Plane & Pilot
Tuesday, December 15, 2009

Muffler Inspection

It’s critical to ensuring a safe flight

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If you were to make a list of the most fun and glamorous aspects of flying, I’d bet that inspecting an aircraft’s muffler wouldn’t be on it. Yet, it’s among the most important steps for ensuring a safe flight.

Muffler systems have two basic designs: crossover and separate. In the crossover type, the exhaust stacks from both banks of exhaust ports are fed to a single muffler and then out the tailpipe; the muffler usually is supported horizontally by the stacks. In the separate type, there’s a muffler and tailpipe for each bank of cylinders; each muffler is supported vertically or at an angle.

In 1968 and 1970, the FAA performed special studies of engine exhaust systems in two- to six-seat light GA aircraft. These studies focused on systems utilizing heat from engine exhaust gases to heat the cabin air, which is done by passing air through a heat-exchange shroud around the muffler core. To be safe, the muffler must remain sealed from the heat shroud, as pinholes or cracks would allow combustion products, including deadly carbon monoxide, to pass into the cabin.

The studies suggested that an FAA-devised testing method could better simulate the stresses to which a muffler and exhaust stacks are normally exposed than the then-used testing methods. In one aircraft model, one hour of operation in the FAA tests was the equivalent of more than 11 hours in the field.

One of the studies reported that time in service before the development of an exhaust system problem varied widely (though the manufacturers and models were disguised, so the study wasn’t useful for making specific predictions about when the muffler should be replaced). In general, though, the FAA found that exhaust systems were exposed to the most stress when the engine was operated at high power during takeoff. Mufflers on engines with turbochargers were exposed to greater stresses than mufflers on normally aspirated engines, because the turbocharging allowed the engines to operate at higher power output during cruise at high altitude.

The FAA also reported that an exhaust system fabricated from metal composed of 32% nickel, 46% iron and 21% chromium would provide enhanced protection from high-temperature oxidation and attack from contaminants produced by combustion. The metal needed to be at least 0.05-inch thick to provide greater strength for handling heat and vibration.

FAA engineers learned that the temperature of the baffles and diffusers inside mufflers probably approached the temperature of the exhaust gases, about 1,500 to 1,600 degrees F, even though the temperature of the metal on the outside surface maxed out at 1,200 degrees F. Oxidation resistance of the standard stainless steel used in exhaust systems became marginal at the temperatures the FAA engineers found inside exhaust assemblies. Uneven airflow of exhaust gases within the system resulted in temperature variations of 300 to 400 degrees within short distances; these variations added to stresses on the metal.

The FAA engineering studies didn’t result in changed requirements for aircraft engine exhaust systems. Careful inspection for pinholes, cracks and other deterioration—and prompt repair or replacement of affected parts—remains the best defense against an accident caused by an exhaust system problem.


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