WWW.SURVIVALEBOOKS.COM<strong>FM</strong> 1-<strong>506</strong>drive the engine during starting. After the engine reachesa self-sustaining speed, the starting system operates as agenerator to supply the electrical system power.The starter-generator unit is basically a shunt generatorwith an additional heavy series winding. This serieswinding is electrically connected to produce a strong fieldand a resulting high torque for starting. Start-generatorunits are desirable from an economical standpoint. Oneunit can perform the function <strong>of</strong> both starter and generator.Additionally, the total weight <strong>of</strong> starting systemcomponents is reduced, and fewer spare parts required8-8
WWW.SURVIVALEBOOKS.COM<strong>FM</strong> 1-<strong>506</strong>CHAPTER 9INSTRUMENT SYSTEMSEngine instruments are mounted on the instrumentpanel in the pilot’s compartment. They are usuallygrouped together. They aid the pilot in monitoring theengine, rotor, or propeller RPM pressures temperaturesand general engine performance. Refer to theengine’s maintenance manual for maintenance, operationallimitations, and color code markings. Instrumentstoday read in vertical scales, digital readouts, and in somecases heads up display (HUD). The HUD systemprojects instrument images on the windshield in front <strong>of</strong>the pilots. In this way, pilots do not have to divert theireyes from the view in front <strong>of</strong> and around them. Thevertical scale <strong>of</strong>fers similar safety conditions. The pilotshave the ability to quickly glance at the instrument for areading. This chapter will discuss the most commonlyused instruments and the newer vertical scale (VIDS).VERTICAL INSTRUMENT DISPLAY SYSTEM(VIDS)The VIDS (Figure 9-1) consists <strong>of</strong> a vertical stripcentral display unit (CDU), two vertical strip pilot displayunits (PDU), and two signal data converters (SDC).These readings are shown by ascending and descendingcolumns <strong>of</strong> multicolored lights (red yellow, and green)measured against vertical scales. The scales operate assegments and light in normal progression. They remainon as the received signal level increases. They go <strong>of</strong>f innormal progression as the received signal level decreases.When scales with red- and amber-coded segments belowgreen-coded receive a signal level <strong>of</strong> zero (bottom scale),the segments light in normal progression and remain on.When the first segment above the red or amber rangegoes on, all red-or amber-coded segments go <strong>of</strong>f. Thesesegments remain <strong>of</strong>f until the received signal level indicatesa reading at or within the red or amber range. Atthat time all red- or amber-coded segments on the scaledisplay goon or <strong>of</strong>f in normal progression. For an increasingindication when a scale with side arrows lights,the corresponding side arrow also lights. As the segmentsgo on, the corresponding side arrows also go on, one at atime. Only the side arrow associated with the highestpercent indication <strong>of</strong> the corresponding scale will be on.For a decreasing indication, scales with side arrows willoperate in the same manner. Only the side arrow associatedwith the highest percent indication <strong>of</strong> the correspondingscale will be on. The CDU and PDUscontain photocells that automatically adjust indicatorlighting to ambient light. If either photocell fails, the lightson the vertical scales or the PDUs and the CDU go <strong>of</strong>f.The DIM knob on the CDU has an override capabiltywhich allows the pilot to manually set the display lightlevel.TACHOMETER SYSTEMThe tachometer provides the pilot with a continuousindication <strong>of</strong> engine RPM. A variety or combination <strong>of</strong>systems may be used on gas turbine engines. Gasproducer to gas generator tachometers, turbine and rotortachometers, and N-I and N-II tachometer are some <strong>of</strong>the tachometer systems used (Figure 9-2).Engine rotor RPM can be sensed by a mechanicallydriven tachometer generator, mechanically driven permanentmagnet, or a pulse pickup which senses passingcompressor, fan blades, or gear teeth. The output orsignal from any <strong>of</strong> the sensors is directed to an appropriateindicator in the cockpit. The indicator iscalibrated to read directly in percent RPM. Dual axialflowcompressor engines are usually provided with twotachometers: one indicates low-pressure compressorspeed (N l), and the other indicates high-pressure compressorspeed (N 2). For the dual-axial and centrifugalcompressors the second tachometer indicates powerturbine speed (N 2).For most axial-flow compressor engines, thetachometer monitors RPM during engine start and indicatesoverspeed condition should one occur. Use <strong>of</strong> thetachometer for setting thrust is not recommended onaxial-flow compressor engines. The low-pressure compressor(N l) tachometer on engine pressure ratio(EPR)-controlled dual-compressor engines may be usedas an approximate reference to set engine thrust in transientand certain other flight conditions. When the N 1tachometer sets engine thrust, the thrust setting should9-1
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