The framework of a nacelle usually consists of structural  members similar to those of the fuselage. Lengthwise  members, such as longerons and stringers, combine with  horizontal/vertical members, such as rings, formers, and  bulkheads, to give the nacelle its shape and structural  integrity. A firewall is incorporated to isolate the engine  compartment from the rest of the aircraft. This is basically a  stainless steel or titanium bulkhead that contains a fire in the  confines of the nacelle rather than letting it spread throughout  the airframe.  [Figure 1-41] Engine mounts are also found in the nacelle. These are  the structural assemblies to which the engine is fastened.  They are usually constructed from chrome/molybdenum  steel tubing in light aircraft and forged chrome/nickel/ molybdenum assemblies in larger aircraft.  [Figure 1-42] The exterior of a nacelle is covered with a skin or fitted with  a cowling which can be opened to access the engine and  components inside. Both are usually made of sheet aluminum  or magnesium alloy with stainless steel or titanium alloys  being used in high-temperature areas, such as around the  exhaust exit. Regardless of the material used, the skin is  typically attached to the framework with rivets. Cowling refers to the detachable panels covering those areas  into which access must be gained regularly, such as the engine  and its accessories. It is designed to provide a smooth airflow  over the nacelle and to protect the engine from damage. Cowl  panels are generally made of aluminum alloy construction.  However, stainless steel is often used as the inner skin aft  of the power section and for cowl flaps and near cowl flap  openings. It is also used for oil cooler ducts. Cowl flaps are  moveable parts of the nacelle cowling that open and close  to regulate engine temperature.  There are many engine cowl designs.  Figure 1-43  shows an  exploded view of the pieces of cowling for a horizontally  Figure 1-38.  Honeycomb wing construction on a large jet transport aircraft. Outboard flap Trailing edge sandwich panels constant-thickness core Spoiler sandwich panel tapered core, solid wedge Trailing edge sandwich panels constant-thickness core Wing leading edge Inboard flap Spoiler sandwich panel tapered core, solid wedge Aileron tab sandwich panel tapered core, Phenolic wedge Aileron tab sandwich panel constant-thickness core Trailing edge wedge sandwich panel tapered core, cord wedge 1-20