration of crlditives in the fuel is critical. Marked deterioration in additive effectiveness may result fran too little or too much cdditive.
CAt11'ION: It SX>uld be recognized that the anti-icin.) a::lditive is in m way a substitute or replacement for carburetor heat. Strict adherence to operating instructions involving the use of carburetor neat shOuld be adhered to at all tiJ'Tles when cperating under atm:::>spheric con::Htions corrlucive to icing.
u. mNCLUSI<l'1S.
a. '!he evidence is clear that carburetor icirY3 arrl aviation gasoline fuel system icing problems are prevented with proper use of aircraft carburetor air heat arrl by good oousekeeping to eliminate water from gasoline arrl the aircraft fuel system.
b. Fuel anti-icing crlditives have been found to have~ beneficial effect on the prevention of fuel system icing when i:roperly bleooed in the fuel systems of aircraft :f()Wered by reciprocating en:Jines.
c. Pue! anti-icing crlditives are rot effective in preventing or reducing carburetor ice under all q::ierating coooitions an::i are oo substitute for tne necessity of carburetor heat or following prescribed flight rranual cperating procedures.
d. The effects arrl recx:rnmeooations described in this circular are general in nature arrl appropriate to rost c:ertificated airplanes. The pilot srould refer to all available cperating instructions a.rd placards pertaining to his airplane to determine whe er any ~cial oonsideration or r;:,r-ocedures apply to its q:ieration.
M. c. Beard
Director of Airworthiness 'par 7 ~Recycled \:I Recyc1a01e