Accident Details
On June 16, 2022, at approximately 0915 central daylight time, a Bellanca 17-30A, registration N39880, was substantially damaged during an accident at Bruce Campbell Field Airport (MBO) in Madison, Mississippi. The pilot and passenger were not injured. The flight was conducted as a personal flight under Title 14 Code of Federal Regulations Part 91.
After departing from MBO, the pilot observed smoke in the cabin and returned to land. While taxiing, the smoke worsened, prompting the pilot to stop the airplane and shut down the engine. The pilot and passenger egressed and noticed a fire on the right side of the engine compartment. The passenger attempted to extinguish the fire with a portable extinguisher but was unsuccessful. Fire department personnel arrived and extinguished the fire. The airplane sustained substantial damage to the forward fuselage.
Postaccident Examination
A postaccident examination revealed that the right-side exhaust system was fractured between the muffler and tailpipe. The muffler and tailpipe assemblies were sent to the National Transportation Safety Board Materials Laboratory for analysis. The laboratory determined that the aft ball tube, a component of the muffler assembly, had separated from the muffler body and remained attached to the tailpipe. The outer surface of the ball tube exhibited rubbing in the regions where it overlapped the tailpipe clamping assembly and the muffler.
The forward end of the ball tube had areas of cut-back material and raised material, both exhibiting rubbing features consistent with failure of a repair weld. Some areas of the ball tube displayed a gray and/or red appearance, indicative of an oxide film formed from repeated exposure to hot exhaust gases.
Maintenance History
In 2007, Alexandria Aircraft, the type certificate holder for the Bellanca 17-30A, issued Service Letter B-110 regarding inspection of the exhaust system. The Federal Aviation Administration subsequently issued Airworthiness Directive (AD) 2008-05-11, mandating compliance with the service letter. The service letter requires inspection of the muffler weld joints for cracks, with an initial inspection within 25 hours and recurring inspections every 50 hours.
Maintenance records indicated that AD 2008-05-11 was complied with during the last annual inspection on May 20, 2022, at an aircraft total time of 2,543.5 hours. According to the owner, the accident occurred about 10 flight hours after that inspection. A maintenance record of the weld repair was not available to the investigation.
The mechanic who performed the last annual inspection stated that he did not notice anything unusual with the exhaust system and remarked that the fracture location was difficult to view. He did not use a borescope to inspect internal welds and was not familiar with the service letter requirement to spray the outside of the muffler with penetrating oil to identify cracks.
Service Bulletin Compliance
Alexandria Aircraft Service Letter B-110 includes the following inspection procedure: spray the outside of the muffler at the riser and ball joint attachment welds with penetrating oil. If a crack exists around the riser or ball joint, the oil will quickly move through the crack, and a wet spot will appear inside the muffler.