Preparation of Engines for Installation
After the decision has been made to remove an engine, the preparation of the replacement engine must be considered. The maintenance procedures and methods used vary widely. Commercial operators, whose maintenance operations require the most efficient and expeditious replacement of aircraft engines, usually rely on a system that utilizes the quick-engine-change assembly (QECA), also sometimes referred to as the engine power package. The QECA is essentially a powerplant and the necessary accessories installed in the engine.
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.
QECA Buildup Method for Changing of Engines
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 fireresistant 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.
Depreservation of an Engine
After the engine has been secured to an engine stand, all covers must be removed from the points where the engine was sealed or closed with ventilatory covers, such as the engine breathers, exhaust outlets, and accessory mounting-pad cover plates. As each cover is removed, inspect the uncovered part of the engine for signs of corrosion. Also, as the dehydrator plugs are removed from each cylinder, make a very careful check of the walls of any cylinder for which the dehydrator plug color indicates an unsafe condition. Care is emphasized in the inspection of the cylinders, even if it is necessary to remove a cylinder.
On radial engines, the inside of the lower cylinders and intake pipes should be carefully checked for the presence of excessive corrosion-preventive compound that has drained from throughout the interior of the engine and settled at these low points. This excessive compound could cause the engine to become damaged from a hydraulic lock (also referred to as liquid-lock) when a starting attempt is made.
The check for excessive corrosion-preventive compound in the cylinders of reciprocating engines can be made as the dehydrator plugs are removed from each cylinder. Much of the compound drains from the spark plug holes of the lower cylinders of a radial engine when the dehydrator plugs are removed. But some of the mixture remains in the cylinder head below the level of the spark plug hole and can be removed with a hand pump. [Figure 8-3] A more positive method is to remove the lower intake pipes and open the intake valve of the cylinder by rotating the crankshaft. This latter method allows the compound to drain from the cylinder through the open intake valve. If excessive compound is present in an upper cylinder, it can be removed with a hand pump.
The oil screens should be removed from the engine and thoroughly washed in an approved solvent to remove all accumulations that could restrict the oil circulation and cause engine failure. After the screens are cleaned, immerse them in clean oil and then reinstall them in the engine.
When the cover has been removed from the intake area, the silica gel desiccant bags (used to remove moisture from the engine in storage) must be removed from the engine area. If the engine uses a propeller, remove the protective covering from the propeller shaft and wash all corrosion-preventive compounds from both the inside and outside surfaces of the shaft. Then, coat the propeller shaft lightly with engine oil. Turbine engines require the removal of several covers on many external areas on the engine.
As a final check, see that the exterior of the engine is clean. Usually a quantity of compound runs out of the engine when the dehydrator plugs and oil screens are removed. To clean the engine, spray it with an approved commercial solvent.
Inspection and Depreservation of Accessories
An engine’s performance is no better than that of its accessories. Though the engine has been completely overhauled and is in top condition, any oversight or error in installing the accessories can result in improper engine operation or even irreparable damage to it.
Before depreserving any of the accessories enclosed with the engine, consult the storage data usually stenciled on the outside of the engine container or the records enclosed with the engine to determine how long the engine and accessories were in storage. Certain accessories that normally accompany an engine from overhaul are considered unsafe for use if their time in storage has exceeded a specified period. This time varies according to the limits prescribed by the manufacturer.
Any accessory that has been removed from an old engine that can be installed on the new one must be given a thorough inspection to determine its condition. This inspection includes a check for general condition, cleanliness, absence of corrosion, and absence of wear as evidenced by excessive play in the moving parts.
Some accessories must be replaced, regardless of their operating time, if the engine is being changed because of internal failure. Such accessories may have been contaminated by metal particles carried into their operating mechanisms by the engine oil that lubricates them.
Before installing any replacement accessory, check it visually for signs of corrosion and for freedom of operation. Always wipe the mounting pad, flange, and coupling clean before mounting the accessory, and install the proper gasket between the mounting pad and the accessory mounting flange. Lubricate the accessory drive shaft if so indicated in the manufacturer’s instructions.