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.
On March 22, 2009, a private pilot took off by himself in a Cessna 150D for a day/VFR local flight in Madison, Ind. He noticed that the engine oil temperature was steadily increasing, so he reduced throttle to see what affect it would have—smoke started coming from under the instrument panel. He was about four minutes from the airport when the engine lost power, and the pilot performed an emergency landing. The plane was substantially damaged, but the pilot escaped serious injury. Examination revealed that the aft section of the muffler had separated from the exhaust assembly, letting hot gases enter the engine compartment. The heat melted the P-lead wire for the magnetos, and allowed it to short to the airplane’s structure. This short meant that electricity was no longer reaching the spark plugs, so the engine quit.
On May 30, 2008, a Piper PA28-235 crashed after takeoff from Highland County Airport in Hillsboro, Ohio. The pilot and passenger were killed. The plane was flying VFR to its home base in New Carlisle, Ohio, after receiving maintenance on its autopilot. According to an FAA inspector, after the maintenance was completed, the airplane initially departed but returned due to smoke in the cockpit. A technician checked the cockpit and autopilot but didn’t find any discrepancies.
On the second takeoff, a witness saw the plane lift off from runway 23 and climb to between 300 and 400 feet before gray smoke started coming out of its right front side. As the smoke blackened, the airplane “veered” to the left; it eventually descended and impacted a tree with its left wing.
In its report, the NTSB quoted the 1973 Piper PA28-235’s flight manual, which advises “the presence of fire is noted through smoke, smell, and heat in the cabin. It is essential that the source of fire be promptly identified through instrument readings, character of the smoke, or other indications, since the action to be taken differs somewhat in each case. The emergency procedures for an in-flight engine fire are: a. Fuel Selector—Off; b. Throttle—Closed; c. Mixture—Idle Cut-Off; d. Heater—Off (in all cases of fire); e. Defroster—Off (in all cases of fire); f. If terrain permits, land immediately.”
The Piper Cherokee’s service manual, dated January 31, 2008, has a 12-page checklist for 50- and 100-hour inspections. Within the list of items dealing with the engine, two items pertain to the exhaust system. One states, “Inspect exhaust stacks, connections and gaskets per Exhaust System Inspection. (See Special Instructions, Procedures.) Replace gaskets as required.” The second item states, “Inspect muffler, heat exchange and baffles per Exhaust System Inspection. (See Special Instructions, Procedures.)”
According to the FAA, the mechanic who had performed the accident airplane’s previous five inspections also had done 42 annual inspections on other aircraft in the same year. He worked out of his hangar and didn’t have any calibrated tools. The checklist he used was one page in length and contained 16 items. One item pertaining to the muffler only had the word “Muffler” with a checkmark next to it.
The muffler and right exhaust stack were examined by the NTSB’s Materials Laboratory in Washington, D.C. A hole was observed in the inwardly deformed lower portion of the muffler’s housing, adjacent to its left end. The left portion of the muffler had deposits and residual burnt insulating material consistent with a fire. The hole’s edges were ragged and displayed radially oriented cracks, predominately on the forward and right edges. Manipulation of the protruding portion of the edge revealed that they could be separated from the muffler by applying as few as three bending cycles. The forward and rear ragged edges of the hole couldn’t be mated, suggesting there was some material missing. A sample of the muffler skin’s inner surface at the rear of the hole displayed areas of an undulating surface consistent with corrosion deterioration. Other areas were covered with brown patches, which, when rubbed with the wooden end of a cotton-tipped applicator, fell off in flakes to reveal a similar undulating surface.
The right exhaust stack was extracted from the muffler to reveal that the middle pipe portion was mechanically damaged, visibly tapered and out-of-round. Examination of the middle pipe’s damaged end revealed several localized areas of inward deformation and a longitudinal cut with inward bends. The out-of-round shape of the exhaust stack’s middle pipe was mirrored in the shape of the muffler’s center pipe.
The NTSB determined that the probable cause of this accident was the pilot’s failure to maintain control of the airplane due to an in-flight fire resulting from a corrosion hole in the muffler that wasn’t identified by a mechanic.
Peter Katz is editor and publisher of NTSB Reporter, an independent monthly update on aircraft accident investigations and other NTSB news. To subscribe, write to: NTSB Reporter, Subscription Dept., P.O. Box 831, White Plains, NY 10602-0831.