Foca&in A SERVICE PUBLICATION OF LOCKHEED-GEORGIA COMPANY A DIVISION OF LOCKHEED CORPORATION Editor Chares 1. Gae Associate Editors James A. Loftin Robert J. R. Rockwood Art Direction & Production Teri L. Mobr Vo. 13, No. 3, Juy-September 1986 CONTENTS 2 Foca Point A.F. Mis, Manager, Avionics Design Department Project Engineering Division 3 New Fue Quantity Indicating System Digita technoogy brings advanced features to the fue quantity indicating system. 11 Automatic Precision Caibration System AMPEL provides a better way to caibrate test equipment. 13 Engine Contro Vibration What causes it, and how to cure it. introducing Digita Technoogy One of the unique things about the Hercues aircraft is its extraordinary adaptabiity. It is fair to say that this remarkabe airifter has been undergoing a process of evoutionary deveopment and improvement ever since the first production aircraft came off the ine.i more than 30 years ago. AL MILLS A recent major update deserves to be noted asaspecia miestone. The new fue quantity indicating system, which is described in this issue of Service News, represents the first fu-scae appication of digita technoogy to a major operating system of the Hercues aircraft. It is no exaggeration to say that the features designed into this new system offer a quantum eap in fue quantity indicating system capabiity. The new system performs the basic function of providing fue quantity indications in a sophisticated, yet straightforward way. A microprocessor buit into each primary fue quantityindicatorusestheeectrica signas received from the tank units to cacuate fue quantity information, which is then presented on a iquid-crysta dispay. But that is just the beginning of the story. The digita indicators are aso abe to dispay error codes which show the presence of eectrica or functiona discrepancies. The system is capabe of discriminating among a wide variety of possibe mafunctions, thus offering rea assistance to the maintenance technician troubeshooting the cause of a fue system probem. In addition, the indicators continuousy check the integrity of their own interna operations. This ensures accurate and dependabe performance at a times. With its versatiity and wide-ranging anaytica capabiities, the new fue quantity indicatingsystem provides an exceent exampe of the immense promiseof microprocessor-based digita technoogy in modern aviation. The digita fue quantity system is a reaity today, but we have ony scratched the surface in rhe possibe uses of this technoogy. As research points the way to further appications of digita technoogy, you can be sure that Lockheed-Georgia Engineering wi be on the forefront of the effort to make these advances part of the everyday operating environment of the aerospace industry. Sincerey Cover: An MC-130E o erate by the 834th AGS of the 1st Specia Operations Wing, Hurburt Fied, Forida, gets some fina touches from SSgt Bruce C. Brege Jr at this year's Voant Rodeo. The back cover shows another view of this premiere internationa airift competition, hed annuay at Pope AFB, N.C. A.F. Mis, Manager Avionics Design Department Project Engineering Division Pubished by Lockheed-Georgia Company. a Division of Lockheed Corporation. Information contained in this issue is considered by Lockheed-Georgia Company Lo be accurate and authoritative; if shoud not be assumed, however, that this materia has received approva from any governmenta agency or miitary service uness it is specificay noted This pubication is for panning and information purposes ony. and it is not to be construed as authority for making changes on aircraft or equipment, or as superseding any estabished operationa or maintenance procedures or poicies. The foowing marks are registered and owned by Lockheed corporation: (0 Hercues: and Written permission must be obtained from Lockheed-Georgia Company before repubishing any materia in this periodica. Address a communications to Editor, Service News, Department 64-31, Zone 278, Lockheed-Georgia Company, Marietta, Georgia, 30063. Copyright 1986 Lockheed Corporation.
by Foyd Posey, Fied Service Representative Mark Cark, Design Engineer DESCRIPTION Most Hercues aircraft Lockheed seria number LAC 5041 and up are now equipped with a digita, computerized fue quantity indicating system. This new system, which is manufactured by Simmonds Precision, repaces the anaog fue quantity indicating system gages used on earier Hercues aircraft and incorporates many advanced features. The digita system utiizes the existing tank units and associated wiring, and the new gages are physicay interchangeabe with the earier anaog servomechanica fue gages (some minor wiring and connector changes are required.) However, it is not possibe to mix the two types. A fue gages on an aircraft must be of the new computerized type or they must a be of the earier anaog type. SYSTEM COMPONENTS Primary Indicators There are eight identica primary indicators, a the same part number, which are used in the fight station in pace of the earier gages. Each primary indicator is a sefcontained, computerized fue gage that independenty measures tank unit and compensator capacitancevaues, computes fue quantity, and dispays the data in both digita and anaog formats. Tank identification is determined by the arrangement of jumper wires in the pugs which connect the individua indicators to the aircraft wiring. Empty caibration is provided by a rear-mounted adjustment (see Figure 1). Since the required span from empty to fu is programmed into the indicator memory, no Lockheed SERVICE NEWS V13N3 3
4 Lockheed SERVICE NEWS V13N3
ANALOG FORMAT DIGITAL FORMAT TANK LOCATION - ERROR CODE Figure 3. Primary indicator Varistor and Indicating Fuse Like other precision eectronic components, the microprocessors that are incorporated in each primary indicator and the totaizer must be protected from votage surges. On the Hercues aircraft, this protection is provided by a surge suppression device (varistor) connected between the AC instrument and fue contro bus and the airframe (ground). A surge suppression protection indicating fuse is ocated on the piot s circuit breaker and fuse pane to indicate a shorted faiure of the varistor. Shoud such a faiure occur, the fuse bows, eectricay opening the circuit between the bus varistor and ground, thus disconnecting the shorted varistor from the bus. If this fuse bows in fight, no action is required by the fight crew. However, at the post-fight inspection, the ground maintenance personne shoud check the varistor to determine if it is shorted, and repace it and the fuse if necessary A spare fuse is provided in the eectrica spares box above the navigator s tabe. FUEL QUANTITY DISPLAY Digita/Anaog Format This new system measures the signas sent by each group of fue probes (tank units) independenty, and dispays fue quantity information in both digita and anaog formats. The digita format consists of three numerics, a decima point, and a mutipier on a iquid crysta dispay (see Figure 3). The anaog format consists of a 51-segment circuar bar graph, which represents the range of 0 to 100 percent of maximum fue quantity (at maximum fue density) in 2-percent increments, with associated reference points at IO-percent increments. Codes A tank code identifying the fue tank to which a particuar gage is connected appears at the bottom eft of the indicator dispay. The main tanks are identified as M, M2, M3, and M4. The externa tanks are identified as E and E2. The auxiiary tanks are identified as A and A2. Error codes (E, EO - E9) which report various types of system or indicator fauts appear at the bottom right of the indicator dispay. Note that the codes for the externa tanks and certain error codes are identica except for position on the dispay: tank codes E and E2 appear at the eft of the dispay, whie error codes E and E2 appear at the right of the dispay. Lockheed SERVICE NEWS V13N3 5
Indicator Lighting Lighting for the digita fue quantity indicators is provided by 28-VDC post ights. The indicator post ights are controed independenty from other overhead pane ights by a dimmer ocated above the copiot s upper circuit breaker pane. SELF-TEST FEATURE The buit-in test (BIT) for the primary indicators verifies that the dispay, and the compensator, and the tank units are operationa. In addition, the test feature isoates and dispays common fue indicating system fauts, which greaty faciitates system maintenance and troubeshooting. Each primary indicator responds to a test switch which causes the indicator dispay to decrease toward zero. Upon reease, the indicator dispays the foowing: A segments ON for approximatey 4 seconds. A segments OFF for approximatey 4 seconds. Any stored error codes that have occurred since power-up (see Figure 4). Any detected system fauts are annunciated on the error code dispay upon competion of the indicator test sequence. Testing a primary indicator wi aso trigger a test sequence of the totaizer indicator or the associated repeater indicator, whichever is in use at the time. The test sequence for the totaizer indicator and repeater indicators is as foows: Fue quantity dispay decreases toward zero. A segments ON for approximatey 4 seconds. A segments OFF for approximatey 4 seconds. Return to norma dispay. SYSTEM MONTORING CAPABILITY Because the capacitance vaues of the compensator and the combined tankunits are measured independenty, the indicator distinguishes between a fauty compensator and wiring, and the tank units and wiring. Further- Figure 4. Indicator sef-test dispay more, the indicators are abe to substitute appropriate vaues mathematicay to correct for defective compensators or defective ow impedance compensator wiring. This enabes the system to provide a safe, usabe fue quantity indication even in the presence of a probem. IMPROVED RELIABILITY The Simmonds computerized fue quantity gaging system uses a 5.68 KHz signa to excite the tank units and compensators. The use of this high frequency significanty increases the ratio of resistance to capacitive reactance over that of the typica 400-Hz system. This makes the new indicating system much ess susceptibe to inaccuracy caused by the effects of fue tank contaminants. The number of fue gaging probems is therefore greaty reduced. SYSTEM ADVANTAGES In summary, the advantages of the new digita computerized fue quantity indicating system are: Soid-state components. Liquid crysta dispay. High-frequency excitation. Buit-in test feature. 6 Lockheed SERVICE NEWS V13N3
Moduar packaging. Tabe 1. Primary Indicator Error Codes Common unit inventory. Fue Qty Dispay Expanation Recommended Action The Simmonds Precision design achieves maximum fexibiity by aowing the buit-in microprocessor to perform a indicator functions. Furthermore, new features can be impemented without hardware changes. TROUBLESHOOTING Troubeshooting the fue quantity indicating system wi in most cases invove the use of the GTF-6 test set or equivaent to verify continuity of fight station to tank wiring, in-tank wiring, and the condition of the tank units and compensators. Note that &troubeshooting activities invoving the fuequantityindicating system, incuding the connection and use of the GTF-6 test set, must be carried out in strict conformity to the safety precautions and other procedures specified in the appicabe authorized maintenance manuas. ERROR CODE ANALYSIS The BIT features incorporated in the Simmonds Precision digita fue quantity indicators provide fight crews and ground maintenance personne with vauabe assistance in accurate troubeshooting of fue quantity indicating system probems. These new indicators reduce downtime and maintenance costs by anayzing the fue quantity indicating system from the fue tank to the fight deck and dispaying error codes which indicate common fauts. The key to obtaining the maximum benefit from this information is, of course, correct interpretation of the indications. PRIMARY INDICATOR The heart of the new digita indicating system is the primary indicator for each tank. The BIT for the primary indicator verifies the operation of the dispay, checks for three separate compensator fauts, four tank unit fauts, indicator caibration faiure, and two eves of interna indicator faiure. Tabe 1 provides an expanation of various primary indicator fue quantity dispays and error messages, and the action that is recommended for each situation. Note that in the tabes and the foowing discussion the abbreviation o-z is used for ow impedance and hi-z for high impedance. E E2 E3 E4 E5 E6 E7 E8 E9 Zero with error code Error code with norma dispay at reduced accuracy Zero with error code Error code with norma dispay at reduced accuracy Zero with error code Error code with norma dispay at reduced accuracy, or zero with error code Zero with error code Error code with norma dispay at reduced accuracy Zero with error code zero or norma dispay with error code, reduced accuracy Bank Open or missing tank unit assemby Tank unit eectrica eakage Hi-Z open or shorted Verify continuity of in-tank wiring to each tank unit. Check hi-2 isoation from shied Check condition of each tank unit. Check hi-z from indicator to tank, o-z wires to tank units, and compensator. Compensator Check o-z wire unit o-z open from indicator to or shorted compensator. Tank nit Check o-z wire open or shorted from indicator to tank units. Compensator faut Shorted tank unit faut Compensator unit eectrica eakage Indicator faut Check condition of compensator. Check hi-z shied continuity. Check condition of each tank unit. Check hi-z shied continuity Check for water or ice in tank units. Check condition of compensator. Check compensator hi-z shied continuity Repace indicator. Indicator micro- Verify 5V at Note: Use ony authorized equipment and procedures in troubeshooting the fue quantity indicating system. PRIMARY INDICATOR CODES Error Code EO - Open or Missing Tank Unit Assemby The capacitance vaue of the tank units is known to have a specific vaue when the tank is empty. f one tank unit or a group of tank units is disconnected from the group, the measured tank unit tota capacitance wi be ess than the expected vaue. Error code EO is indicative of an in-tank wiring probem. The recommended maintenance action is to verify the continuity of the in-tank wiring, and to check hi-z isoation from airframe or shied. Lockheed SERVICE NEWS V13N3 7
Error Code E - Tank Unit Eectrica Leakage The resistance of the tank units instaed in each tank is periodicay checked by means of an automatic testing procedure carried out by the indicator microprocessor. When excessive current eakage is detected, error code E is dispayed. When this condition is present, fue quantity continues to be dispayed, but with 2.0 percent or ess error imposed by eectrica eakage. Error condition E indicates an in-tank condition. The recommended maintenance action is to drain the tank and check the tank units for contamination. This error code is sometimes caused by a two-point ground on the shied, aowing circuating currents. Note that if the measured resistance is severey out of imits, or a shorted tank unit is present, error code E6 wi be dispayed. Error Code E2 - Hi-Z Open or Shorted Both tank units and the compensator share a common hi-2 signa ine. If an in-range signa is not detected on the hi-z signa ine after norma excitation of both the tank unit and the compensator, the hi-z signa ine is either open, shorted to shied, or shorted to ground (airframe). When this condition is present, the fue quantity dispay wi go to zero. Error code E2 normay indicates a wiring probem in the coaxia wire and connectors in the aircraft wiring harness between the indicator and the tank. The recommended maintenance action is to check the hi-z signa wire to airframe and hi-z to shied. Check for hi-z continuity, indicator to tank unit. Error code E2 is dispayed when tank unit o-z and compensator o-z are both open at the same time, or when there is an open cabe connector in both ines. Error Code E3 - Compensator Lo-Z Open or Shorted Since the indicator microprocessors independenty excite the compensators and measure both the excitation and the return signa, it is possibe to identify specific wiring probems with compensator o-z wiring. If the o-z signa ine is open, a return signa wi not be detected. If the o-z signa ine is shorted to ground, the detected excitation signa wi be zero. When these conditions are present, fue quantity wi continue to be dispayed and the indicator microprocessor wi use a stored vaue equivaent to a nomina fue dieectric constant in pace of the measured compensator vaue. Error code E3 indicates wiring probems. The recommended maintenance action is to check the wiring harness from the indicator to the tank (specificay, the compensator o-z signa wire). Verify continuity and isoation from ground. Error Code E4 - Tank Unit Lo-Z Open or Shorted Each indicator microprocessor independenty excites the tank unit sensors and detects an open or short in the tank unit excitation ine simiar to error code E3. When this condition is present, the fue quantity dispay wi go to zero. Error code E4 indicates an open or short on the o-z signa ine to the tank units. Error Code E5 - Compensator Faut Each compensator is designed to measure a maximum sensor vaue. If themeasured vaueexceeds the maximum vaue, error code E5 is dispayed. Error code E5 is dispayed if conductivity across the compensator is high enough to cause a change of more than approximatey 25 percent (gage accuracy is sti within 2.9 percent) in K -1, which represents the dieectric constant used to cacuate the fue mass. When this condition is present, fue quantity wi continue to be dispayed and the indicator microprocessor wi use a nomina stored vaue for K -1. Error code ES indicates foreign matter, water, ice, or excessive contamination in the compensator. The recommended maintenance action is to check the fue for contamination. If no probemis found, thencheck hi-z shied continuity and the hi-z shied to ground. If the shied is okay, physicay check the condition of the compensator. Note that even if the faut is ceared, once E5 is set, it may take up to 30 seconds to cear. Error Code E6 - Shorted tank unit The tank units are designed to a specific capacitance range. If the design capacitance range is exceeded, E6 is dispayed. E6is aso dispayed if conductivity hi-z to o-z is high enough to cause more than 2.0 percent error at empty. When this condition is present, the fue quantity dispay goes to zero. 8 Lockheed SERVICE NEWS V13N3
Error code E6 indicates tank unit faiure, or drops of water or ice between tank unit tubes. The recommended maintenance action is to check the condition of the individua tank units by test set or inspection. If the tank units check out correcty, test hi-z shied continuity and ground isoation, and test for stray capacitance hi-z to IO-Z. Error Code E7 - Compensator Leakage The indicator microprocessor for each tank measures the exact extent of resistive contamination in the compensator by periodicay using a different excitation frequency and comparing the resut with measurements made at the norma excitation frequency. Excessive compensator current eakage wi cause error code E7 to be dispayed. When this condition is present, fue quantity continues to be dispayed. If the eakage of current is arge enough to cause a significant error in fue quantity determination, the indicator microprocessor wi use a nomina stored vaue for K -1 in pace of the measured compensator vaue. Error code E7 indicates that water or other impurities are buiding up in the sump area where the compensator is ocated. The recommended maintenance action is to check the fue for contamination and the condition of the compensator. Check aso for an open hi-z shied to compensator. An uncovered or partiay covered compensator may cause an error code E7. If the fue eve in a given tank is ess than 20% of that tank s maximum, an E7 error code may occur. This is considered norma operation. At the start of refueing an empty tank, E7 may dispay unti the compensator is fuy covered. Error Code ES - Indicator Interna Faiure In addition to the detection of system fauts, each indicator checks its own interna functiona eements. Error code ES reports interna faiure of the indicator. The recommended action is to repace the indicator. Error Code E9 - Interna Caibration Faut The sef-caibration circuitry is continuousy checked. If a faut occurs which causes a noninearity in the signa processing circuitry or a faiure of the system reference eements, E9 wi be dispayed. Error code E9 indicates an indicator faiure. The recommended maintenance action is to repace the indicator. Bank Dispay - Indicator Microprocessor Faut The circuit is designed so that if the indicator microprocessor fais the dispay wi bank. The maintenance action is to check that power is correcty appied to the indicator. If power is norma, repace the indicator. MEA0 Dispayed and No Error Code The indicator is unabe to perform tank ID (identify its associated tank). Check for good pug connection and wiring. If they check out correcty, repace the indicators. REPEATER INDICATORS The repeater indicators continuousy conduct BIT and dispay appropriate error codes for detected faiures. BIT, incuding a dispay test, is aso activated when the primary indicator is tested by remote test switch. The BIT features, recommended maintenance actions, and dispay status during the error conditions for the repeater indicator are summarized in Tabe 2: Tabe 2. Fue Qty Dispay Zero with error code Foows primary indicator Bank Repeater Indicator Error Codes Expanation Recommended Action Open or missing Verify correct digita input operation of primary indicator. Check wiring between primary and repeater indicator. Primary indica- Check primary tor is dispay- indicator. ing error code Indicator faiure or oss of power Verify 115v at connector Repace indicator if required. Note: IJse ony authorized equipment and procedures in troubeshooting the fue quantity indicating system. REPEATER INDICATOR CODES Error Code EO - Open or Missing Digita Input Error code EO is dispayed whenever the digita signa from the primary indicator is not being received. A primary indicator may be inoperative, or a wiring probem may exist between the indicators. A primary indicator error code E3, compensator o-z faut, may aso cause this error in the repeater indicator. 1 Lockheed SERVICE NEWS V13N3 9
Error Code E - Received Primary Indicator Error Error code E does not indicate an error within the repeater indicator. It does show that an error code from a primary tank indicator has been received, indicating that a probem is being reported by that indicator. Bank - Indicator Faiure The entire dispay is banked when there is a major faiure of the indicator, which indicates either tota faiure of the indicator microprocessor or the power suppy, or the tota faiure of the iquid crysta dispay. TOTALIZER INDICATOR/SUMMATION UNIT The BIT features, recommended maintenance actions, and dispay status during the error conditions for the totaizer indicator and totaizer summation unit are summarized in Tabe 3. Tabe 3. Totaizer Indicator Error Codes TOTALIZER INDICATOR CODES Error Code EO - Open or Missing Digita Input Error code EO is dispayed whenever the digita signa from the primary indicator is inoperative. A primary indicator error code E3, compensator o-z faut, may aso cause this error code in the totaizer. Error Code E - Received Primary Indicator Error Error code E does not indicate an error within the totaizer indicator. It does show that an error code from a primary tank indicator has been received, indicating that a probem is being reported by that indicator. Bank - Indicator Faiure The entire dispay is banked when there is a major faiure of the indicator, which resuts from tota faiure of the indicator microprocessor or the power suppy, or the tota faiure of the iquid crysta dispay. Fue Qty Dispay Tota fue quantity dispay minus the fue quantity of the tank associated with the EO error code Foows primary indicator Bank Bank Expanation Open/missing digita input Recommended Action Verify correct operation of primary indicators. Check wiring between primary and totaizer indicators. Primary indica- Check primary tor is dispaying indicator. error code Indicator faiure or oss of power Verify 15V at connector. Repace indicator if required. Note: Use ony authorized equipment and procedures in troubeshooting the fue quantity indicating system. 10 Lockheed SERVICE NEWS V13N3
by Terry James, Production Liaison Engineer, Senior The increasing sophistication of eectronic test equipment technoogy has made it impractica to train individua technicians on the caibration verification requirements for every piece of equipment sent to the metroogy ab. Contributing to the probem is the fact that whie test equipment is becoming more compex, it is aso becoming more reiabe. The technician in the metroogy ab is therefore unikey to see the equipment very frequenty, which adds to his difficuty in staying current on a requirements. The Practica Soution The use of automatic test equipment within the metroogy ab offers a practica soution to the need for accurate, methodica, and consistent testing with minimum training. Automatic testing offers many advantages over manua testing: Convenient organization of a series of tests. Fast test setup and execution. Data ogging of test resuts for easy comparison with earier tests. Repeatabiity, independent of the operator. Enter APMEL PN 3402854- Automated Precision Measurement Eectronics Laboratory To meet these requirements, Lockheed-Georgia Company has designed and buit the Automated Precision Measurement Eectronics Laboratory (APMEL). APMEL is an automated test and caibration system taiored to provide the user with the capabiity to caibrate the test equipment. used in connection with aircraft maintenance with a high degree of precision. Lockheed SERVICE NEWS V13N3 11
The hardware empoyed in this system is off-the-shef aboratory standard, produced and supported by the major equipment manufacturers. The software is in a high-eve anguage which permits the user to modify, revise, or procure programs to fit his changing needs. APMEL supports most genera-purpose test instruments. It has been speciay designed to permit the addition of both hardware and software to meet the specific needs of the individua customer. Mutipe Instruments The system makes use of the foowing programmabe stimuus or measurement instruments: function generator, digita mutimeter, scope caibration generator, AC and DC caibrator, audio anayzer, measuring receiver, RF signa generators, and rubidium osciator. Controer with Software The controer is the heart of the system. The controer is fundamentay a computer which tes the instruments what to do, coects the resuts, and processes them. It is equipped with a genera-purpose interface bus (GPIB) port capabe of communicating with as many as 28 other instruments; it aso contains two RS-232-C interface ports for data communications with items such as terminas, printers, or other computers. The controer is equipped with a magnetic tape drive for program and data storage and 512K bytes of memory. These features make the controer a very fexibe and powerfu device, capabe of processing compicated programs quicky and accuratey. At the same time, it is easy to use, and assists the operator with simpe step-by-step instructions iustrated by coor graphics dispays. Computer Peripheras An ink jet printer has been provided to produce a hard copy printout of each test run. The printout incudes test item performance versus programmed acceptance imits, and a pass/fai status for each caibration point. The record aows the user to deveop a historica data fie on each piece of equipment tested. ERASE keys, a 14-key numeric keypad, four specia function keys, and eight programmabe function keys. Dispay A dispay provides the user with instructions for setup and operation of the system in both text and graphics. It provides a review of intermediate resuts and monitors system operations. The termina can dispay 64 distinct coor mixtures. Up to 16coors can bedispayed simutaneousy in the graphics area, with an additiona 8 coors in the text area. Coor enhances thevisua impact and the information content of both graphics and text. System Interface The entire system is mounted in a singe three-bay cabinet, incuding a front-mounted work bench (20 inches by 5.5 feet). A eectrica connections are made within the cabinet for thecontroer, measuring instruments, and peripheras. The measuring instruments are connected on two separate GPIBs to the controer. One bus contains the meter and oscioscope caibration equipment and the other bus contains the RF instruments. These buses and the instrument interfaces aow connection of the system components to the controer. The appication software which contros the system and provides instruction to the operator is suppied on tape cassettes for each item of equipment to be caibrated. For More Information If you woud ike additiona technica or procurement information about the PN 3402854- Automated Precision Measurement Eectronics Laboratory, pease direct your inquiries to the Manager, Suppy Saes and Contracts Department at the foowing address. Lockheed-Georgia Company Suppy Saes and Contracts Department Dept. 65-11, Zone 451 Marietta, Georgia 30063 Teephone (404) 424-4214 TELEX 804263 LOC CUST SUPPL Keyboard A keyboard enabes the user to send commands or information to the system. The keys incude ASCI1 upper case, ower case, and contro characters, BREAK and 12 Lockheed SERVICE NEWS V13N3
by G.T. Waker, Fied Service Representative Vibration with a frequency approximatey equivaent to propeer RPM can appear in the throtte or condition evers during fight. When this happens, it is usuay isoated to the contros of one engine. The vibration may manifest itsef in the throtte, the condition ever, or in both at the same time. This is not an uncommon occurrence and shoud not be considered a safety of fight item; nevertheess, vibration in engine contros shoud aways be investigated. Note that an out-of-baance condition in the engine occurs at such a high frequency that it is usuay not detectabe by the fight crew. An unbaanced propeer, on the other hand, wi cause a detectabe movement of the Lockheed SERVICE NEWS V13N3 13
nacee and possiby some other airframe components. The causes of engine contro ever vibration can be, but are not imited to, one or more of the foowing: Worn gimba bushings. Improper engine contros cabe tension.. Worn contro puey bearings and rod ends. Worn cutch and cutch bearings at engine firewa quick disconnect. Worn attaching hardware in the engine contro system. Improper propeer phase ange setting. Sagging aft engine mount. With these possibe causes in mind, a ogica sequence of corrective actions that begins with the easiest to accompish and progresses to the more difficut and timeconsuming shoud be pursued. The first step shoud be to verify that the tension on the power pant contro cabes, both in the airframe and in the nacees, is within the toerances specified in the authorized maintenance manua. Pay particuar attention to the readings between opposing cabes. Where appropriate, they shoud be as neary equa as possibe. Worn gimba bushings are another common cause of controever vibration. Check gimba bushings shown in Figure 2. At the same time, check puey bearing PN POK. If any wear is evident, repace a the affected parts. The cutches and cutch bearings at the firewa quick disconnect (shown in Figure 3) are often overooked during inspection. A sight amount of wear on either the cutch or cutch bearing can cause contro ever vibration. Improper propeer phase ange settings can aso cause contro ever vibration. The inboard engines are more frequenty affected by propeer phase ange misadjustment than the outboard engines. The severity of vibration is determined by the amount of error in phase Figure 1. Checking cae tension 14 Lockheed SERVICE NEWS V13N3
A fina step is to check the rear engine mount. Some maintenance organizations routiney repace rear engine mounts when vibration probems are reported, but this is not necessariy a cure-a. If avaiabe, use a Lord Corporation PN LT27 too to check the mount for permanent set. Defective rear engine mounts have been the cause of vibration fet in the contros in some instances; however, experience has shown that repacement of engine mounts-in particuar good ones-does not aways sove the probem. More often than not, a combination of factors is invoved. A systematic approach that takes a of the possibe causes of contro ever vibration into consideration is therefore ikey to sove the probem with the east expenditure of time and effort. Figure 2. Throtte and condition ever gimba assemby. ange setting in the tube type synchrophaser. Note that it is not possibe to make fied adjustments of phase ange settings on the soid-state synchrophaser, but these adjustments are possibe on the tube type synchrophaser, using the proper equipment. Figure 4. Rear engine mount. -Figure 3. Engine contros firewa quick disconnect. Lockheed SERVICE NEWS V13N3 15