GENERAL AVIATION SAFETY SENSE

14A

PISTON ENGINE ICING

1 INTRODUCTION

a Although this leaflet is mainlyaddressed to aeroplane operations,much of its content applies equallyto piston-engined helicopters andgyroplanes.

b Piston engine induction systemicing, commonly referred to ascarburettor icing, can occur even onwarm days, particularly if they arehumid. It can be so severe that unlesscorrect action is taken the enginemay stop (especially at low powersettings during descent, approach orduring helicopter autorotation).

c Every year there are severalaccidents in the UK where engineinduction system icing may havebeen a factor. Unfortunately theevidence rapidly disappears.

d Some aircraft/enginecombinations are more prone toicing than others and this should beborne in mind when flying differentaircraft types.

2 TYPES OF ICING

There are three main types ofinduction system icing:

a Carburettor Icing

The most common, earliest to show,and the most serious, is carb icingcaused by the sudden temperaturedrop due to fuel vaporisation andpressure reduction at the carburettor

venturi. The temperature drop of20–30°C results in atmosphericmoisture turning into ice whichgradually blocks the venturi. Thisupsets the fuel/air ratio causing aprogressive, smooth loss of powerand slowly ‘strangles’ the engine.Conventional float type carburettorsare more prone to icing thanpressure jet types.

b Fuel Icing

Less common, is fuel icing which isthe result of water, held insuspension in the fuel, precipitatingand freezing in the induction piping,especially in the elbows formed bybends.

c Impact Ice

Ice which builds up on air intakes,filters, alternate air valves etc iscalled impact ice. It forms on theaircraft in snow, sleet, sub-zero cloud

GENERAL AVIATION SAFETY SENSE LEAFLET 14A

PISTON ENGINE ICING

Throttlebutterfly

Fuel

To engine

Air

Ice in carburettor venturi

Fuel icing (less common)

BUILD-UP OF ICING IN INDUCTION SYSTEM

CAA Carto DO C(G)6 Drg No 8805b 23-11-84 10-5-90

and rain, (if either the rain or theaircraft is below zero °C). This type oficing can affect fuel injection systemsas well as carburettors. In general,impact ice is the main hazard forturbocharged engines.

d Testing has shown that because ofits greater and seasonally variablevolatility and higher water content,carb icing is more likely whenMOGAS is used.

e Engines at reduced power settingsare more prone to icing becauseengine induction temperatures arelower. Also, the partially closedbutterfly can more easily berestricted by the ice build-up.

Note: For the sake of simplicity, in the restof this leaflet, the term Carb Icingincludes Induction Icing and Carb Heatincludes Alternate Air.

3 ATMOSPHERIC CONDITIONS

a Carb icing is not restricted to coldweather, and will occur on warmdays if the humidity is high,especially at low power settings.Flight tests have produced seriousicing at descent power with theambient (not surface) temperatureover 25°C, even with relativehumidity as low as 30%. At cruisepower, icing occurred at 20°C whenthe humidity was 60% or more.(Cold, clear winter days are less of ahazard than humid summer daysbecause cold air holds less moisturethan warm air.) In the UnitedKingdom and Europe where highhumidity is common, pilots must beconstantly on the alert for thepossibility of carb icing and takecorrective action before anirretrievable situation arises. If thereis an engine failure due to carb icing,the engine may not re-start and even

if it does, the delay could be critical.

b Carb icing can occur even in clearair and is therefore more dangerousdue to the lack of visual warning. Incloud the risk of icing may be higherbut the pilot is less likely to becaught unawares.

c Specific warnings of inductionsystem icing are not normallyincluded in aviation weatherforecasts and you must be preparedto deal with it on the basis of yourknowledge and experience. Whendewpoint information is notavailable, assume high humidityparticularly when:

— the surface and low level visibilityis poor, especially in the earlymorning and late evening, andparticularly when near a largearea of water;

— the ground is wet (even with dew)and the wind is light;

— just below cloud base or betweencloud layers (highest liquid watercontent is at cloud tops);

— in precipitation, especially ifpersistent;

— in clear air where cloud or fogmay have just dispersed;

— in cloud and fog, these beingwater droplets; hence the relativehumidity should be assumed to be100%.

d The chart overleaf shows the widerange of ambient conditions wherethe formation of carb icing is mostlikely. Particular note should betaken of the much greater risk ofserious icing with descent power. Thecloser the temperature and dewpointreadings, the greater the relativehumidity.

4 RECOGNITION AND GENERALPRACTICES

Paragraphs 4 and 5 are intended as ageneral guide to assist you to avoidicing, but reference should be madeto the relevant sections of Pilot'sOperating Handbook or FlightManual for specific proceduresrelated to the particularairframe/engine combinations.These may vary for a different modelof the same aircraft type.

a With a fixed pitch propeller, aslight drop in rpm and airspeed arethe most likely indication of theonset of carb icing. This loss of rpm

can be smooth and gradual and theusual reaction is to open the throttleslightly to compensate. However, thisaction, whilst restoring the powerhides the loss. As the icing builds up,rough running, vibration, loss ofairspeed and ultimately stoppage ofthe engine may follow. The primarydetection instrument is the rpmgauge in conjunction with the ASI.

b With a constant speed propeller,and in a helicopter, the loss ofpower would have to be large beforea reduction in rpm occurs. Onset oficing is even more insidious, butthere will be a drop in manifoldpressure and reduction in airspeed in

Serious icing - any power

Moderate ic ing - cruise powerSerious icing - descent power

Serious icing - descent power

Light icing - cruise or descent power World Wide

NW Europe

100%

Rel

ative

Hum

idity

+30

+20

+10

0 Dewpoint 0C

-10

-20

+30 +40+20+100-10-20

80%

60%

40%

20%

FOG/C

LOUD

Approximateupper limitsof dewpoint

CARB ICING CHART

level flight. Thus, in this case theprimary detection instrument is themanifold pressure gauge.

c If fitted, an exhaust gastemperature gauge will show anoticeable decrease in temperaturebefore any significant decrease inengine and aircraft performance.

d Carb icing is cleared by the pilotselecting an alternative air sourcewhich supplies air, (heated in anexhaust heat exchanger) which meltsthe ice obstruction. This source by-passes the normal intake filter.

e Engines with fuel injectiongenerally have an alternate airintake located within the enginecowling via a valve downstream fromthe normal air intake. This alternateair is warmed by engine heat, eventhough it does not normally passthrough a heat exchanger.

f Always use full heat whenevercarb heat is applied, partial hot airshould only be used if an intaketemperature gauge is fitted and onlythen in accordance with the FlightManual or Pilot's OperatingHandbook. Partial heating caninduce carb icing because it may meltimpact ice particles (which wouldotherwise pass into the enginewithout causing trouble) but notprevent the resultant mixture fromfreezing when it passes through theinduction system; partial heat canraise the induction air temperatureinto the critical range.

g Hot air should be selected:

• whenever a drop in rpm ormanifold pressure is experienced,

• when icing conditions aresuspected, or

• when flying in conditions withinthe high probability rangesindicated in the chart.

But always be aware that hot air,whilst selected, reduces enginepower. This reduction may be criticalin certain flight phases. Unlessexpressly permitted, (or necessary),the continuous use of hot air shouldbe avoided. It should be selected forlong enough to pre-empt the loss ofengine power or restore the enginepower to the original level.

h If a loss of power is due to icing,and the use of hot air disperses it, re-selection of cold air should producean increase in rpm or manifoldpressure over the earlier reading.This is a useful check to see whetherice is forming. If it is, monitor theengine instruments as it may re-occur. Lack of carb icing will meanthat there will be no increase in rpmor manifold pressure beyond thatnoted prior to the use of hot air.

i Remember, selection of hot air,when ice is present, may at firstmake the situation appear worse dueto an increase in rough running asthe ice melts and passes through theengine. If this happens thetemptation to return to cold air mustbe resisted so that the hot air hastime to clear the ice. This time maybe in the region of 15 seconds, whichwill, in the event, feel like a verylong time!

5 PILOT PROCEDURES

a Maintenance

Periodically check the carb heatingsystem and controls for propercondition and operation. Payparticular attention to the conditionof seals which may have deterioratedallowing the hot air to becomediluted by cold air.

b Start Up

Start up with the carb heat control inthe COLD position.

c Taxying

Generally, the use of carb heat is notrecommended while taxying becausethe air is usually unfiltered when inthe HOT position. However, if it isnecessary – USE IT.

d Ground Run-Up

Check that there is a significantpower decrease when hot air isselected (typically 75–100 rpm or3–5" of manifold pressure) and thatpower is regained when cold air is re-selected. If it is suspected that ice ispresent, the hot position should beselected until the ice has cleared andnormal power is restored.

e Immediately Prior to Take-Off

Since icing can occur when taxyingwith low power settings, or when theengine is idling, select carb heat ONfor 5 seconds and then OFF,immediately before take off to clearany build-up. If the aircraft is keptwaiting at the holding point inconditions of high humidity, it maybe necessary to carry out the run-updrill more than once to clear icewhich may have formed.

f Take-Off

Take-off should only be commencedwhen you are sure the engine isdeveloping full power. When at fullpower and as airspeed is building,you must check that the full throttlerpm and/or manifold pressure is asexpected. Carburettor heat mustNOT be used during take-off unlessspecifically authorized in the FlightManual or Pilots OperatingHandbook.

g Climb

Be alert for symptoms of carb icing,

especially when visible moisture ispresent or if conditions are in thehigh probability ranges in the chart.

h Cruise

Monitor appropriate engineinstruments for any changes whichcould indicate icing. Make a carbheat check at least every 10 minutes,(more frequently if conditions areconducive to icing). Use full heat andnote the warning of para 4 (e), itmay take up to 15 seconds to clearthe ice and the engine will continueto run roughly as the ice melts andpasses through the engine. If theicing is so severe that the engine hasdied, keep the hot air selected asresidual heat in the rapidly coolingexhaust may be effective. In all cases,it is vital to select carb heat beforeany selector valves or linkages arefrozen solid by an accumulation ofice around them. Avoid clouds asmuch as possible, note; that fewpiston engined aircraft are clearedfor flight in icing conditions.

i Descent and Approach

Carb icing is much more likely atreduced power, so select hot airbefore, rather than after, power isreduced for the descent, andespecially for a practice forcedlanding or a helicopter autorotation,ie, before the exhaust starts to cool.(This also allows a check that no ice ispresent and that the carb heat is stillworking.) Maintain FULL heat duringlong periods of flight with reducedpower settings. At intervals of about500 ft or more frequently ifconditions require, increase power tocruise setting to warm the engineand to provide sufficient heat tomelt any ice.

j Downwind

Ensure that the downwind checkincludes the following check:

— Note the RPM/Manifold Pressure

— Apply Full Carb heat for about15 seconds and note the reducedindication.

— Return Carb heat to Cold. TheRPM/Manifold Pressure will returnto the earlier indication if therewas no icing. If it is higher – icingwas present.

k Base Leg and Final Approach

Unless otherwise stated in the Pilot'sOperating Handbook orFlight Manual, the HOT positionshould be selected on base leg whenpower is reduced. On some engineinstallations, to ensure better engineresponse and to permit a go-aroundto be initiated without delay, it isrecommended that the carb heat bereturned to COLD at about 200/300 fton finals.

l Go-around or Touch and Go

Ensure the carb heat is COLD, ideallybefore, or simultaneously as power isapplied for a go-around.

m After Landing

Return to the COLD setting beforetaxying, if not already set COLD,(para k).

THE SUMMARY IS OVERLEAF

6 SUMMARY

— Icing forms stealthily.

— Some aircraft/enginecombinations are more susceptiblethan others.

— Icing may occur in warm humidconditions and is a possibility atany time of the year in the UK.

— Mogas makes carb icing morelikely.

— Low power settings, such as in adescent or in the circuit, are moreprone to give carb icing.

— Use full carb heat frequently whenflying in conditions where carbicing is likely. Remember that theRPM gauge is your primaryindication for a fixed pitchpropeller; manifold pressure forvariable pitch.

— Treat the carb heat as an ON/OFFcontrol – either full hot or fullcold.

— It takes time for the heat to workand the engine may run roughlywhile the ice is clearing.

— Timely use of appropriateprocedures can PREVENT THISPROBLEM.

FINALLYIn the event of carb heat system failure in flight:

• Keep out of icing conditions.

• Maintain high throttle setting – full throttle if possible.

• Weaken the mixture slightly.

• Land as soon as reasonably possible.

PREVENTION IS BETTER THAN CUREOther leaflets in this series:

1C Good Airmanship Guide2B Care of Passengers3C Winter Flying5D VFR Navigation6C Aerodrome Sense7B Aeroplane Performance8D Air Traffic Services Outside

Controlled Airspace9A Weight and Balance

10A Bird Avoidance11 Interception Procedures12C Strip Sense13A Collision Avoidance15B Wake Vortex16A Balloon Airmanship Guide17B Helicopter Airmanship18A Military Low Flying19 Aerobatics20A VFR Flight Plans21A Ditching22 Radiotelephony23 Pilots: It’s your Decision24 Pilot Health

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ISSN 0266-1519

© Civil Aviation Authority 2000

Prepared by the Safety Promotion Section and theCorporate Communications Department of the CivilAviation Authority.