The Silent Killer: Threats from exhaust heaters

Deaths from carbon monoxide leaking into an aircraft from a defective exhaust system are not common, but do occur from time to time.


I just came across a report of two deaths from carbon monoxide leaking into an aircraft from a defective exhaust system. These type of accidents are not common but do occur from time to time. We need to be constantly reminded of this silent killer and the threat it presents to small aircraft operators, technicians, and repair facilities. This was a tragic loss that did not have to happen.

The accident

The accident occurred during a one hour planned flight to deliver a Piper Lance to a customer. The two men aboard were very experienced pilot flight examiners. The aircraft was observed over-flying the destination airport and continuing flight until it ran a tank dry, lost power and descended in a turn until impact. The heavy wing with fuel will usually drop forcing the uncontrolled aircraft into a spiral descent.

The report stated that the scene was grim but with both bodies intact and no blood anywhere. They were both unconscious at impact. The autopsy found that one man had a 71 percent and the other a 65 percent carbon monoxide blood saturation level. Anything over 50 percent saturation is considered the cause of death. Remember the movie and book by Robin Cook, "Coma"? The fictional story explained clearly what effect carbon monoxide has on the body. These men were probably unconscious within the first 30 minutes of the flight. This accident was totally avoidable.

The cause

Investigators found the heater muff and muffler badly corroded. This allowed exhaust gases to seep into the heater duct. The aircraft was 25 years old and the system was allowing exhaust gases to seep into the cockpit for some time. This can occur even with the heater control valve turned off.

If you have ever experienced a leak in an exhaust heater system, you know it could have been your last. I clearly remember the odor and smell from an exhaust heater as a kid in our family '37 Ford sedan. You pulled a cable and turned on a fan to start the heat flowing. This was very similar to small aircraft heaters. The heater was very efficient. The hot air came through a duct on the lower right hand side of the firewall. You knew right away that exhaust was coming into the heater duct by the odor. I called it the suicide heater. My father disabled this heater for good as soon as the odor was apparent. We all know that CO (carbon monoxide) does not have any odor in its pure form, but when mixed with exhaust gases and oil what you smell is the common exhaust pipe odor. This odor may give you a clue to the danger but it may not be present at all when only small amounts of exhaust gas are admitted to the cockpit. As pilots and mechanics we are all familiar with various odors in the cockpit that we sort of take for granted. It could be our last smell however.

Technicians alert

Technicians should be familiar with the two common combustion heat systems: the exhaust muff type, where both carburetor heat and cabin heat can be supplied and the gas combustion heater common in most piston twins and some singles. Gas combustion heaters are usually regularly pressure checked for combustion gas leaks into the heat system. ADs are usually controlling here.

Both of these sources use the heat from burned fuel and of course, one of the gases produced is CO, a deadly poison when ingested into your body. Your red blood cells absorb CO like a sponge! ("Coma"). The red blood cells have such an affinity for CO that it replaces the oxygen molecules very quickly. You start to feel drowsy and a short time later you are unconscious. It only takes a few minutes to saturate your blood system. Your body is deprived of oxygen and you expire.

There is no doubt in my mind that the technicians or repair facility who performed the last annual inspection on this aircraft and anyone else who recently performed maintenance on this aircraft will be in for a wrongful death lawsuit. The local district attorney may even consider a criminal negligence charge against the responsible party or parties.

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