engine failure can occur.  [Figure 3-43]  As opposed to other  failures, erosion and carburization caused by the extreme  thermal conditions are the primary causes of internal failures.  Engine backfiring and combustion of unburned fuel within the  exhaust system are probable contributing factors. In addition,  local hot-spot areas caused by uneven exhaust gas flow can  result in burning, bulging, or rupture of the outer muffler wall.  On exhaust systems equipped with augmentor tubes, the  augmentor tubes should be inspected at regular intervals for  proper alignment, security of attachment, and general overall  condition. Even where augmentor tubes do not contain heat  exchanger surfaces, they should be inspected for cracks  along with the remainder of the exhaust system. Cracks  in augmentor tubes can present a fire or carbon monoxide  hazard by allowing exhaust gases to enter the nacelle, wing,  or cabin areas. When a turbocharger or a turbosupercharger system is  included, the engine exhaust system operates under greatly  increased pressure and temperature conditions. Extra  precautions should be taken in exhaust system care and  maintenance. During high-pressure altitude operation, the  exhaust system pressure is maintained at or near sea level  values. Due to the pressure differential, any leaks in the system  allow the exhaust gases to escape with torch-like intensity  that can severely damage adjacent structures. A common  cause of malfunction is coke deposits (carbon buildup) in the  wastegate unit causing erratic system operation. Excessive  deposit buildups may cause the wastegate valve to stick in  the “closed” position, causing an overboost condition. Coke  deposit buildup in the turbo itself causes a gradual loss of  power in flight and low manifold pressure reading prior to  takeoff. Experience has shown that periodic de-coking, or  removal of carbon deposits, is necessary to maintain peak  efficiency. Clean, repair, overhaul, and adjust the system  components and controls in accordance with the applicable  manufacturer’s instructions. It is generally recommended that exhaust stacks, mufflers,  tailpipes, etc., be replaced with new or reconditioned  components rather than repaired. Welded repairs to exhaust  systems are complicated by the difficulty of accurately  identifying the base metal so that the proper repair materials  can be selected. Changes in composition and grain structure  of the original base metal further complicate the repair.  However, when welded repairs are necessary, the original  contours should be retained; the exhaust system alignment  must not be warped or otherwise affected. Repairs or sloppy  weld beads that protrude internally are not acceptable as they  cause local hot spots and may restrict exhaust gas flow. The  proper hardware and clamps should always be used when  repairing or replacing exhaust system components. Steel or  low temperature, self-locking nuts should not be substituted  for brass or special high temperature locknuts used by the  manufacturer. Old gaskets should never be re-used. When  disassembly is necessary, gaskets should be replaced with  new ones of the same type provided by the manufacturer. Turbine engines have several different types of exhaust  nozzles depending upon the type of engine. Turboshaft  engines in helicopters can have an exhaust nozzle that forms  a divergent duct. This type of nozzle would not provide any  thrust, all engine power going to rotate the rotors, improving  helicopter hovering abilities. Turbofan engines tend to fall  into either ducted fan of unducted fan engines. Ducted fan  engines take the fan airflow and direct it through closed  ducts along the engine. Then, it flows into a common exhaust  nozzle. The core exhaust flow and the fan flow mix and flow  from the engine through this mixed nozzle. The unducted fan  has two nozzles, one for the fan airflow and one for the core  airflow. These both flow to ambient air separate from each  other and have separate nozzles.  [Figure 3-44] The unducted engine or the separate nozzle engine handles  high amounts of airflow. The fan air which creates most of the  thrust (80–85 percent total thrust) must be directed through  Figure 3-43.  An example of internal muffler failure. Muffler failure  can be caused by erosion and carbonization, which in turn can lead  to breakage blocking exhaust flow. 3-26 Turbine Engine Exhaust Nozzles Exhaust System Repairs Exhaust Systems with Turbocharger Augmentor Exhaust System