Textron Lycoming O-290-D2 - Carburetor Heat Control Usage

LYCOMINGOPERATOR’SMANUAL SECTION3
O235ANDO290SERIES OPERATINGINSTRUCTIONS
WARNING
REFER TO THE PILOT’S OPERATING HANDBOOK OR AIRFRAME
MANUFACTURER’S MANUAL FOR ADDITIONAL INSTRUCTIONS ON THE USE OF
CARBURETOR HEAT CONTROL. INSTRUCTIONS FOUND IN EITHER PUBLICATION
SUPERSEDETHEFOLLOWINGINFORMATION.
F. UseofCarburetorHeatControl–Undercertainmoistatmosphericconditions(generallyatarelative
humidity of50% or greater) andattemperaturesof20°t o 90°F it ispossib le for icet o form inthe
inductionsystem.Eveninsummerweathericemayform.Thisisduetothehighairvelocitythrough
the carburetor venturi and the absorption of heat from this air by vaporization of t he fuel. The
temperatureinthemixturechambermay dropasmuchas70°Fbelow thetemperatureoftheincoming
air.Ifthisaircontainsalargeamountofmo isture,thecoo lingprocesscancau seprecipitationinthe
formofice.Iceformationgenerallybeginsinthevicinity ofthebutterflyandmaybuilduptosuchan
extentthatadropinpoweroutputcouldresult.Ininstallationsequ ippedw ithfixedpitchpropellers,a
loss of power is reflected by a drop in manifold pressure and RPM. In installationsequ ipped with
constantspeedpropellers,alossofpowerisreflectedbyadropinmanifoldpressure.Ifnotcorrected,
thisconditionmaycausecompleteeng inestoppage.
Toavoidthis,allinstallationsareequippedwithasystemforpreheatingtheincomingairsupplyto
thecarburetor.Inthiswaysufficientheatisaddedtoreplacetheheatlossofvaporizationoffue l,and
themixingchambertemperaturecannotdroptothefreezingpointofwater(32°F).Theairpreheateris
a tube or jacket through which the exhaust pipe from one or more cylinders is passed, andtheair
flo wing over these sur faces is raised to the required temperature before entering the carburetor.
Consistentlyhightemperaturesaretobeavoidedbecauseofalossofpowerandadecidedvariationof
mixture. High charge temperatures a lso favor detonation and pr eignition, both of which are to be
avoidedifnormalservicelifeistobeexpectedfromtheengine.Thefo llowingoutlineisthepr oper
methodofutilizingthecarburetorheatcontrol.
(1)Ground Operation – Use of the carburetor air heat on the ground must be held to an absolute
minimum.Onsomeinstallatio nstheairdoesnotpassthroughtheair filter,anddirtandforeign
substancescanbetakenintotheenginewiththeresultantcylinderandpistonringwear.Only use
carburetorairheatonthegroundtomakecertainitisfunctioningpro perly.
(2)TakeOff–Setthe carburetorheatinfullcoldposition.Fortakeoffandfullthrottleoperationthe
possibilityofexpansionorthrottleicingat widethrottleopeningsisveryremote.
(3)Climbing–Whenclimbingatpartthrottlepowersettingsof80%orabove,setthecarburetor heat
controlint hefu llcoldposition;however,ifitisnecessarytousecarburetorheattopreventicingit
is possible for e ngine roughness tooccur dueto the overrich fuel/air mixture produced by the
additional carburetor  heat. When this happe ns, lean the mixture with the mixture contro l only
enoughtoproducesmo othengineo peration.Donotcontinuetousecarburetorheatafterflightis
outoficingconditions,andreturnmixturetofullrichwhencarburetorheatisremoved.
(4)Flight Operation – During normal flight, leave the carburetor air heat control in the full cold
position.Ondamp,cloudy,foggyorhazydays,regardlessoftheoutsideairte mperature,bealert
forlossofpower.Thiswillbeevidencedbyanu naccountablelossinmanifoldpressureorRPM
orboth,dependingonwhetheraconstantspeedorfixedpitchpropellerisinstalledontheaircraft.
RevisedSeptember2008 35