The fuselage is one of the principal structural units of the airplane. It houses the crew, passengers, cargo, instruments, and other essential equipment. Most present day airplanes have a fuselage made of a combination of truss and monocoque design. In the truss type construction, strength and rigidity are obtained by joining tubing (steel or aluminum) to produce a series of triangular shapes, called trusses. In monocoque construction, rigs, formers, and bulkheads of varying sizes give shape and strength to the stressed skin fuselage (Fig. 2-2).
On single engine airplanes the engine is usually attached to the front of the fuselage. There is a fireproof partition between the rear of the engine and the cockpit or cabin to protect the pilot and passengers from accidental engine fires. This partition is called a firewall and is usually made of a high heat resistant, stainless steel.