Other operators of aircraft equipped with reciprocating  engines sometimes use a different replacement method in  these repair facilities because engine changes often occur  at random intervals. Such replacement engines may be  partially or wholly built up with the necessary accessories  and subassemblies, or they may be stored as received from  the manufacturer in packing boxes, cases, or cans and are  uncrated and built up for installation only when needed to  replace an engine. Because the QECA system is most commonly used with large  turbine engines used in the airlines, such engines are used to  describe QECA buildup and installation procedures. Many  of these procedures are applicable to all other methods of  engine buildup and installation. The following study of QECA buildup is not designed  to outline procedures to follow in a practical application;  always use those recommended by the manufacturer.  The procedures included in this chapter provide a logical  sequence in following a QECA and its components through  the stages of a typical buildup to gain a better understanding  of units and systems interconnection. The components of a  QECA are illustrated in  Figure 8-1.  As shown, the QECA  consists of several units. On many aircraft, the engines are  mounted in streamlined housings called nacelles that extend  from the wings. These nacelles are divided into two main  sections: wing nacelle and engine nacelle. The wing nacelle  is that portion of the nacelle that is attached to the wing  structure. The engine nacelle is that portion of the nacelle  that is constructed separately from the wing. Also, the wing  nacelles normally contain lines and units of the oil, fuel, and  hydraulic systems, as well as linkages and other controls for  the operation of the engine. The firewall is usually the foremost bulkhead of the engine  nacelle and differs from most other aircraft bulkheads in that  it is constructed of stainless steel or some other fire-resistant  material.  [Figure 8-2]  The primary purpose of the firewall is to  confine any engine fire to the engine nacelle. It also provides  a mounting surface for units within the engine nacelle and a  point of disconnect for lines, linkages, and electrical wiring  that are routed between the engine and the aircraft. Without  this firewall, an engine fire would have ready access to the  interior of the aircraft. Since the consequences of an engine  fire are obvious, the necessity of sealing all unused openings  in the firewall cannot be overstressed. An aircraft engine and its accessories that have been in  storage must undergo careful depreservation and inspection  before they may be installed in an aircraft. This involves  more than removing an engine from its container and bolting  it to the aircraft. If the engine is stored in a pressurized metal  container, the air valve should be opened to bleed off the  air pressure. Depending upon the size of the valve, the air  pressure should bleed off in somewhat less than 30 minutes. Prepare the container for opening by removing the bolts that  hold the two sections together. Then, attach a hoist to the  “hoisting points” and lift the top section clear of the container  and place it away from the work area. If the engine is installed  in a wooden shipping case, it is necessary to carefully break  the seal of the protective envelope and fold it down around  the engine. Remove the dehydrating agent or desiccant bags  and the humidity indicator from the outside of the engine.  Also, remove and set safely aside any accessories that are  not installed on the engine but are mounted on a special  stand or otherwise installed inside the protective envelope  with the engine.  QECA Buildup Method for Changing of Engines Figure 8-2.  Typical firewall with components mounted on it. Figure 8-1.  Open cowling view of a typical power package. 8-4