CARBURETTOR ICING
A GENERAL AVIATION SAFETY COUNCIL PAPER
Carburettor icing is said by many to be a known killer. It can cause engine stoppage or loss of power, leaving the pilot of a single engine aircraft facing a forced landing on whatever terrain happens to be beneath at the time and the eventual outcome will then depend upon a combination of the pilot’s skill and luck. In many cases skill and luck will be sufficient for the event to conclude in the survival of the aircraft’s occupants and no damage to the aircraft. In other cases the conclusion will be occupants with only light or no injuries and a damaged aircraft. In some, however, skill and luck will be insufficient to prevent serious injuries or fatalities.
The Air Accident Investigation Branch (AAIB) reports a continuing toll of accidents where carburettor ice is a suspected factor. During the period 1998 to 2002 the average of such accidents was between six and ten per year and no doubt there are many more carburettor icing events that, thankfully, did not result in injury or damage and therefore did not need to be investigated. In 2002 AAIB investigated 11 carburettor ice related accidents.
The likelihood of formation of carburettor ice depends principally upon the ambient humidity. A carburettor has the ability to reduce the temperature of the air induced through it by as much as 35 degrees C. If there is dampness in the air (i.e. it is humid) then in the temperatures found in the UK there is a strong possibility of the moisture in the air condensing on the interior of the carburettor and forming ice there, particularly on the butterfly valve. High humidity is a normal feature of the UK’s moist maritime climate and consequently a high incidence of carburettor icing is encountered in this country. Just below the cloud base, where much cross country flying takes place in the UK, the humidity is likely to be very high.
The subject of carburettor ice is hedged about with a great deal of uncertainty for GA pilots. The certification requirements for carburettor heat systems for single engined GA aircraft are limited, the principal requirement under JAR 23.1093 (a)(3) being simply that at 60% maximum continuous power and in stated ambient conditions there is a heat rise of 56 degrees C (100 F). The JARVLA requirements are even less prescriptive; more rigour would be desirable in arriving at the requirements that would best meet the needs of GA. A well known graph that shows the degrees of carburettor icing to be expected in differing temperatures and humidities is commonly encountered in discussions of carburettor icing. Its provenance is unknown although it is understood to have originated in Australia and was validated for the CAA by Loughborough University of Technology in 1996. There is uncertainty as to whether it applies to all engines and to all fuels.
Pilots’ Operating Handbooks (POHs) seldom offer much information about carburettor icing and where manually applied carburettor heat is fitted the POH usually leaves pilots to make their own minds up as to when and for how long they should apply carburettor heat. Consequently pilots have no real idea whether to apply carburettor heating for 30 seconds or two minutes. Nor do they have any advice as to whether a reduction caused by carburettor ice of, say, 100 RPM or an equivalent amount of manifold pressure, represents a serious and imminent danger or merely an early warning of trouble in the offing. At this point, how long an application of carburettor heat will be needed to clear the ice? Should power be increased? Assuming that ambient conditions and engine settings remain unchanged, how quickly is carburettor icing likely to build up again to the same degree?
The CAA has sponsored further research specifically addressing ice accretion rates and the time taken to dispel that ice. The authority hopes that the research will help to identify the effectiveness of current carburettor heat systems and procedures and inform a review of the need for any further requirements of operational changes. The General Aviation Safety Council (GASCo) welcomes the CAA’s initiative.
Current regulations allow manufacturers to produce aircraft and engines with a manually applied carburettor heat system where they cannot quantify it performance in all likely conditions. In many cases, the system will not work for a significant proportion of engine control settings and there are only vague instructions as to the use of the system. With experience, pilots who have become familiar with a particular aircraft and engine combination may eventually acquire relevant practical carburettor icing knowledge but that will offer no assistance to the pilot unfamiliar with the particular combination. Hard detailed information is seldom to be found in a POH, although it should be available.
GASCo believes that the time has arrived for a major initiative throughout GA in Britain to address the problem of carburettor icing and for the general introduction of permanent defences against it. There are twenty eight members of the GASCo Council. The members include the representative bodies of all the significant General Aviation activities in the United Kingdom, relevant learned societies and Government Departments concerned with General Aviation. A list of the member organisations of GASCo is included in the Appendix.
This paper does not address the relatively rarer induction icing problems associated with impact icing at the induction inlet, icing of ram air devices on some piston engines nor ice formation at very low temperatures of water contained within fuel.
Aircraft engines can offer a variety of defences against carburettor icing, as follows:
Manually applied carburettor heat
Carburettor piston engines are often fitted with a carburettor heat device that warms the air before it reaches the carburettor. With the air heated to a sufficiently high temperature it will still remain above freezing level after subsequent cooling in the carburettor and ice will therefore not form. On Continental, Lycoming and many other aircraft engines the heat is derived from an airbox surrounding part of the exhaust system. The current American engines use hot air straight off an exhaust heat exchanger. This is a very fierce heat and is intended to work almost instantly: it works well if the principle is understood.
Heating the air in a manually applied carburettor heat system usually reduces the volumetric efficiency of the engine, causing a power loss of anything up to 15%. To get around this loss of efficiency the engines are normally run without carburettor heat being applied. Application of carburettor heat is made by manual operation of a knob or switch and, typically, pilots of aeroplanes are trained to apply carburettor heat for at least 10 seconds at about five minute intervals in the cruise. Applications must also be made before descending, during final approach and before take off. These devices theoretically provide a more or less complete defence against carburettor icing. In practice, however, pilots forget to apply heat on all occasions that their training has taught them and sometimes they pay a heavy price for their forgetfulness. Pilots used to flying aircraft not fitted with manual carburettor heat sometimes forget altogether to apply heat when flying an aircraft that has it. Every issue of Flight Safety, GASCo’s quarterly magazine, includes a competition and one that was run in 2002 was devoted to carburettor ice. The degree of misinformation that was evidenced in the replies was quite disturbing: the majority of respondents – and they presumably would not have entered the competition if they had entertained many doubts about their knowledge of the subject – believed that air temperature rather than humidity was the major determinant of carb ice vulnerability and that if an engine faltered when carb heat was selected then it should be deselected. It is clear that a great deal more could usefully be done to improve GA pilots’ knowledge about carburettor ice and how to deal with it.
Certain conditions can exist in which intermittent, even if regular, operation of carburettor heat is ineffective, and carburettor heat needs to be applied all the time and even when pilots comply absolutely with their training they can still come to grief. AAIB Bulletin 4/2003, Ref. EW/G2002/09/12 tells of a Jabiru pilot who flew into clear air downwind of the Drax power station at 1,500 ft. Power was lost and in the subsequent forced landing on a recently seeded flat field the nose wheel dug in at slow speed, the new microlight overturned and was badly damaged. That pilot and the world of GA generally have now learned that even when in clear air form, the exhaust from cooling towers will have an extremely high humidity and this may persist for some miles downwind. The AAIB concludes that it is highly likely that carburettor icing caused the loss of power in this case. Whatever the theoretical reliability of manually applied carburettor heat, the accident records show that reliance upon the pilot applying heat in a sufficiently timely manner is sometimes ineffective. In the Robinson R22 helicopter – a type that currently sells better than any other aircraft type – the carburettor icing problem is addressed by the provision of manually applied carburettor heat backed up with a carburettor temperature gauge. This is discussed below.
Some aircraft are fitted with a gauge that reports the temperature within the carburettor. If the carburettor temperature is below freezing point then there is a risk of carburettor icing although the degree of risk will depend upon the humidity of the air. A pilot can adjust the setting of the carburettor heat control so as to maintain a carburettor temperature above freezing and thus eliminate the risk. However, a change of throttle setting, mixture setting or ambient temperature may require adjustment of the carburettor heat setting. There will be an inclination on the part of the pilot to aim for a temperature just above freezing so as to reduce the loss of volumetric efficiency to a minimum. However, just a small drop in carburettor temperature subsequently may produce conditions highly conducive to carburettor icing. These instruments can be inaccurate and before the first start up of the day pilots should check the reading of an OAT gauge on the ground and compare it with the carburettor temperature gauge. The device is passive and requires monitoring but it is likely to increase the likelihood of carburettor heat being applied conscientiously. One such device, however, did not prevent a Robinson R22 pilot’s death by way of carburettor icing in December 2002 (AAIB Bulletin 10/2001. EW.C2000/12/3)
A 100% reliable ice formation warning device coupled to a wholly effective warning system (pilots sometimes postpone attention to warning horns until it is too late) could be the near complete answer to the problem. On receiving the warning the pilot applies carburettor heat and continues to do so until the warning ceases. The ‘Iceman’ is probably the best known carburettor ice warning device available at present. A probe within the carburettor reports the actual formation of ice and activates warning devices – a flashing light and/or warning horn that can be piped into the headset if required. A degree of endorsement of the Iceman is implied by the CAA’s willingness to lessen modification approval charges for those fitting the device. We have encountered anecdotal objections that the Iceman carburettor probe is prone to becoming fouled with oil or other matter, causing false alarms, although the device includes a zeroing facility that is intended to cope with partial fouling of the probe. There are also unconfirmed allegations that the device does not work satisfactorily at low throttle settings, and this is the usual setting on the approach to land. Firm evidence from existing users needs to be assembled about the reliability in use of this device and if the Iceman is in fact reliable and trouble free then its use should be actively encouraged. If not, its improvement or other availability of a 100% reliable ice detector would be very welcome.
The example is often quoted of the DHC Chipmunk which, when in military use, had its carburettor heat wired permanently on. This inevitably caused a permanent loss in engine power but it is instructive to reflect that the military decided that the loss in engine power and efficiency was a worthwhile trade off against the potential loss of aircraft resulting from carburettor icing. Considering that pilots with greater aptitude than the average GA pilot were operating these aircraft it is arguable that the military solution might have considerable relevance to GA operations and aircraft generally. Gipsy and Renault engines use heat from the side of the crankcase and if not selected will come on automatically with certain throttle settings. This heated air which is the air that has cooled the cylinders is less intense and has worked well with very little input from the pilots over the years.
There are engine aspiration issues such as air filtration to be considered when the permanently selected heat solution is applied to an installation originally intended for manual heat application. There is also the question of whether the loss of maximum obtainable power might leave some aircraft dangerously under powered and unable to cope safely with a go around from an aborted landing. Furthermore, for engines with a high compression ratio, the application of hot air can lead to detonation and putting such engines permanently into hot air mode is clearly unacceptable. Equally, forgetting to return the carb heat to cold when going around with a high compression engine can also cause detonation and therefore manually applied carb heat appears to be an unsatisfactory arrangement for high compression engines in any case. Two stroke Rotax engines of the air cooled types, using the Bing 54 carburettor, can be fitted with an electric carburettor heater powered directly from the engine generator. An alternative example of permanently applied carburettor heat is to be found in the 912 versions of the Rotax engine, which are liquid cooled and can be fitted with a carburettor heater that derives heat constantly from the engine coolant. In the British climate this heat is applied permanently and the suppliers claim that as the amount of heat applied to the carburettor (not directly to the induction air in this case) is small, the loss of volumetric efficiency is negligible.
Carburettor installations that are ice free
There are carburettor engine installations not fitted with a dedicated air or carburettor heater that do not suffer from carburettor icing. These usually route the induction air past warm parts of the engine causing sufficient rise in the induced air temperature to avoid carburettor ice. This therefore is a very similar solution to wiring manually applied carburettor heat permanently on. However, air filtration and other issues can be considered ab initio. The Limbach engine (a derivative of the Volkswagen Beetle engine) makes an interesting case in point. In some installations the engine is permanently proof against carburettor icing, with no actions being required of the pilot, and in others it is fitted with carburettor heat. The latter arrangement is found in many Druine Turbulents where the engine ( an ‘Ardem’ version) in this installation is notorious for suffering carburettor icing at the least provocation whenever heat is not being applied and is known to have led to at least one fatality.
Fuel injection engines
Not having a carburettor these engines do not suffer from carburettor icing. They are often fitted with an ‘alternate air’ control, which seems similar to a carburettor heat knob, but this is in fact intended to cope with the case of snow impacting or ice forming against the induction air filter and blocking it. This is not likely to be a situation encountered by most GA aircraft in the UK. Fuel injection seems to be a complete answer to the carburettor icing problem and engines of 150 HP and upwards are frequently fitted with fuel injection. GASCo is encouraged to see the spread of the popularity of fuel injection. The new Cessna 172s and the new Robinson Raven R44s are both fitted as standard with fuel injection engines, as are many other new aircraft. This is by no means universal, however, and we are disappointed to report that the new Piper Warrior III still has a normally aspirated engine, even when offered on the British market. A carburettor ice detector is offered as an option for an extra $1,300. The latest Robinson R22 unfortunately still has a carburettor and manually applied carburettor heat with a temperature gauge fitted as standard. There can be a significant cost penalty in opting for petrol injection, however. Signature Aircraft Engineering at Oxford (formerly CSE Aviation) quote (August 2003) an extra cost of $4,623 for fuel injection when supplying the IO 360 LZA as fitted to the new Cessna 172 as compared with the O 360 A4M 180 HP carburettor version, as fitted to the Piper Archer.
Diesel or compression ignition engines seem likely to become a significant part of the European piston engine market for aeroplanes before long. One of their attractions is proof against carburettor icing as they have no carburettor.
Fuel additives and Teflon coating of carburettor interiors
The National Research Council of Canada produced a paper in 1970, Aircraft Carburettor Icing Studies, which dealt with research into the prevention of carburettor icing either by the introduction of fuel additives or by coating the interior of a carburettor with Teflon. An engine was run at what seemed to be the throttle setting that produced the greatest vulnerability to carburettor icing and water was sprayed into the induction system. The conclusion was that the use of ethylene glycol monomethyl ether (EGME) at 0.10 to 0.15% by volume proved to be the most effective additive and that Teflon coating was also relatively effective. A combination of both solutions seemed to be absolutely proof against icing. In spite of the apparent promise of these findings we have not discovered any subsequent commercial application of either of these solutions nor are we aware of any subsequent research along the same lines. We have discussed the fuel additive issue with a technical expert at Shell but that company does not market EGME either as a constituent of avgas or as an additive. It does market isopropyl alcohol (IPA) as an additive for both jet fuel and avgas , but this is intended to deal with the potential for water contained within fuel (there is usually some) from forming ice crystals when aircraft are operated at temperatures a good deal below freezing. IPA is not offered as a solution for carburettor icing.
Further research
We understand that the CAA intends to carry out some research into possible measures against carburettor icing and we welcome this.
CONCLUSIONS
1. GASCo believes that the present toll of deaths, injuries and aircraft write offs connected with carburettor icing could be reduced significantly, and this is an eminently desirable aim.
2. Experience shows that reliance on the intermittent application by the pilot of carburettor heat is an inherently unsatisfactory solution as GA pilots cannot all be relied upon to apply heat conscientiously on all occasions. Much needs to be done to improve the level of knowledge about this issue amongst GA pilots and instructors, both when training and subsequently. GASCo would be happy to become involved in such an initiative.
3. So far as new aircraft are concerned a number of proven ways exist to remove the likelihood of carburettor icing occurring. Fuel injection, induction systems that heat the air continually, and heated carburettors are all in regular and satisfactory use. Diesel engines may well join them. GASCo would prefer to see better measures, other than manually operated carburettor heat, incorporated in new and replacement engines where there are practical and cost effective ways of achieving this.
4. For the existing fleet and engines which have not yet reached the need for replacement or overhaul GASCo believes that, so far as those engines that rely on manually applied carburettor heat are concerned, more research is required into ways making these engines safer in the hands of inherently forgetful pilots. Permanently applied carburettor heat may be the best solution for some aircraft and a proven 100% reliable carburettor ice detector may answer best for others. No final conclusion can be made without further authoritative research. GASCo would be willing to become involved in any such research, subject to the availability of resources and a suggestion worth exploring is to organise a competition of College and University projects to bring some bright young thinking to bear.
5. Where manually applied carburettor heat is fitted, the POH should give detailed and specific advice as to its operation, particularly as regards rates of carburettor ice accretion to a dangerous degree and rates of elimination. These should be given for varying ambient conditions and engine settings.
References
AAIB Bulletins. AAIB website: www.aaib.dft.gov.uk
Piston Engine Icing. CAA Safety Sense Leaflet No 14. www.caa.co.uk/publications through “general aviation” or LASORS 2004.
JAR 23.1093 (a)(3) on Induction System Icing Protection
CAA British Civil Airworthiness Requirements. Sub-section K5-Power’
Aircraft Carburettor Icing Studies
by L Gardner and G Moon, Division of Mechanical Engineering, National Research
Council of Canada. Information on obtaining this
document from CISTI's Help Desk/Judy Parisien. Tel 001 613/993-9206, Fax 001
613/998-2399
The Elimination of Carburettor Icing in Civil Aircraft. March 1994. A report by Ricardo Engineering to the CAA under contract no. 7D/S/1137.
Induction Icing by Miles McCallum. Flyer magazine, January 2003.
Rotax engine carburettor heaters. Skydrive website: www.skydrive.co.uk
---------------------
THE GENERAL AVIATION SAFETY COUNCIL - GASCo
GASCo is a charitable body whose objective is to foster the development of General Aviation in the United Kingdom along safe lines by encouraging competence, safety and good airmanship among General Aviation pilots and operators and all concerned with General Aviation activities. General Aviation includes all forms of aviation other than scheduled and non scheduled airline operation, and military operations.
There are twenty nine members of the GASCo Council. The members include the representative bodies of all the significant General Aviation activities in the United Kingdom, relevant learned societies and Government Departments concerned with General Aviation. A list of the member organisations of GASCo is included below.
It is estimated that GASCo represents through its member organisations approximately 100,000 individuals who are actively involved in General Aviation. The principal activities with which the member organisations are concerned include pilot training, business aircraft users, air taxi, aircraft maintenance and trading, aerial work, air traffic control, airport owners and a wide range of General Aviation recreational activities. The recreational activities include the operation of powered aircraft, gliders, hang gliders, paragliders, microlight aircraft, balloons, parachutes and model aircraft. General Aviation as represented by GASCo and its member organisations is considered to represent a substantial level of economic activity. General Aviation aircraft operate in controlled and uncontrolled airspace and use major international airports, regional airports and a wide range of local airfields. All the member organisations of GASCo give a very high priority to the safety of their activities.
Members of GASCo:
Airport Operators Association,
Aircraft Owners and Pilots Association,
Association of Aviation Medical Examiners,
British Aerobatic Association,
British Balloon and Airship Club,
British Gliding Association,
British Hang Gliding & Paragliding Association,
British Helicopter Advisory Board,
British Microlight Aircraft Association,
British Model Flying Association,
British Parachute Association,
Civil Aviation Authority,
UK Flight Safety Committee,
Flying Farmers Association,
General Aviation Manufacturers and Traders Association,
Guild of Air Pilots and Air Navigators,
Guild of Air Traffic Control Officers,
Helicopter Club of Great Britain,
Historic Aircraft Association,
LDA (Aviation) Limited,
The Meteorological Office,
Ministry of Defence (Air),
Mission Aviation Fellowship Europe,
Popular Flying Association,
PPL/IR Europe,
Royal Aeronautical Society,
Royal Institute of Navigation.