Air Safety – The Dangers of Carbon Monoxide

The toxicity of carbon monoxide is well understood. A concentration of only 0.06% in air inhaled can cause unconsciousness within two hours; 0.1% can cause death in as short a time as an hour, depending on the physical condition of the person affected. And even a concentration of carbon monoxide as small as 0.02% can have perceptible effects over a period of an hour and a half. The effect on pilots is particularly dangerous because it is so insidious — a pilot may suffer from impaired vision and judgement, reduced alertness and reasoning power, yet he may be completely unaware that his efficiency is deteriorating.

And so the gradual carbon monoxide poisoning process goes on undetected — in the worst cases until it is too late: too late because the pilot has already become unconscious, or too late because he has been reduced to a state in which he does not recognise what is happening to him, or is simply incapable of handling the situation that now confronts him.

In the early days of enclosed cockpit aircraft, there were a number of accidents resulting from this cause, which ultimately led to the adoption of design specifications aimed at keeping cockpit air contamination within safe limits. Today, carbon monoxide contamination checks are a routine part of the type certification for new aircraft types. But even with these safeguards, there have been cases where new aircraft have later proved faulty in this respect. In most of these instances, the type certifications had been carried out on a prototype aircraft, and subsequent production models later underwent minor modifications which adversely affected their cabin sealing or ventilation.

A typical case involved a single engined type in which a small hole had been cut in the cabin floor for a camera installation. The hole did not affect the structural integrity of the aircraft, but it provided a means by which cabin air could become seriously contaminated with carbon monoxide from the engine exhaust. In another single engined aircraft, exhaust fumes were able to enter an area of reduced pressure in the fuselage through an unsealed tail wheel strut aperture, again resulting in serious carbon monoxide contamination in the cabin. In this instance the source of the cabin contamination was discovered only after prolonged investigation. In yet another case, a modification to the louvres of an engine cowling caused contamination of the air flowing around the cabin cold air vent in the side of the fuselage, and allowed carbon monoxide to be directly drawn into the cabin when the cold air vent was opened. And though the official BASI report on the February 1994 Cessna 182 accident is not yet available, it is believed that cabin contamination occurred because the luggage compartment door was being held open to drop dingo baits during a property inspection flight, causing exhaust gases to swirl back through it into the cabin.

Because of numerous incidents of this sort, the Civil Aviation Authority requires carbon monoxide contamination checks to be carried out, not only on new aircraft types during type certification, but also on any aircraft that has been modified in a way that could possibly produce results similar to those described.

It might be argued that the cases mentioned are isolated and do not have a great deal of application to the vast majority of normal light aircraft operations. Yet they do serve to illustrate how easily unsuspected carbon monoxide contamination can occur in an aircraft.

This brings us to a particular item of equipment fitted to most light aircraft which, although fully meeting design specifications, is an ever present source of treachery and danger if it is not carefully maintained — the aircraft cabin heater and the engine exhaust system associated with it. Most light aircraft cabin heaters utilise a heat exchanger which consists essentially of a sheet metal jacket wrapped around part of the engine exhaust pipe or exhaust muffler. Ram air is admitted to the jacket, usually through a flexible hose leading from an air intake at the front of the engine nacelle, heated in the jacket by the engine exhaust, and then ducted to outlets at various points inside the cabin. It is obvious that if a crack or a hole develops between the exhaust pipe and heater jacket, the ram air will become contaminated and carbon monoxide will be piped to the cabin hot air outlets.

Clearly, the danger will be greatest during the winter months when cold weather encourages full use of the cabin heater and the other cabin air sources are kept closed. But it is as well to remember that carbon monoxide can also enter the cabin at any time through the various small openings in the firewall, if there are gas leakages in other parts of the engine exhaust system. Cracks and holes can develop — and exhaust gaskets can blow — in a relatively short time and, for this reason, exhaust manifold and cabin heater assemblies should be inspected for defects, including blown gaskets, at regular intervals. Some manufacturers in fact recommend inspections as often as every 25 hours flying.

On one occasion a pilot on cross country flight in western NSW found himself suffering from inexplicable nausea, and a tight feeling across his forehead. As time went on he had difficulty in focusing his eyes, his sense of balance deteriorated, he became incapable of working out even simple navigational problems, and he felt a compelling urge to fly closer and closer to the ground. Fortunately the pilot was wise enough to realise something was seriously wrong and landed the aircraft at a strip which was a few nautical miles closer than his destination. Here he walked around for half an hour until he felt better, then flew on to Bourke where he immediately declared the aircraft unserviceable. When the exhaust system was inspected, a small hole was found in the heat exchanger. The hole had been made by an excessively long PK type attaching screw, which had pierced the inner wall of the heat exchanger.

The best protection against carbon monoxide poisoning is to be alert, as this pilot was, to its early symptoms — the smell of engine exhaust gases, a generally sluggish feeling, drowsiness, tightness across the forehead, headaches. If you do experience these early warning signs, assume that carbon monoxide is getting into the cabin and take all reasonable precautions: Shut off the cabin heater immediately and open a cabin window or other fresh air source. Refrain from smoking, because this also increases the carbon monoxide level in the body, and land at the first suitable aerodrome. Do not take off again until all effects of the carbon monoxide have subsided. At the first opportunity, have the aircraft checked for carbon monoxide contamination, either by a thorough physical inspection of the suspected components, or by having the cabin air sample tested under operational conditions. The test equipment used for this purpose today is an electronic, hand held, “hot wire” instrument which measures the level of carbon monoxide present through changes in electrical resistance and indicates the reading on a calibrated dial.

In past years, such tests were accomplished using a manually operated air sampler featuring a replaceable glass indicator tube containing yellow silica gel crystals. To use the tester, the sealed ends of the indicator tube were broken off and the tube inserted in a spring loaded holder on the instrument. An air sample was then drawn through the tube by squeezing the rubber bulb on the end of the instrument and allowing it to reinflate. If the sampled air contained carbon monoxide, the yellow silica gel turned green, the depth of colour being directly proportional to the concentration of carbon monoxide in the air. To measure this concentration, the discolouration of the chemical was compared with a colour scale mounted beside the indicator tube. This simple tester could indicate concentrations of carbon monoxide from 0.001 to 0.100%.

Extremely economical, but nonetheless effective derivations of this type of manual tester are available today in the form of small plastic cards to which a thin “patch” of carbon monoxide sensitive crystals, light brown in colour, is attached. The crystals look very much like a small disk of sandpaper in the middle of the card. The card is carried in the aircraft cabin, preferably in an easy to see position, eg. somewhere on the instrument panel, and if the cabin air should happen to become contaminated by carbon monoxide, the crystals will turn dark brown. While this may not give any accurate indication of the percentage of carbon monoxide contamination present, it at least provides a warning that all is not well and that the matter should be investigated further. These cards are generally available from aviation supplies shops, and cost only a few dollars. No light aircraft should be without one!

If for this or any other reason you have cause to suspect carbon monoxide contamination in your aircraft, do not delay arranging an operational check with the electronic test instrument. And make it a habit to have your cabin heater inspected regularly. The time and money spent is good insurance!