Engine

Unit 1: Engines Engine and APU Components and System Operation Objective 1a: Without reference, identify basic facts and terms relating to engine and APU components and system operation. A minimum of 11 out of 15 responses must be correct. INTRODUCTION The F-108 engine is a high bypass turbofan engine used to power the majority of C-135 aircraft. Four engines are mounted and suspended individually below the wings. In this lesson, we will touch on engine components and the auxiliary power unit (APU) and their operation. INFORMATION Engine Cowlings (Fan and Fan Duct Cowls) The engine cowlings are hinged to the engine strut along the upper edge and latched together along the lower edge (Table 1-1, Figure 1-1). Guide pins and holes ensure positive alignment. There are two types of engine cowlings that will be covered: Fan cowls (2 each): Fan duct cowl (2 each): 1. Located forward exterior section of each engine. 1. Located on aft exterior section of each engine. 2. Streamline the engine. 2. Streamlines engine. 3. Allows for maintenance access. 3. Allows for maintenance access. 4. There are doors that provide access to the starter control valve and for oil servicing. Table and Figure 1-1 Fan Cowl and Fan Duct Cowl CUI

F-108 High Bypass Turbo Fan Fan Frame Module (Fan Case) The fan frame module (fan case) (Figure 1-2) is the first major section of the engine we will discuss. The fan frame module is the main support structure of the fan assembly. It provides a mount for the accessory section and the variable bleed valve system, which will be discussed shortly. Figure 1-2 Fan Case Compressor Section Another major section of the engine is the compressor section. The compressor section is divided into two parts, low pressure compressor (LPC or N1) and high pressure compressor (HPC or N2). The LPC (N1) consists of four stages: a 68-inch diameter fan and three reduced diameter booster stages (Figure 1-3). The 68 inch fan provides bypass airflow and the first stage of core engine compression. The N1 is connected to and driven by the low pressure turbine (LPT) by a shaft. The HPC (N2) is driven by and connected to the high pressure turbine (HPT) by a hollow shaft that rotates around the N1 shaft. The N2 consists of nine stages: one stage of variable inlet guide vanes (IGVs), three stages of variable stator vanes (VSVs), and five additional stages (Figure 1-3).

Combustion Chamber The combustion chamber is another main section of the engine. The combustion chamber utilizes an annular ring type construction. The main purpose of the combustion chamber is to provide an area for mixing compressed air and fuel. Turbine Section The turbine section (Figure 1-4) is also a main section of the engine. The turbine section consists of a high pressure turbine (HPT) and a low pressure turbine (LPT). The HPT consists of a single stage and is responsible for driving the HPC (N2) via a hollow shaft. The LPT consists of four stages and is responsible for driving the LPC (N1) via a shaft as well.

Exhaust Section The exhaust section is considered a main section of the engine as well. The exhaust section utilizes a convergent design that accelerates exhaust gases to increase thrust. Accessory Section The final main section of the engine is the accessory section (Figure 1-5). The accessory section consists of the transfer gear box (TGB) (Figure 1-6) and the accessory gear box (AGB). The TGB is driven by a shaft geared to the N2. The TGB also provides a mount for the starter. The TGB is connected to and drives the AGB. There are five components mounted on and driven by the AGB. These components are the fuel pump, lubrication unit, integrated drive generator (IDG), control alternator, hydraulic pump. Engine Systems Engine Fuel System The engine fuel system includes 4 parts as follows: ENGINE DRIVEN FUEL PUMP. The engine driven fuel pump (Figure 1-7) is mounted on the AGB. This is a two-stage pump (high/low pressure) that delivers fuel to the main engine control.

MAIN ENGINE CONTROL (MEC). The MEC is mounted on the engine fuel pump (Figure 1-7) and acts as a speed governor. The MEC meters fuel to 20 primary swirl fuel nozzles which spray atomized fuel into combustion chamber. The amount of fuel metered by the MEC is based on throttle position and airflow conditions. Figure 1-7 Fuel Pump and MEC POWER MANAGEMENT CONTROL (PMC). The PMC (Figure 1-8) is mounted on the right side of the fan frame module (fan case). The PMC computes and maintains a constant thrust setting based on throttle position. It is powered by the control alternator on the AGB.

SERVO FUEL HEATER AND MAIN FUEL/OIL HEAT EXCHANGER. The servo fuel heater and main fuel/oil heat exchanger is two separate but interconnected units (Figure 1-9). This unit cools engine oil by using fuel as a cooling medium. Figure 1-9 Main Fuel/Oil Heat Exchanger Engine Oil System The engine oil system is completely self-contained. The engine oil system on each engine lubricates, cools, and cleans engine bearings and gears installed in the engine. OIL TANK. Each engine has an oil tank mounted on the left side of the fan frame module (fan case). The tank has a capacity of 5 gallons. An access door on the left fan cowl of each engine allows access to the oil tank; servicing is accomplished through a filler cap located at the top of the tank. LUBRICATING UNIT. The lubricating unit (Figure 1-10) contains oil pumps, relief valves, bypass valves, and filters with clogging indicators. The lubricating unit also contains four magnetic plugs (chip detectors) that detect internal engine wear/failure.

Starter and Ignition Systems Switches on the pilot’s forward instrument panel (Figure 1-11) control the starting and ignition systems. These systems are used to start the engines either for a ground maintenance run or for an aircraft launch. Figure 1-11 Starter and Ignition Switches IGNITION SYSTEM. The purpose of the ignition system is to ignite the fuel/air mixture in the combustion section. Two ignition exciters (Figure 1-12) provide a high voltage electrical current to the spark igniter plugs. The two spark igniter plugs extend into the combustion section at the 4 o’clock and 8 o’clock positions and change the high voltage electrical current into a spark. Figure 1-12 Ignition Exciters ENGINE STARTER SYSTEM. The engine starter system uses an air turbine starter that is pneumatically driven by bleed air. This bleed air comes from either the QSAS/APU, ground cart, or another operating engine (Figure 1-13). An electrically controlled starter control valve directs bleed air to the starter. In the event that the starter control valve fails to electrically open, the starter control valve may be opened or closed manually.

Variable Bleed Valve System The variable bleed valve system is there for compressor stability and prevents surging/stalling during acceleration and deceleration of the engine. The variable bleed valves (Figure 1-14) dump excess compressor air overboard through twelve bleed valves. Figure 1-14 Variable Bleed Valves Fire and Overheat Detection System F-108 engines have a continuous dual element fire/overheat detecting system (Figure 1-15). The continuous dual element system has the capability of providing both overheat and fire detection indications.

FIRE DETECTION CONTROL UNIT (FDCU). The FDCU (Figure 1-16) is located on the electrical equipment rack (next to boom operator’s forward position) and monitors the fire/overheat detection system. The FDCU is also equipped with a built-in-test (BIT) function so that the system can be tested. Figure 1-16 FDCU INDICATOR AND WARNING LIGHTS. There are three indicator/warning lights (Figure 1-17) for the fire/overheat detection system per engine that are located on the glare shield. A red “fire” light in the Thandle indicates an engine fire. An amber “compartment hot” light indicates an engine over heat condition exists. Lastly, the extinguisher switch (EXT) illuminates when the fire extinguisher system is armed.

ENGINE FIRE AND OVERHEAT DETECTION TEST SWITCH. This switch is located on copilot’s exterior light control panel (Figure 1-18). It checks the system and illuminates all indicator lights (if operable). Figure 1-18 Test Switch FIRE EXTINGUISHING SYSTEM. Two halon bottles are located in each INBD (#2 and #3) engine strut. Both bottles in each strut support the engines on the applicable wings. For example, the two halon bottles in the #2 engine strut support the #1 and #2 engines in the event of a fire. The fire extinguisher pressure gauges are located on the OUTBD side of the INBD (#2 and #3) engine struts (Figure 1-19). The pressure in the bottles is affected by the ambient air temperature. Figure 1-19 Pressure Gauges Engine fire extinguisher discharge indicators (discs) are located on the INBD side of the INBD engine struts (Figure 1-20). The discharge indicators will rupture to indicate thermal relief or intentional discharge. Each halon bottle has a red and a yellow indicator (disc). Red indicates thermal relief and yellow indicates that the bottle has been intentionally discharged.

The fire extinguishing system is operated by first pulling the T-handles to arm the explosive squibs. Once the system is armed, the pilot can press the extinguisher switch (EXT) to blow the squib (explosive) and discharge the halon to the appropriate engine. Quick Start APU System (QSAS) The QSAS is located in the aft section of the cargo compartment. The QSAS provides bleed air for engine starts, cabin heating, and electrical power for ground use. The QSAS consists of two gas turbine units mounted in a stainless steel enclosure. A single generator is installed on one of the two gas turbine units to provide electrical power. The QSAS can be started from three locations: the QSAS control panel (Figure 1-21), the pilot’s forward instrument panel (Figure 1-22), and the crew entry door (Figure 1-23).

The QSAS has two hydraulic accumulators (Figure 1-24) (one for each gas turbine unit) that store fluid under pressure to open/close the inlet/exhaust doors and start the gas turbine units. The accumulators are normally pressurized by the right hydraulic system or by a hand pump. The hydraulic accumulators are precharged with nitrogen or clean, dry air.

The QSAS has a surge dampener (Figure 1-25) that collects accumulator pressure. This surge dampener meters pressure to the starter and is pre-charged by nitrogen or clean, dry air. The QSAS accumulator pressure normally operates the inlet/exhaust doors (Figure 1-26), but, if necessary, these doors can be operated manually. Figure 1-26 QSAS Inlet and Exhaust Doors Jet Fuel Starter (JFS) System Specialty aircraft equipped with F-108 engines that have no room for a QSAS/APU have a JFS system (Figure 1-27). This self-contained unit is mounted on the #3 engine to allow the engine to be started without any AGE support.