AircraftOwner Online

Carbon monoxide (CO) is a silent killer. We all know that and we have all heard that. Do pilots really understand the frequency and significance of CO in the cockpit?

    We introduced low level digital carbon monoxide detectors to general aviation in the late 1990s because of a rash of CO accidents in GA aircraft. This was in part due to the availability of new detector technology and concern over the lack of CO awareness in aviation. After the introduction of our detectors, we were engulfed by stories from our customers with near miss carbon monoxide problems.

    First, consider a bit of human physiology. Carbon monoxide is a tasteless, odorless gas emitted from the incomplete combustion of carbon fuels—particularly gasoline. The CO molecule binds the normally oxygen carrying hemoglobin molecule in red blood cells eight times stronger than oxygen so it displaces the oxygen. In simple terms, a person who has CO poisoning is suffocating on a cellular level in spite of breathing normally.

    Probably the most common first sign of carbon monoxide poisoning is headache and nausea. Cognitive disability is next followed by coma and death. Unfortunately, the aircraft environment makes this worse because the relative hypoxia (or low oxygen) associated with altitude has a synergistic ill effect on the occupants of the cabin.

    Probably the most interesting finding we have made from all the field reports from pilots who bought our CO detectors was the muffler cuff, which we suspected would be the most frequent source of CO, was the least likely culprit. Our guess is that this is because the mufflers are inspected once a year and cracks are found early. Other places on the firewall which might have cracks may not be checked as carefully.

Here are some examples of sources of CO reported to Aeromedix:

    Before his death, Scotty Crossfield reported that his 210 had high CO levels. He took another pilot up with him one day to try to find the source. The other pilot flew while Scotty crawled all over the aircraft with our detector to locate where the CO was coming from.

To his surprise, the levels were higher in the back seat than the front. Crossfield determined that the CO was being sucked into the cabin from the tail cone through the rear bulkhead. Part of this is due to the Venturi effect that creates a relative negative pressure in the cabin. He cured the problem by sealing the bulkhead.

    Normally, twin engine aircraft do not have much of a CO risk but listen to this story. A Beech 18 driver had one of our detectors on the floor of his aircraft in front of the co pilot’s seat. While he was waiting in line to take off, he heard the alarm. When he read the detector, it showed over a 100 ppm which can have debilitating effects in minutes. The source was determined to be exhaust which was being funneled into the fresh air source of the cabin due to the aircraft’s relationship to the wind while waiting in line to leave.

    In another situation, a friend who owned a Maule who had never had a CO problem was given a replacement detector for one that was lost. On his first flight with the detector, he started getting very high readings. After landing, he found that one of the inspection covers on the bottom of the fuselage had come off and the cabin was being filled with CO through the three inch hole.

    Two questions about CO detectors come up all the time. One is whether the chemical spot detectors sold by many pilot shops are any good. The short answer is no. These spots turn color so late in the exposure that by the time they change, the occupants are either incapacitated or dead.

       The problem with the hardware store type detectors is due to Underwriters Laboratory restrictions, these detectors cannot read below 35 ppm and cannot alarm below 50 ppm. This is due to too many false alarms complaints from fire departments. There is no such thing as a false alarm in an aircraft at 10,000 feet so that is why I recommend a low level detector. You do not want to just know when the levels are high. You want to know when they are low before they get high!

       Flying lean of peak also reduces the production of CO to almost zero so there is another reason to join the church of LOP. You can read an in depth article by Mike Bush about CO and CO detectors at Aeromedix.com.