Inspection of General Aviation Aircraft Exhaust Systems
Exhaust system failure has caused a number of serious accidents in the past. Some of the problems that can arise out of an exhaust system failure are carbon monoxide poisoning, partial or complete loss of engine power, and fire.
With these hazards, the Federal Aviation Administration (FAA) has released Advisory Circular (AC) No. 91-59A in July 2007, canceling its 1982 predecessor, AC No. 91-59. AC 91-59A focuses on the hazards that a poorly maintained aircraft exhaust system can pose to safety. It further provides the problems that should be expected as it recommends the pilots and the mechanics to perform maintenance, including routine inspection of the exhaust systems.
According to the AD, potential exhaust failures include the following:
- Escape of exhaust gas into the cabin, possibly through the cabin heat system, when there is muffler or heat exchanger leakage.
- Material failures in components of heat exchangers and mufflers that function as both, leading to leakage of the exhaust gas directly into the cabin or through the cabin heat system.
- Partial or full engine power loss caused by loose baffles, cones, or diffusers on mufflers and heat exchangers that partially or completely block the exhaust gas outlet flow. This condition may occur intermittently if internal components are loose within the muffler and move around during subsequent flights.
- Impingement heating or torching of the surrounding structure can occur in any area where exhaust system components exist or are breached and may lead to structural failure or fire conditions. Torching is of particular concern on turbocharged engines, which operate at higher exhaust gas temperatures and pressures.
To prevent these problems from occurring, the FAA mandates that all inspections, checks, and processes be done in accordance with the recommendations of the exhaust system’s and the aircraft’s manufacturer. Also, regular inspections are a must – annual, progressive, and 100-hour inspections. When defects are found, repairs should also be done as prescribed. Moreover, daily pre-flight inspections by the owner or the operator should be performed to ensure safety.
Here are some signs that the FAA has set to see potential exhaust system problems:
- Signs of exhaust system leakage on external surfaces include a flat gray, gray-white, or light gray powder or a sooty appearance. Signs of exhaust system and aircraft structural deterioration include warping, deformation, thinning, collapse, dents, cracking, tears, separation, scaling, weld separation, discoloration, corrosion, metal pitting, or burn-through.
- Signs of exhaust system leakage may appear on external joints, flex-joints, slip-joints, clamps, or couplings.
- Improper installation, including misalignment of exhaust stacks, ball joints, and/or connections can appear as abnormal wear or warped, broken, loose, or missing fasteners, clamps, gaskets, or seals.
- Airplanes that have had backfires or unburned fuel in the exhaust system may have damage such as weld failure to the baffles and cracks, particularly in areas where exhaust gasses could leak into cabin air heat exchangers.
Finally, the FAA recommends the following inspection procedure:
- Perform thorough preflight and subsequent repetitive inspections of the exhaust system.
- Perform daily visual inspection on the aircraft.
- Thoroughly inspect exhaust system components.
- Pressure test your aircraft’s exhaust system
- Partially remove exhaust system components for better inspection and checks
- Use a borescope for internal inspection
- Inspect turbocharger system components
- Perform complete inspection of aircraft exhaust system prior to cold weather operations
- Inspect installation of any exhaust system measuring components
- Avoid using stainless steel to repair a carbon steel system for it can cause severe localized corrosion.
For the complete and detailed inspection procedures plus information on repairs and overhauls, please read the full text of the Advisory Circular from FAA or give us a call so we can explain further.