Aviation Safety Letter 2/2004
Exhaust System Failures: A Severe Flight Hazard
All aircraft systems deserve respect - especially the exhaust systems. They are crucial to your engine's performance as they help it breathe in the air-fuel mixture that will give you the rated power. In addition, the exhaust system supplies you with carburetor and cabin heat necessary for your comfort, as well as preventing carburetor ice built-up. Many two-stroke light aircraft engines depend on the specifically-designed, tuned exhaust system in order to ensure that it will give the rated power. As soon as any modification to the system occurs, the pilot will notice that the engine is not performing adequately. On four-stroke engines, a small exhaust crack will most likely fail to show a decrease in power. Think of the exhaust system as a channel, much like a river, through which air flows like water. Exhaust gases flow and create a partial vacuum behind them that assists the engine on the following intake stroke to admit the new air mixture, allowing you the best fuel-air mixture for best combustion and power.
What often gets the least attention during inspection? What is the last engine accessory light-aircraft owners who do their own maintenance review during the aircraft inspection? You guessed it, the exhaust system. They are often taken for granted. Here are two tragedies that could have been prevented.
A young instructor and a pilot friend had set out to practice touch and go on an early July evening. Both were in their mid-twenties and had a wonderful future ahead of them. They had performed two landings and had taken off again to perform another circuit when the flight service station (FSS) specialist called them on the radio to tell them that they were trailing smoke. At the same time they acknowledged the radio transmission, smoke started pouring into the Taylorcraft BC-12D cabin. They proceeded immediately to return for landing, but the cabin was quickly engulfed in flames. Control of the aircraft was lost, and soon after it fell to the ground. The investigation revealed that both pilots had suffocated and likely had died before reaching the ground. The cause was a failure of the exhaust system. The aircraft had been put through its annual inspection just a few hours before the crash but the engineer had failed to see the well-hidden crack that started at the exhaust pipe flange. Had the exhaust system been removed from the engine for inspection, it is very likely that the crack would have been seen and the sooth marks around its opening would have been noticed.
In another case, two friends set out for their annual trout fishing trip in early summer. The aircraft was the revered Piper Super Cub and had received its annual inspection just days before. It was late Friday afternoon when they loaded all of their gear aboard the plane, filed the required flight notification (FLNOT) with the pilot's wife and took off for camping at their favorite fishing spot. Monday, when they failed to return, search and rescue (SAR) was notified and found the aircraft along its intended track in the woods. The aircraft seemed to simply have flown into the trees, as it had left a trail of broken branches on the forest rooftops before impact. The investigation revealed that both passengers had lost consciousness in flight and the aircraft continued until it gradually lost altitude and made contact with the top of the trees which slowed it down and it crashed. Unknowingly, both passengers had become intoxicated from carbon monoxide fumes expelled by a broken exhaust pipe. It had entered silently and surreptitiously contaminated the cabin. In both cases, the engineers who performed the annual inspections were very familiar with these specific aircraft and it is possible that some complacency may have allowed for the events to occur.
Aircraft and engine manufacturers, as well as Transport Canada and other civil aviation authorities, insist that the exhaust system receive a very thorough inspection at least once a year. There are airworthiness directives (AD) such as CF-90-03R2 that give specific instructions on how to proceed in the inspection. This information is available on the Transport Canada Web site at http://wwwapps3.tc.gc.ca/Saf-Sec-Sur/2/cawis-swimn/logon-cs0101.asp?lang=E&rand=. The Federal Aviation Administration (FAA) has numerous documents available online for inspecting exhaust systems. Take a look at http://www.faa.gov/regulations_policies/advisory_circulars/index.cfm/go/document.information/documentID/74468.
Exhaust system parts fail because of metal fatigue, corrosive environments, continuous stress at flange mating areas, vibrations, repeated cycles of heating and cooling, looseness of components and other factors such as material thickness, material compatibility fabrication methods, etc. Tests performed on aircraft have shown that cracks may appear after between 100 and 200 hours of operation. One half of the failures noted were on the heat exchanger surfaces used for carburetor and cabin heat. Apart from cabin contamination, failure of the heat exchanger surfaces may allow for gases to be drawn into the induction system, causing overheating and power loss. Erosion and carbonizing of the surfaces are the primary causes of internal failure. Any lead pencil mark left on exhaust pipes or any exhaust system part is likely to lead to a premature crack, as the lead causes a heat concentration that degrades the base metal and weakens it through carbonizing. The efficiency of the engine and exhaust system depends on you; always give it your best.
Carbon monoxide detectors are available for most airplanes. They can be of a type that is passive, such as a type that has a chemical patch that reacts to any cabin environment contamination of carbon monoxide. This type has a limited life and should be replaced annually. There are active carbon monoxide detectors that use a chemical detector with associated integrated electronic circuitry, a light and an audible warning system to alert the pilot and passengers of contamination. This system either works with the help of a 9-V battery or is pre-wired into the aircraft electrical system through the help of a supplemental type certificate (STC). Any carbon monoxide detector can help reduce the risk of cabin contamination by this deadly odorless gas. Look into the matter before setting out on your next flight.
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