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Aircraft Maintenance: Digging deep for a poisonous gas leak

A remarkable thing happened while we were testing the airplane at the beginning of our annual inspection. The Lightspeed Delta Zulu headset we were testing that day started warning us about carbon monoxide.

Internally, the muffler seemed to be in good condition. Photo courtesy of Jeff Simon.

This was remarkable for two reasons: first, because we hadn’t noticed any CO warnings during prior operations, and second, because it was the headset that first alerted us to the danger (as opposed to our portable CO detector).

The experience kicked off a “find the exhaust leak” adventure between my son Jake (a newly minted A&P) and me. The CO levels reported by the Delta Zulu directly related to the cycling of the cabin heat control in the cockpit. Since our annual inspection happens in the fall, this is often the first time we test the heat for the season.

We began with a borescope inspection of the muffler from the tailpipe. Surprisingly, this didn’t show an obvious smoking gun. Our exhaust system wasn’t very old and looked quite good from the inside. The flame cone looked fine, and we didn’t see anything too obvious with the borescope. In fact, when we first removed the muffler shroud for the external inspection, everything initially looked to be in excellent condition. It wasn’t until we examined the underside of the muffler that we discovered a deformed and cracked section directly in line with where the exhaust gases enter the muffler at an angle.

Superficial visual inspections can often miss exhaust leaks. That’s why it’s critically important to remove the muffler shroud and perform a “soap bubble” pressure check anytime that an area even looks slightly suspicious. Blowing filtered air into the exhaust while spraying a soap solution on the welds and other suspect areas will quickly show bubbles at any location air is leaking out. If air can get out during the test, exhaust gases surely can as well.

With the shroud removed, the muffler failure was obvious. Photo courtesy of Jeff Simon.

Knowing that we didn’t want to remain grounded waiting on our muffler issue, I immediately called Aerospace Welding Minneapolis Inc. (AWI) to see how quickly we could get a repair or replacement. AWI’s Jake Larson found all the parts we needed in stock, including upgraded Inconel components. According to Larson, repairing mufflers is becoming less common given the benefits of immediate shipping and the cost of labor balancing out the comparison between new and repaired components. They can still repair what you have, but in many cases (such as ours) shipping out a brand-new replacement as soon as the call comes in is better for the customer. In fact, I had the replacement muffler in hand before we even had the annual inspection completed. We didn’t lose a day in the process.

As with many things in aviation maintenance, one discovery led to another. As we removed the muffler for replacement, we discovered a cracked exhaust bracket at the mounting hole. The crack was completely hidden by the fasteners and wouldn’t have been discovered until the bracket failed entirely. Tuned in to concerns about CO, we looked for other areas where exhaust gases could be entering the cabin in flight and ended up repairing some seals between the cabin and the wheel wells. It’s not the first time that the repair of one failure led to the prevention of others.

A few things to consider during exhaust inspections:

  • Always include an electronic CO test as part of annual inspection runups.
  • Carefully inspect around “hot spots” in the design of exhaust components, such as heat-transfer studs and welds.
  • Broken or severely deformed flame cones are cause for muffler repair or replacement.
  • If you have a symmetrical muffler, consider rotating it slightly at annual to distribute the heat stress and get a bit longer life.
  • Disassemble and lubricate exhaust slip joints to reduce physical stress on the components.
  • Check exhaust mounting hardware and other support components carefully.
  • Use a soap or smoke test to check for leaks in the system.
  • Seal cabin access points, such as gear wells, where exterior exhaust gases could enter the cockpit during flight.

It turns out that my experience with the Delta Zulu was not unique. When I met with Lightspeed Executive Vice President of Marketing and Development Teresa DeMers at the Sun ‘n Fun Aerospace Expo in Lakeland, Florida, she explained the amazing data the company is seeing from its “opt-in” Personal Safety Data Partnership Program using the Delta Zulu. Using data collected from the Delta Zulu app over 5,000 flights, the company has discovered that 3 percent of those flights experienced at least temporary CO levels reaching 35 ppm and 1 percent of flights registered over 70 ppm at some point. This is alarming, considering how dangerous CO can be and how its effects can be virtually undetectable to humans until it’s too late. Clearly, our industry needs to pay more attention to exhaust system maintenance.

The data demonstrates how critical it is to use a modern CO detector in the cockpit regardless of the season. We’ve come a long way from the passive “brown dot” methods of detecting CO. Modern electronic detectors can help you identify and fix CO problems at the earliest stages, before they have the potential to become deadly. And now, with the advent of “safety wearables” such as Lightspeed’s Delta Zulu, CO detection can be as simple as putting on a headset. Until next time, I hope you and your families remain safe and healthy, and I wish you blue skies.

Jeff Simon
Jeff Simon
Jeff Simon is an A&P mechanic, IA, pilot, and aircraft owner. He has spent the last 22 years promoting owner-assisted aircraft maintenance and created the first inspection tool for geared alternator couplings available at Jeff is also the creator of SocialFlight, the free mobile app and website that maps more than 20,000 aviation events, hundred-dollar hamburger destinations, and also offers educational aviation videos. Free apps are available for iOS and Android devices, and users can also visit
Topics: Aircraft Maintenance
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