THE KILLER GAS
by H. Dean Chamberlain
As cold weather approaches, so does the threat of carbon monoxide (CO) poisoning in many general aviation aircraft. The deadly connection between cold weather and CO poisoning is simple. In many single-engine aircraft, the most common way to provide cabin heat is by heating outside ram air by routing it through a heat-exchanger shroud that fits over the aircraft’s exhaust pipe. It is a simple and effective system that works very well. The problem is if there is any leakage of engine exhaust within the shroud area, some of the exhaust gas will be ducted into the cabin.
That exhaust gas could contain deadly carbon monoxide. Compounding the problem is the fact that as the outside air temperature starts dropping, pilots and their passengers tend to keep the window and other aircraft fresh air vents closed. The result can be a cabin full of exhaust gas containing deadly carbon monoxide, a gas that is colorless, odorless, and tasteless. Adding to the problem is the fact that the human body likes carbon monoxide.
The gas is readily absorbed by the oxygen-carrying red blood cells in the body which then reduces the blood’s ability to carry oxygen throughout the body to sustain life. Not only does the body readily absorb CO, the body is reluctant to release CO once it is absorbed by the blood. The result is, it takes a long while for anyone exposed to CO poisoning to recover completely once they are exposed to fresh air.
Because of the body’s affinity for CO, CO poisoning can also occur slowly over an extended period of time. You don’t have to have a sudden, large exposure of the gas to be at risk. Although such an exposure may kill you because of a lack of oxygen, your body can also become saturated to dangerous or deadly levels of CO over an extended period of time by absorbing small levels of the gas throughout the period. An example of this would be breathing a slight amount of CO throughout a long flight. As the amount of CO slowly builds up in the blood, the person may not realize he or she is at risk. This is why CO poisoning is called the silent death because not only can’t you detect the presence of the gas with your normal senses, it does have this deadly, cumulative effect.
Then, like hypoxia, which is just another term for a lack of oxygen, the person suffering from CO poisoning starts losing the ability to determine why he or she is having a problem. Last year a general aviation aircraft crashed and killed its two occupants because the people on board could not respond to advice from air traffic controllers on how to save their lives. It is believed the pilot and his 71 year-old mother were suffering from CO poisoning. According to newspaper accounts of the accident, the pilot passed out and his mother, also a pilot, tried to fly the aircraft. During the two-hour flight ordeal, the controllers tried to talk the woman to a landing in Connecticut, but she was becoming increasingly groggy and tired. Before the controllers realized she might be suffering from carbon monoxide poisoning, she, too, passed out. The plane continued to fly. Finally, while being chased by another aircraft and National Guard helicopters trying to help, the aircraft flew to New Hampshire and crashed. According to one of the newspaper articles, the four pilots in the civilian chase plane trying to help the woman could only watch "helplessly" as the aircraft "...plummeted to the ground...." According to the reports, by the time the air traffic controllers suggested the woman turn off the cabin heat and ventilate the cabin, it was too late. The woman had passed out before she could respond to the safety suggestions from the controllers.
So, what can you do to protect yourself if you fly an aircraft that uses an exhaust shroud as its heating source? You can do several things.
First, realize that CO-related accidents in aircraft are rare. Whether this is because pilots keep their aircraft’s heating system in good mechanical condition, recognize the symptoms of CO poisoning and take appropriate action, or accidents caused by CO poisoning are blamed on other causes, we don’t know. What we do know are some of the ways to reduce your exposure to CO poisoning this winter.
The first way is to have your aircraft’s heating system thoroughly inspected by a knowledgeable maintenance technician, someone who knows and understands your particular aircraft. Although we are focusing on shrouded exhaust heating systems, some aircraft, particularly some light twin aircraft use an auxiliary gasoline heater in the nose to provide cabin heat. These types of heaters can also produce CO gas if not properly maintained and operated. Regardless of the type of heating system on board, have your maintenance technician inspect it for proper operation, air tightness, and exhaust leaks. The critical element is to make sure that no exhaust gas or contaminants can become mixed with the heated fresh air entering your cockpit area.
The second way is to be able to recognize the effects of carbon monoxide poisoning. Because the results of carbon monoxide poisoning result in hypoxia, the signs of hypoxia are common to both. Symptoms of headache, drowsiness, throbbing temples, nausea, fatigue, inattentiveness, tiredness, a blue coloration (cyanosis) are a few of the symptoms someone may experience with CO poisoning.
Once you know how to detect the effects of CO in the cockpit, the rest is easy. If you smell exhaust gas or suspect you might be exposed to carbon monoxide gas or feel any of the above symptoms, you need to turn off your heater and vent the cockpit. Although you may be somewhat incapacitated, you should decide if you need to land as soon as practical, or, if you are able, continue the flight to your destination. Since it is hard to tell how much CO you may have in your body or how much you might absorb if you continue your flight, it might be prudent to land as soon as practical, get some fresh air on the ground, and have your aircraft’s heating system checked before continuing your flight.
Because of the dangers of CO, manufacturers have developed several different types of CO detectors for different applications. The most common seen in small aircraft are the small "dot" or "button" chemical-type of detector that changes color when exposed to CO. If you buy one of the small chemical type of CO detector, you need to make sure you follow the manufacturer’s directions and change the detector as recommended.
Because any type of carbon-based fuel such as gasoline or jet fuel can produce CO when not properly burned in the combustion process, any time any type of carbon-based fuel engine is running there is the risk of CO being formed. That risk exists not only in flight but on the ground as well. For example, a gasoline powered tug is running outside a small hangar or an aircraft engine is being tested in a hangar or a small gasoline engine preheater is being used to warm an aircraft’s engine before the aircraft is moved from its hangar can all expose anyone within the enclosed areas to the danger of CO poisoning. Since the effects of CO are cumulative over time, even this type of exposure can put a pilot at risk. For example, let’s add in a planned late night flight. We will say the pilot smokes. The pilot is planning on flying at 10,000 feet MSL. Throw in a little CO, and, when all of this is computed in the pilot’s body, that pilot is at risk for the dangers of CO - and, to be really precise, to the danger of hypoxia.
The moral of this article is for everyone to recognize the dangers of CO and to take appropriate precautions. A careful review of your aircraft’s operating handbook with particular emphasis on the safe operation and maintenance of its heating system is a step in the right direction. A careful review of the medical aspects of the dangers of CO is another step. Then because people involved in aviation just don’t work on or fly aircraft, the final step is to remember that you can be exposed to carbon monoxide in your home, in your car, in your boat, in your hangar, or in your aircraft. If you are a camper or hunter for example, each year there are several reported deaths where campers or hunters are found dead in their tent, cabin, or RV from carbon monoxide after their heaters malfunctioned.
If you are involved in an off-airport survival situation such as a crash or precautionary landing or something as simple as being stranded in your car on the way home from the airport because of deep snow, and you are using a candle or some type of flame producing device for heat or light in your aircraft, car, or some type of confined space, you need to remember that not only might you be exposed to carbon monoxide, but that the burning candle or flame can also consume the available oxygen in the space. You must remember that too much carbon monoxide or too little oxygen can both kill. The secret is too avoid the first and have plenty of the latter.
Have a safe winter of flying.
PS: When traveling about, remember to carry a good emergency survival kit based upon your local conditions. Who knows? You just might never have to need it.
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