Twenty percent of partial power failures or power losses are the result of bad internal exhaust baffles
What part on your airplane isn't likely to be a prime topic of hangar flying sessions, must be leak-checked during each annual inspection, was at the center of a controversy that resulted in new procedures for implementing airworthiness directives, and glows red-hot during cruise power operations? If your answer is the airplane's exhaust system, go to the front of the takeoff line.
321 stainless steel and Inconel
Although there are still some mild steel exhaust systems used on a few homebuilt airplanes, 321 stainless steel — a chromium-nickel steel — is used for almost all of today's general aviation exhaust systems. It exhibits good strength and performance at temperatures from 800 to 1,600 degrees Fahrenheit; it's easily weldable and formable; and it's relatively inexpensive. If a 321 stainless steel exhaust system is treated like all the other engine accessories, and overhauled during the engine overhaul, there's a good chance that it will last until the next engine overhaul. Inconel 601, although about three times as expensive as 321 stainless, has a greater ability to withstand higher temperatures. Until recently, Inconel has been used exclusively for the high-temperature portions of turbocharged airplane exhaust systems. Because of Inconel's expense, these exhaust systems are hybrids, with the low-temperature part of the system employing 321 stainless steel and the high-temperature portions using Inconel.
Agents of change
When the FAA in mid-1996 proposed a new airworthiness directive (AD) targeting Cessna turbocharged twin-engine exhaust systems, 321 stainless and Inconel were thrust into the limelight. Although there had been an AD in place since 1975 (AD 75-23-08) that required recurring inspections of the exhaust systems in the turbocharged twin-engine Cessna fleet, a rash of fatal accidents in 1995 and 1996 was traced to inadequate exhaust system maintenance and inspection. The accidents prompted the FAA to notify owners of affected Cessnas that a new AD would be written with more stringent performance procedures.
By the time that AD 2000-01-16 was finally issued in early 2000, the FAA had realized that the depth and breadth of exhaust and airplane technical knowledge amassed by aircraft owners' groups had been invaluable in the formulation of a workable AD.
A focused and concentrated effort by the Cessna Pilots Association to communicate with and educate its members provided the technical data. AOPA helped members of the owners' associations, industry representatives, and other affected parties join together to ensure that they were represented in the AD process. Other significant contributors to this effort were Steve Rapp of Parts Exchange, Larry Ball of the Twin Cessna Flyers group, and the experts at Ram Aircraft. This group effort was so effective that the FAA was surprised to receive more than 350 written comments related to the AD. To the FAA's credit, it listened, liked what it heard, and implemented sweeping changes in the system for creating ADs. Today the FAA often seeks technical input from owners' groups during the AD creation process.
One of the peripheral benefits of this process was the widespread dissemination of information concerning the advantages and disadvantages of Inconel and stainless steel. In addition, new data emphasizing the importance of limiting the high-end temperatures of 321 stainless steel systems were discovered.
The magic bullet?
Inconel 601 is undoubtedly able to withstand higher temperatures than 321 stainless. But Inconel has its own share of problems. Because of its high nickel content, the interior walls of Inconel pipes are affected by a corrosion interaction with the sulfur in the exhaust gases. This process, known as sulfidation, is undetectable by looking at the outside surface of the pipe. As a result of the sulfidation corrosion, the failure mode of Inconel is for tiny pinhole-size leaks to open up during flight. If one pinhole is detected, it's recommended that the exhaust system be removed and sent to an overhaul shop for evaluation. If there is ever any unusual discoloration on the cowling paint, or any evidence of exhaust leakage during a preflight inspection, seek the advice of an A&P mechanic before further flight.
There are limits
According to Alan Caddy, a metallurgical engineer who wrote an article in the March 1997 issue of the Cessna Pilots Association Magazine, 321 stainless steel suffers a shortened life and increased deterioration whenever it's exposed to exhaust temperatures in excess of 1,600 degrees F. While this upper limit should pose no problem for normally aspirated engines, most turbocharged airplanes can exceed the 1,600-degree limit, especially during high-altitude operations. A practice recommended by the Cessna Pilots Association is to lean by reference to peak EGTs until the turbine inlet temperature (TIT) reaches 1,600 degrees F. When temperatures reach this level, abandon the EGT reference and adjust the fuel mixture to keep the TIT below the 1,600-degree limit.
321 stainless steel, as opposed to 601 Inconel, is very easy to inspect in the field since long-term exposure to the exhaust gases will result in gradual changes to the metal structure. A worn-out pipe is characterized by a bluish-black color, and the appearance of small eruptions on the surface of the pipe. Severely worn pipes no longer have a smooth outer surface but look lumpy and saggy as the result of the metal actually "flowing" as it loses strength. When a pipe gets to this point, the metal has deteriorated so much that the system is no longer repairable or airworthy.
Parts and pieces
Airplane engines, with their individually removable cylinders, are connected to the exhaust system via individual pipes that bolt onto each cylinder. The cylinder pipes are termed risers and have flat steel flanges welded onto the cylinder end. A gasket is installed between the cylinder and flange, and it is held in place by special high-heat nuts threaded onto studs that project from each cylinder.
The risers then join manifolds called collectors, either by a slip-joint connection consisting of a smaller-diameter pipe sliding into a larger-diameter pipe or by being held in alignment and position with specially shaped clamp halves that are bolted together, surrounding and capturing raised beads that are formed in the risers and manifolds. The hot gases from the individual cylinders are contained, collected, and routed overboard.
Heat exchange
Simple systems have been developed to extract heat from the exhausting gases for heating the cabin and providing carburetor heat. These systems route the hot exhaust gases through a rudimentary heat exchanger (which some pilots mistakenly think is a muffler) before exhausting them overboard. In most cases this heat exchanger looks like a welded metal can surrounded by a thin stainless steel cover. In most heat exchangers there are devices, such as perforated metal cones, that are welded to the inlet end of the "muffler" to aid in heat extraction by slowing down and spreading the hot exhaust gases against the inside surface of the can. Ram air is directed to the heated space between the outer surface of the can and the surrounding shroud. After the air circulates around the heated can, it is routed to the cabin heater, carburetor heat duct, defroster, or, if it's not needed, out the bottom of the engine compartment.
These internal baffles, or cones, should be visually checked every three or four months for condition. This can be done by shining a flashlight up the exhaust pipe and visually checking the condition of the internal parts. Any internal baffle that appears to be wavy, distorted, or eroded should be repaired. If eroded or weak baffle material breaks off and plugs the exit pipe during a flight, it will seriously limit the engine's power output. In Advisory Circular (AC) 91.59, the FAA writes that 20 percent of partial power failures or power losses are the result of internal baffle failure. Fortunately, baffle replacement is a routine procedure for exhaust system repair shops. Wall Colmonoy, a leading manufacturer of exhaust system components for many airplane manufacturers, is currently welding Inconel baffles in some of their muff…oops, I mean heat exchangers.
Fatal leakage
During inspections, mechanics are required to test the heat exchanger for leakage. This inspection is no one's favorite job, because a surrounding shroud must be removed and access is often difficult. The inspection can be done visually using just a flashlight, but most shops are more comfortable with a method that pressurizes the heat exchanger can by pumping compressed air into the tailpipe while spraying the outside of the can with soapy water. Any leaks, even the barely visible ones, will blow easily detected bubbles.
If any leaks are missed, carbon monoxide gas, a product of incomplete combustion in internal combustion engines, may enter the cabin through the cabin heater system. Since this deadly gas is colorless, odorless, and fatal, all pilots should invest in a carbon monoxide detector and carry it in their airplane at all times. There are some very capable units for under $50.
Maintenance and failures
While many pilots aren't interested in learning the hands-on skills that are required to inspect a complete exhaust system, a good visual preflight inspection of the outer cowl before every flight helps spot leakage, which is the number one exhaust system problem. Grayish or brownish trails streaming aft from ventilating louvers or any discoloration on the cowl aft of the engine compartment can signal to pilots that there is an exhaust leak. Unless the exhaust system components are eroded, bent, or cracked, small leaks are relatively inexpensive to fix.
Why all this insistence that all leaks be fixed? Simply because exhaust gases are extremely hot (1,700 degrees F or higher) and corrosive. In an engine compartment, the close proximity of critical system components such as fuel lines and oil lines means that there really is no such thing as a noncritical exhaust system leak. Even a small leak between the steel exhaust pipe flange and the aluminum cylinder exhaust port flange will quickly erode the aluminum flanges of the cylinder head. If the leak is caught soon enough, the cylinder exhaust port can be resurfaced without removing the cylinder. Repairing severely eroded cylinder exhaust flanges requires cylinder removal, which is expensive and time consuming. If the leak is not discovered or is ignored, and the resulting erosion goes on too long, the cylinder flange will require welding to replace metal lost as a result of the leak.
Free power
The airplane manufacturers work with the engine company and exhaust system engineers to create an exhaust system that is reliable, lightweight, fits the airframe and cowl design, and is adaptable to ancillary functions, such as carburetor and cabin heat. Since many of today's systems were designed when aviation fuel was inexpensive, concerns about the efficiency of the exhaust system/engine installation were often pretty far down the list.
Lycon ( www.lycon.com), an engine rebuilder in Visalia, California, conducted a series of test-cell checks for Power Flow Systems of Daytona Beach, Florida. During these tests it was found that the power output of a 150-horsepower Lycoming O-320 engine was 134 hp when the "stock" Cessna 172 exhaust system was installed. Robin Thomas, owner of Power Flow Systems, and his staff have developed a number of free-breathing exhaust systems that bolt on, replacing power-robbing factory systems. After bolting one of his systems to the Lycon O-320 mentioned above, the engine developed 157 hp — a gain of 23 hp!
Thomas presently has STCed systems for most Lycoming-powered 150- and 180-hp Cessna 172, 177, 177A, and 177B airplanes, and Piper PA–28-140s. Testing is under way with plans to add systems for the Cessna 182, 172R, and 172S models; Grumman AA5B Tiger, Cheetah, and Traveler; and the Mooney M20C. More information can be obtained on the Web site ( www.powerflowsystems.com).
Exhaust system parts should not be the cause of any problems if they are overhauled at engine TBO like any other engine accessory. Because there are no moving parts in normally aspirated installations, a cursory visual check is often the only attention they receive during an engine overhaul. An airplane exhaust system is one that demands respect because, unlike other critical airplane systems, there is no redundancy where your exhaust system is concerned.
Links to additional information about exhaust systems may be found on AOPA Online ( www.aopa.org/pilot/links/2001/links0103.shtml). E-mail the author at [email protected].
Companies mentioned in "Airframe & Powerplant: Exhausted, and Often Forgotten":
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