Recip Technology: Heating the Aircraft Cabin

As winter approaches here in the Northern Hemisphere, it’s time to prepare for operating aircraft in colder temperatures. For many single-engine aircraft powered by reciprocating engines, winter flying means heating the aircraft cabin by way of the exhaust system and a heat exchanger. The warm cabin air is a direct result of fresh air passing over and around the aircraft muffler or heat exchanger.

The importance of maintaining the condition of the exhaust system and associated components can not be overstated. A poorly maintained system can cause dangerous exhaust fumes to make their way into the cabin and ultimately can cause carbon monoxide poisoning to the unsuspecting pilot and passengers. Over the years there have been numerous articles, technical bulletins, and Airworthiness Directives relating to exhaust systems.

In 2009 the Federal Aviation Administration tasked Wichita State University to conduct research that focused on carbon monoxide safety issues as they apply to general aviation aircraft. The full report titled “Detection and Prevention of Carbon Monoxide Exposure in General Aviation Aircraft, Document No. DOT/FAA/AR-09/49, dated October 2009” can be found at the National Technical Information Services web site at http://www.tc.faa.gov/its/worldpac/techrpt/ar0949.pdf.

FAA Special Airworthiness Information Bulletin

As a result of this study and report, the FAA released another Special Airworthiness Information Bulletin (SAIB) CE-10-33, dated May 7, 2010, and a subsequent revision CE-10-33R1, dated Aug. 16, 2010, to communicate an airworthiness concern with exhaust system heat exchangers that are used for heating the aircraft cabin.

The following excerpt comes directly from the SAIB:

The report shows that after researching National Transportation Safety Board (NTSB) accidents related to carbon monoxide (CO) poisoning, the muffler system was the top source of CO. For the CO-related cases where the muffler was identified as the source of the CO leakage, 92 percent had a muffler with more than 1,000 hours of service.

The SAIB goes on to recommend the following four items:

  1. Replace the mufflers on reciprocating engine-powered airplanes that use an exhaust system heat exchanger for cabin heat with more than 1,000 hours on the muffler and at each 1,000-hour interval, unless the manufacturer recommends or FAA regulations require a more frequent replacement.
  2. Review and continue to follow the guidance for exhaust system inspections and maintenance in SAIB CE-04-22, dated Dec. 17, 2003, and Advisory Circular 43-16A, Aviation Maintenance Alert (AMA), issued October 2006, All Powered Models, Carbon Monoxide Poisoning Potential.
  3. Use CO detectors while operating your aircraft as recommended by SAIB CE-10-19R1, dated March 17, 2010.
  4. Continue to inspect the complete engine exhaust system during 100-hour/annual inspections and at inspection intervals recommended by the aircraft and engine manufacturers following their applicable maintenance manual instructions.

AMT talks to the experts

To understand more on exhaust system inspection and maintenance best practices, AMT visited Aerospace Welding Minneapolis Inc. (AWI), an FAA-approved repair station specializing in repair and overhaul of general aviation aircraft exhaust systems. Tom Heid, the company’s president, shares, “Over the years there has been progressive information supporting the importance of this subject. The concern is that the importance of the issue is not going away.”

Heid went on to explain how normal wear and tear damage to a muffler occurs on inside, and as a result, the inside of an exhaust component is always in worse condition than the outside. This is due to the chemical reactions taking place in the metal when it is continually exposed to exhaust gases and heat. Visually inspecting the outside of a muffler may not be good enough. The importance of thoroughly inspecting the inside of a muffler can not be overstated.

An example I was shown was a muffler from a Piper PA28-236 with a crack near the end of the muffler can. “The swirling actions of the exhaust gases near the end-caps of the muffler can erode the metal away on the inside and eventually cause a crack,” says Heid. He went on to explain that mufflers are made using thin material; some are .035- to .040-inch thick material. It doesn’t take much thinning to fatigue the material and crack.

AWI recommends that in the field, pressure checking a muffler is the best way to find cracks and ensure a good muffler. Advisory Circular 91-59A dated July 23, 2007, and SAIB CE-04-22 dated Dec. 17, 2003 both describe this recommended method. During exhaust system inspections, pressurizing the system slightly can aid in the detection of cracks. You can apply 2 to 3 pounds per square inch (psi) of pressure using regulated shop air, or a shop vacuum inserted in the exhaust pipe, and using a solution of soapy water sprayed on the system, any cracks will be found as they form a bubble on the surface of the metal.

Ensuring that internal muffler baffles are in good shape requires you to dissemble the system and carefully look inside of it. Take time to consider what conditions you are seeing in order to determine if the baffles are intact, loose, bent, or sagging.

When asked what recommendations AWI has for technicians making repairs to mufflers, Heid shared the following: “You really must ensure you have a clean surface in the area of the crack, both outside and inside of the muffler. If you can’t achieve this the welded repair will not last long. Next, make sure you use the correct welding rod material for the material that the muffler is made from. And, the welder should have some experience in welding thin-gauge material.”

Best practices for exhaust system inspection and installation

When checking the condition of an exhaust system it is very important to remove all shrouds and shields from the muffler and exhaust pipes in order to access all areas of the system. Use of a flashlight and inspection mirror to shine into pipes and check areas difficult to directly view is a must. If needed, you can even use a borescope to examine the internal components such as baffles and tubes.

Look for signs of exhaust leakage. Examine engine compartment surfaces located next to the exhaust system for signs of discoloration or exhaust soot. Examine the exhaust system components for exhaust colored stains around welds, clamps, and flanges. Check all connections for proper fit. Inspect all surfaces for bulges, distortions, and cracks which are signs of metal fatigue. Examine bends in pipes for pitting and thinning of material. Use an awl to probe material in suspected thin material or weak spots.

Carefully inspect internal components for wear, pitting, cracks, and broken baffles. Corrosion may be occurring on a component that looks good on the outside. Damaged, bent or missing heat studs, fins, or other heat sink material can cause uneven heating of the muffler surface, leading to thin spots and holes in the muffler can. If the muffler has internal baffles or tubes check to determine if they are damaged or missing. Broken baffles may become dislodged and restrict the exhaust outlet and can cause loss of power. Damaged or missing baffles means the muffler needs to be replaced.

During exhaust system installation, AWI recommends that you do not force fit parts together as this can cause stress, lead to cracking, and shorten the life of the exhaust system components. Make sure that all parts are properly aligned during installation. Use an anti-seize compound for high temperatures on all slip-joints during assembly, and disassemble and re-apply during the annual inspection. Loosely mount the system on the engine and then tighten all connections to the manufacturer’s specifications. Retighten everything after an engine run where the exhaust system has a chance to heat up and expand and contract. And finally, do not reuse exhaust system gaskets.

The above recommendations can be found on the AWI web site. As always when accomplishing any maintenance and inspection tasks, closely follow the aircraft and/or engine manufacturer’s instructions. Proper exhaust system inspection and maintenance is important and these best practices just might save a life someday. AMT

Information for this article was provided by Aerospace Welding Minneapolis Inc., an FAA and EASA approved repair station specializing in repair and overhaul of aircraft exhaust systems and related components, engine mounts, and other welded components. The AWI parts catalogue which is available on its web site contains helpful inspection and maintenance tips relating to specific part number exhaust systems. For more information call (800) 597-4315 or visit www.awi-ami.com.

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