Magnetic Compasses and Vacuum Systems

HYD ELEC AC ELEC DC BLUE GREEN YELLOW TSS TM MCNM BUS SENRO CTL ESS BUS EMER BUS BAT 1 BAT 2 BAT 3 3000 PSI 3000 PSI 3000 TAI TA2 27 V OFF 27 V 0 0 GENBUS 1 BUS 1CYRD ELEC 1 10 1 00 40 A 40 A GEM1 GEM2 OFF OFF APU GEM ESS BUS 115 V 400 Hz 100.1 100.2 TA1 ESSTA TA2 28 V 28 V 28 V AIR BLEED AIR COND H CAPT CAB PRESS 15 0 C F C F 20 RAM AIR 0 LO HI LO HI ANTI ANTI ICE ICE 1515 C H FWD 15 15 C HMID 1515 P V/S CAS ALT PSI FT/MIN FT 10 2 10 UP 0 DW 0 0 2 00.0 0 SYS1 SYS2 PSI PSI °C °C APU C H AFT 11 1511 15 C FWD AFT SFTY BATTERY GYSO LP HP HP LP FUELENG 1 JPU ENG 2 APU FLT CTL LEF: 1000 TOTAL: 120.10 SPLA AQL AQL DW: CG: APU DRM DFF 2 1 1 24 3 SPD 35 4 57 6 MAX 6 7 CTR L NOSE R AIL UP AIL 6 41 6 41 OUTER 47 51 OUTER N EST % °CEST 662122 ELEV STAD 0 0 DW 24 54 24 54 85 AUD INNER INNERTHIN 0 200 34.3 Traditional gauges are not utilized, other than as a standby combination engine gauge in case of total system failure.

EICAS is also a two-monitor, two-computer system with a display select panel. Both monitors receive information from the same computer. The second computer serves as a standby. Digital and analog inputs from the engine and airframe systems are continuously monitored. Caution and warning lights, as well as aural tones, are incorporated. [Figure 10-124] EICAS provides full time primary engine parameters (EPR, N1, EGT) on the top, primary monitor. Advisories and warning are also shown there. Secondary engine parameters and nonengine system status are displayed on the bottom screen. The lower screen is also used for maintenance diagnosis when the aircraft is on the ground. Color coding is used, as well as message prioritizing.

The display select panel allows the pilot to choose which computer is actively supplying information. It also controls the display of secondary engine information and system status displays on the lower monitor. EICAS has a unique feature that automatically records the parameters of a failure event to be reviewed afterwards by maintenance personnel. Pilots that suspect a problem may be occurring during flight can press the event record button on the display select panel. This also records the parameters for that flight period to be studied later by maintenance. Hydraulic, electrical, environmental, performance, and APU data are examples of what may be recorded.

10-71 WLDP FAULT LOAD DSPL TEST LOAD DSPL FWC 1 FAULT FWC 2 FAULT TEST TEST RESET BITE DISPLAY INHIB OVER BITE INPUTS INPUTS ECAM EFIS Aircraft systems inputs test button Inhibited warnings override switch Displays switch Signal generator test button Flight warning computer test buttons and annunciation lights annunciating faults in the ECAM system. EICAS uses BITE for systems and components. A maintenance panel is included for technicians. From this panel, when the aircraft is on the ground, push-button switches display information pertinent to various systems for analysis. [Figure 10-125] Flight Management System (FMS) The highest level of automated flight system is the flight FMS.

Companies flying aircraft for hire have special results they wish to achieve. On-time performance, fuel conservation, and long engine and component life all contribute to profitability. An FMS helps achieve these results by operating the aircraft with greater precision than possible by a human pilot alone. A FMS can be thought of as a master computer system that has control over all other systems, computerized and otherwise. As such, it coordinates the adjustment of flight, engine, and airframe parameters either automatically or by instructing the pilot how to do so. Literally, all aspects of the flight are considered, from preflight planning to pulling up to the jet-way upon landing, including in-flight amendments to planned courses of action.

The main component of an FMS is the flight management computer (FMC). It communicates with the EICAS or ECAM, the ADC, the thrust management computer that controls the autothrottle functions, the EIFIS symbol generators, the automatic flight control system, the inertial reference system, collision avoidance systems, and all of the radio navigational aids via data busses. [Figure 10-126] The interface to the system is a control display unit (CDU) that is normally located forward on the center pedestal in the flight deck. It contains a full alphanumeric keypad, a CRT or LCD display/work screen, status and condition annunciators, and specialized function keys. [Figure 10-127] The typical FMS uses two FMS FMCs that operate independently as the pilot’s unit and the copilot’s unit.

However, they do crosstalk through the data busses. In normal operation, the pilot and copilot divide the workload, with the pilot’s CDU set to supervise and interface with operational parameters and the copilot’s CDU handling navigational chores. This is optional at the flightcrew’s discretion. If a main component fails (e.g., an FMC or a CDU), the remaining operational units continue to operate with full control and without system compromise. Each flight of an aircraft has vertical, horizontal, and navigational components, which are maintained by manipulating the engine and airframe controls. While doing so, numerous options are available to the pilot. Rate of climb, thrust settings, EPR levels, airspeed, descent rates, and other terms can be varied. Commercial air carriers use the FMC to establish guidelines by which flights can be flown. Usually, these promote the company’s goals for fuel and equipment conservation. The pilot need only enter variables as requested and respond to suggested alternatives as the FMC presents them.

The FMC has stored in its database literally hundreds of flight plans with predetermined operational parameters that can be selected and implemented. Integration with NA V-COM aids allows the FMS to change radio frequencies as the flight plan is enacted. Internal computations using direct input from fuel flow and fuel quantity systems allow the FMC to carry out lean operations or pursue other objectives, such as high performance operations if making up time is paramount on a particular flight. Weather and traffic considerations are also integrated. The FMS can handle all variables automatically but communicates via the CDU screen to present its planned action, gain consensus, or ask for an input or decision.

As with the monitoring systems, FMS includes BITE. The FMC continuously monitors its entire systems and inputs for faults during operation. Maintenance personal can retrieve system generated and pilot recorded fault messages. They may also access maintenance pages that call out line replaceable units (LRUs) to which faults have been traced by the BITE system. Follow manufacturers’ procedures for interfacing with maintenance data information. 10-72 Standby engine indicators Aural warning Upper DU Lower DU Warning & cautions Engine primary displays Engine secondary or status display or maintenance display Discrete caution & warning lights R computer L computer Display switching Maintenance panel Display select panel Other system discretes FCC MCDP TMC interface FEC interface FMC interface RAD altitude interface ADC interface Engine sensors N1 Oil pressure N2 Oil quantity N3 Oil temperature EPR Vibration EGT FF System sensors Hydraulic quantity & pressure ADC hydraulic system temperature Control surface positions Electical system: volts, amps, frequency Generator drive temperature ECS temps APU EGT RPM Brake temperature Data buses Warnings & Cautions Annunciator Panels An annunciator panel is a panel of warning lights in plain sight of the pilot. These lights are identified by the name of the system they represent and are usually covered with colored lenses to show the meaning of the condition they announce. Annunciator panels are installed for two purposes: to display current conditions and to notify of unsatisfactory conditions. Standardized colors are used to differentiate between visual messages. For example, the color green indicates a satisfactory condition. Yellow is used to caution of a serious condition that requires further monitoring. Red is the color for an unsatisfactory condition which needs immediate attention or action. Whether part of the instrument face or of a visual warning system, these colors give quick-reference information to the pilot.

Most aircraft include annunciator lights that illuminate when an event demanding attention occurs. These use the aforementioned colors in a variety of presentations. Individual lights near the associated flight deck instrument or a collective display of lights for various systems in a central location are common. Words label each light or are part of the light itself to identify any problem quickly and plainly. On complex aircraft, the status of numerous systems and components must be known and maintained. Centralized warning systems have been developed to annunciate critical messages concerning a multitude of systems and components in a simplified, organized manner. Often, this will be done by locating a single annunciator panel somewhere on the instrument panel. These analog aircraft warning systems may look different in various aircraft and depend on manufacturer 10-73 ECS MSG ELEC HYD PERF APU CONF MCDP ENG EXCD AUTO MANUAL REC ERASE TEST DISPLAY SELECT EICAS MAINT EVENT READ Environmental control systems and maintenance message formats Engine exceedances BITE test switch for self-test routine Configuration and maintenance control/display panel Electrical and hydraulic systems format Performance and auxiliary power unit formats Selects data from auto or manual event in memory Erases stored data currently displayed Records real-time data currently displayed (in manual event) preference and the systems installed. [Figure 10-128] EFIS provide for annunciation of advisory and warning messages as part of its flight control and monitoring capabilities, as previously described. Usually, the primary display unit is designated as the location to display annunciations.

Master caution lights are used to draw the attention of the crew to a critical situation in addition to an annunciator that describes the problem. These master caution lights are centrally wired and illuminate whenever any of the participating systems or components require attention. Once notified, the pilot may cancel the master caution, but a dedicated system or component annunciator light stays illuminated until the situation that caused the warning is rectified. Cancelling resets the master caution lights to warn of a subsequent fault event even before the initial fault is corrected. [Figure 10-129] Press to test is available for the entire annunciator system, which energizes all warning circuitry and lights to confirm readiness. Often, this test exposes the need to replace the tiny light bulbs that are used in the system.

Aural Warning Systems Aircraft aural warning systems work in conjunction with illuminated annunciator systems. They audibly inform the pilot of a situation requiring attention. Various tones and phrases sound in the flight deck to alert the crew when certain conditions exist. For example, an aircraft with retractable landing gear uses an aural warning system to alert the crew to an unsafe condition. A bell sounds if the throttle is retarded and the landing gear is not in a down and locked condition. A typical transport category aircraft has an aural warning system that alerts the pilot with audio signals for the following: abnormal takeoff, landing, pressurization, mach airspeed conditions, an engine or wheel well fire, calls from the crew call system, collision avoidance recommendations, and more. Figure 10-130 shows some of the problems that trigger aural warnings and the action to be taken to correct the situation.

Clocks Whether called a clock or a chronometer, an FAA-approved time indicator is required in the flight deck of IFR-certified 10-74 Flight crew FMC CDU Controls and indicators Data buses Engine control & monitoring (indicating & alerting) Radio NAV VOR ADF DME ILS RAD. ALT ATC transponder WXR GPWS ADC TMC IRS AFCS FMC Navigation database Operational program EFIS symbol generators Control panels AFCS mode IRS mode ILS EFIS ATC VOR/DME RDMI ADF WXR EFIS ADI HSI Maintenance control and display Antennae Control surface servos Database loading Autothrottle servos Sensing probes Discrete inputs from aircraft systems Fuel quantity & fuel flow data management of the flight.

aircraft. Pilots use a clock during flight to time maneuvers and for navigational purposes. The clock is usually mounted near the flight instrument group, often near the turn coordinator. It indicates hours, minutes, and seconds. For many years, the mechanical 8-day clock was the standard aircraft timekeeping device largely because it continues to run without electrical power as long as it has been hand wound. The mechanical 8-day clock is reliable and accurate enough for its intended use. Some mechanical aircraft clocks feature a push-button elapsed time feature. [Figure 10-131] As electrical systems developed into the reliable, highly redundant systems that exist today, use of an electric clock to replace the mechanical clock began. An electric clock is an analog devise that may also have an elapsed time feature. It can be wired to the battery or battery bus. Thus, it continues to operate in the event of a power failure. Electric aircraft clocks are often used in multiengine aircraft where complete loss of electrical power is unlikely.

Many modern aircraft have a digital electronic clock with LED readout. This device comes with the advantages of low power consumption and high reliability due to the lack of moving parts. It is also very accurate. Solid-state electronics allow for expanded features, such as elapsed time, flight 10-75 10-76 9 2 1 9 0 6 3 0 6 3 M I 0 0 0 1 8 2 9 9 1 9 1 28 37 456 0 0 0 0 50 100 120 140 160 200 400 350 250 1 2 4 6.5 Up Vert Speed Down 6.5 4 21 0 1 2 3 456 7 8 99 2 1 9 AP AT ALT NAV AUX 0 10 20 30 40 50 60 70 80 90 100 5 2 0 10 20 30 40 50 60 70 80 90 100 5 2 0 10 20 30 40 50 60 70 80 90 100 5 2 0 10 20 30 40 50 60 70 80 90 100 5 2 0 10 0 2030 40 50 60 70 80 90 100 110 10 0 2030 40 50 60 70 80 90 100 110 10 0 2030 40 50 60 70 80 90 100 110 10 0 2030 40 50 60 70 80 90 100 110 10 0 20 30 40 50 60 70 1 3 0 10 0 20 30 40 50 60 70 1 3 0 10 0 20 30 40 50 60 70 1 3 0 10 0 20 30 40 50 60 70 1 3 0 1 4 7 10 13 16 19 3 0 3 1 4 7 10 13 16 19 3 0 3 1 4 7 10 13 16 19 3 0 3 1 4 7 10 13 16 19 3 0 3 14.2 NM 10 20 30 40 50 60 70 10 20 30 40 50 60 70 10 20 30 40 50 60 70 0 10 20 30 0 10 20 30 20 10 20 10 F S 10 LNV1 VALT 14.2----- NAV Master warning and master caution Annunciator panel LEFT GEAR is supported by the master caution system.

panel. time that starts automatically upon takeoff, a stop watch, and memories for all functions. Some even have temperature and date readouts. Although wired into the aircraft’s electrical system, electronic digital clocks may include a small independent battery inside the unit that operates the device should aircraft electrical power fail. [Figure 10-132] On aircraft with fully digital computerized instrument systems utilizing flat panel displays, the computer’s internal clock, or a GPS clock, can be used with a digital time readout usually located somewhere on the primary flight display. Instrument Housings & Handling Various materials are used to protect the inner workings of aircraft instruments, as well as to enhance the performance of the instrument and other equipment mounted in the immediate vicinity. Instrument cases can be one piece or multipiece. Aluminum alloy, magnesium alloy, steel, iron, and plastic are all common materials for case construction.

Electric instruments usually have a steel or iron alloy case to contain electromagnetic flux caused by current flow inside. Examples of Aircraft Aural Warnings Stage of Operation Warning System Warning Signal Cause of Warning Signal Activation Corrective action Takeoff Flight control Intermittent Throttles are advanced and any of the Correct the aircraft to horn following conditions exist: proper takeoff conditions 1. Speed brakes are not down 2. Flaps are not in takeoff range 3. Auxiliary power exhaust door is open 4. Stabilizer is not in the takeoff setting In flight Mach warning Clacker Equivalent airspeed or mach number Decrease aircraft speed exceeds limits In flight Pressurization Intermittent If cabin pressure becomes equal to Correct the condition horn atmospheric pressure at the specific altitude (altitude at time of occurrence) Landing Landing gear Continuous Landing gear is not down and locked when Raise flaps; advance horn flaps are less than full up and throttle is throttle retarded to idle Any stage Fire warning Continuous Any overheat condition or fire in any engine or 1. Lower the heat in the bell nacelle, or main wheel or nose wheel well, the area where in the APU engine, or any compartment having fire F/W was activated warning system installed 2. Signal may be silenced Whenever the fire warning system is tested pushing the F/W bell cutout switch or the APU cutout switch Any stage Communications High chime Any time captain’s call button is pressed at Release button; if button external power panel forward or rearward remains locked in, pull cabin attendant’s panel button out 12 6 8 DAYS 9 3 Despite their rugged outward appearance, all instruments, especially analog mechanical instruments, should be handled with special care and should never be dropped. A crack in an airtight instrument case renders it unairworthy. Ports should never be blown into and should be plugged until the instrument is installed. Cage all gyro instruments until mounted in the instrument panel. Observe all cautions written on the instrument housing and follow the manufacturer’s instruction + °C UP D SET B 1 hr upDIM F.

T. TIME E.T. ZERODAVTRON M811B s t o p RUN for proper handling and shipping, as well as installation. Instrument Installations & Markings Instrument Panels Instrument panels are usually made from sheet aluminum alloy and are painted a dark, nonglare color. They sometimes contain subpanels for easier access to the backs of instruments during maintenance. Instrument panels are usually shock- 10-77 mounted to absorb low-frequency, high-amplitude shocks. The mounts absorb most of the vertical and horizontal vibration but permit the instruments to operate under conditions of minor vibration. Bonding straps are used to ensure electrical continuity from the panel to the airframe.

[Figure 10-133] The type and number of shock mounts to be used for instrument panels are determined by the weight of the unit. Shock-mounted instrument panels should be free to move in all directions and have sufficient clearance to avoid striking the supporting structure. When a panel does not have adequate clearance, inspect the shock mounts for looseness, cracks, or deterioration. Airframe structure Instrument panel Bonding strap Instrument panel layout is seemingly random on older aircraft. The advent of instrument flight made the flight instruments of critical importance when flying without outside reference to the horizon or ground. As a result, the basic T arrangement for flight instruments was adopted, as mentioned in the beginning of this chapter. [Figure 10-4] Electronic flight instrument systems and digital flight deck displays have kept the same basic T arrangement for flight instrument and data presentations. The flight instruments and basic T are located directly in front of the pilot and copilot’s seats. Some light aircraft have only one full set of flight instruments that are located in front of the left seat.

The location of engine instruments and navigation instruments varies. Ideally, they should be accessible to both the pilot and copilot. Numerous variations exist to utilize the limited space in the center of the instrument panel and still provide accessibility by the flight crew to all pertinent instruments. On large aircraft, a center pedestal and overhead panels help create more space. On small aircraft, the engine instruments are often moved to allow navigation instruments and radios to occupy the center of the instrument panel. [Figure 10-134] On modern aircraft, EFIS and digital flight information systems reduce panel clutter and allow easier access to all instruments by both crewmembers. Controllable display panels provide the ability to select from pages of information that, when not displayed, are completely gone from view and use no instrument panel space.

Engine instruments Navigation instruments Flight instruments Basic “T” layout radios primarily to the right, which is the center of the instrument panel. This arrangement is commonly on light aircraft to be flown by a single pilot. 10-78 Instrument Mounting The method of mounting instruments in their respective panels depends on the design of the instrument case. In one design, the bezel is flanged in such a manner that the instrument can be flush mounted in its cutout from the rear of the panel. Integral, self-locking nuts are provided at the rear faces of the flange corners to receive mounting screws from the front of the panel. The flanged-type instrument can also be mounted to the front of the panel. In this case, nut-plates are usually installed in the panel itself. Nonferrous screws are usually used to mount the instruments.

There are also instrument mounting systems where the instruments are flangeless. A special clamp, shaped and dimensioned to fit the instrument case, is permanently secured to the rear face of the panel. The instrument is slid into the panel from the front and into the clamp. The clamp’s tightening screw is accessible from the front side of the panel. [Figure 10-135] Regardless of how an instrument is mounted, it should not be touching or be so close as to touch another instrument during the shock of landing. Instrument Power Requirements Many aircraft instruments require electric power for operation. Even nonelectric instruments may include electric lighting. Only a limited amount of electricity is produced by the aircraft’s electric generator(s). It is imperative that the electric load of the instruments, radios, and other equipment on board the aircraft does not exceed this amount.

Electric devices, including instruments, have power ratings. These show what voltage is required to correctly operate the unit and the amount of amperage it draws when operating to capacity. The rating must be checked before installing any component. Replacement of a component with one that has the same power rating is recommended to ensure the potential electric load of the installed equipment remains within the limits the aircraft manufacturer intended. Adding a component with a different rating or installing a completely new component may require a load check be performed. This is essentially an on the ground operational check to ensure the electrical system can supply all of the electricity consuming devices installed on the aircraft. Follow the manufacturer’s instructions on how to perform this check.

Instrument Range Markings Many instruments contain colored markings on the dial face to indicate, at a glance, whether a particular system or component is within a range of operation that is safe and desirable or if an undesirable condition exists. These markings are put on the instrument by the original equipment manufacturer in accordance with the Aircraft Specifications in the Type Certificate Data Sheet. Data describing these Front mounted Rear mounted Clamp mounted Nut plates mounted in instrument Nut plates mounted in panel Strap tightened by clamp Clamp mounted on instrument panel flangeless (bottom).

limitations can also sometimes be found in the aircraft manufacturer’s operating and maintenance manuals and in the flight manual. Occasionally, the aircraft technician may find it necessary to apply these marking to an approved replacement instrument on which they do not appear. It is crucial that the instrument be marked correctly and only in accordance with approved data. The marking may be placed on the cover glass of the instrument with paint or decals. A white slippage mark is made to extend from the glass to the instrument case. Should the glass rotate in the bezel, the marking will no longer be aligned properly with the calibrated instrument dial. The broken slippage mark indicates this to the pilot or technician.

The colors used as range markings are red, yellow, green, blue, or white. The markings can be in the form of an arc or a radial line. Red is used to indicate maximum and minimum 10-79 ranges; operations beyond these markings are dangerous and should be avoided. Green indicates the normal operating range. Yellow is used to indicate caution. Blue and white are used on airspeed indicators to define specific conditions. [Figures 10-136 and 10-137] Maintenance of Instruments & Instrument Systems An FAA airframe and powerplant (A&P) technician is not qualified to do internal maintenance on instruments and related line replaceable units discussed in this chapter. This must be carried out at facilities equipped with the specialized equipment needed to perform the maintenance properly.

Qualified technicians with specialized training and intimate knowledge of instruments perform this type of work, usually under repair station certification. However, certified airframe technicians and A&P technicians are charged with a wide variety of maintenance functions related to instruments and instrument systems. Installation, removal, inspection, troubleshooting, and functional checks are all performed in the field by certified personnel. It is also a responsibility of the certified technician holding an airframe rating to know what maintenance is required and to access the approved procedures for meeting those requirements.

In the following paragraphs, various maintenance and servicing procedures and suggestions are given. The discussion follows the order in which the various instruments and instrument systems were presented throughout this chapter. This is not meant to represent all of the maintenance required by any of the instruments or instruments systems. The aircraft manufacturer’s and instrument manufacturer’s approved maintenance documents should always be consulted for required maintenance and servicing instructions. FAA regulations must also be observed. Instrument Range marking Airspeed indicator White arc bottom top Green arc bottom top Blue radial line Yellow arc bottom top Red radial line Carburetor air temperature Green arc Yellow arc Red radial line Cylinder head temperature Green arc Yellow arc Red radial line Manifold pressure gauge Green arc Yellow arc Red radial line Fuel pressure gauge Green arc Yellow arc Red radial line Oil pressure gauge Green arc Yellow arc Red radial line Flap operating range Flaps-down stall speed Maximum airspeed for flaps-down flight Normal operating range Flaps-up stall speed Maximum airspeed for rough air Best single-engine rate-of-climb airspeed Structural warning area Maximum airspeed for rough air Never-exceed airspeed Normal operating range Range in which carburetor ice is most likely to form Maximum allowable inlet air temperature Normal operating range Operation approved for limited time Never-exceed temperature Normal operating range Precautionary range Maximum permissible manifold absolute pressure Normal operating range Precautionary range Maximum and/or minimum permissible fuel pressure Normal operating range Precautionary range Maximum and/or minimum permissible oil pressure Instrument Range marking Oil temperature gauge Green arc Yellow arc Red radial line Tachometer (reciprocating engine) Green arc Yellow arc Red arc Red radial line Tachometer (turbine engine) Green arc Yellow arc Red radial line Tachometer (helicopter) Engine tachometer Green arc Yellow arc Red radial line Rotor tachometer Green arc Red radial line Torque indicator Green arc Yellow arc Red radial line Exhaust gas temperature indicator (turbine engine) Green arc Yellow arc Red radial line Gas producer N1 tachometer (turboshaft helicopter) Green arc Yellow arc Red radial line Normal operating range Precautionary range Maximum and/or minimum permissible oil temperature Normal operating range Precautionary range Restricted operating range Maximum permissible rotational speed Normal operating range Precautionary range Maximum permissible rotational speed Normal operating range Precautionary range Maximum permissible rotational speed Normal operating range Maximum and minimum rotor speed for power-off operational conditions Normal operating range Precautionary range Maximum permissible torque pressure Normal operating range Precautionary range Maximum permissible gas temperature Normal operating range Precautionary range Maximum permissible rotational speed 10-80 AIR SPEED KNOTS 40 60 80 100 120 140 160 180 200 220 240 markings.

Altimeter Tests When an aircraft is to be operated under IFR, an altimeter test must have been performed within the previous 24 months. Title 14 of the Code of Federal Regulations (14 CFR) part 91, section 91.411, requires this test, as well as tests on the pitot-static system and on the automatic pressure altitude reporting system. The certified airframe or A&P mechanic is not qualified to perform the altimeter inspections. They must be conducted by either the manufacturer or a certified repair station. 14 CFR part 43, Appendix E details the requirements for these tests. Pitot-Static System Maintenance & Tests Water trapped in a pitot static system may cause inaccurate or intermittent indications on the pitot-static flight instruments.

This is especially a problem if the water freezes in flight. Many systems are fitted with drains at the low points in the system to remove any moisture during maintenance. Lacking this, dry compressed air or nitrogen may be blown through the lines of the system. Always disconnect all pitot- static instruments before doing so and always blow from the instrument end of the system towards the pitot and static ports. This procedure must be followed by a leak check described below. Systems with drains can be drained without requiring a leak check. Upon completion, the technician must ensure that the drains are closed and made secure in accordance with approved maintenance procedures.

Aircraft pitot-static systems must be tested for leaks after the installation of any component parts or when system malfunction is suspected. It must also be tested every 24 months if on an IFR certified aircraft intended to be flown as such as called out in 14 CFR part 91, section 91.411. Certified airframe and A&P technicians may perform this test. The method of leak testing depends on the type of aircraft, its pitot-static system, and the testing equipment available. [Figure 10-138] Essentially, a testing device is connected into the static system at the static vent end, and pressure is reduced in the system by the amount required to indicate 1,000 feet on the altimeter. Then, the system is sealed and observed for 1 minute. A loss of altitude of more than 100 feet is not permissible. If a leak exists, a systematic check of portions of the system is conducted until the leak is isolated. Most leaks occur at fittings. The pitot portion of the pitot-static system is checked in a similar fashion. Follow the manufacturer’s instructions when performing all pitot-static system checks.

In all cases, pressure and suction must be applied and released slowly to avoid damage to the aircraft instruments. Pitot-static system leak check units usually have their own built-in altimeters. This allows a functional cross-check of the aircraft’s altimeter with the calibrated test unit’s altimeter while performing the static system check. However, this does not meet the requirements of 14 CFR part 91, section 91.411 for altimeter tests. Upon completion of the leak test, be sure that the system is returned to the normal flight configuration. If it is necessary to block off various portions of a system, check to be sure that all blanking plugs, adaptors, or pieces of adhesive tape have been removed.

Tachometer Maintenance Tachometer indicators should be checked for loose glass, chipped scale markings, or loose pointers. The difference in indications between readings taken before and after lightly tapping the instrument should not exceed approximately 15 rpm. This value may vary, depending on the tolerance established by the indicator manufacturer. Both tachometer generator and indicator should be inspected for tightness of mechanical and electrical connections, security of mounting, and general condition. For detailed maintenance procedures, the manufacturer’s instructions should always be consulted.

When an engine equipped with an electrical tachometer is running at idle rpm, the tachometer indicator pointers may fluctuate and read low. This is an indication that the synchronous motor is not synchronized with the generator output. As the engine speed is increased, the motor should synchronize and register the rpm correctly. The rpm at which synchronization occurs varies with the design of the tachometer system. If the instrument pointer(s) oscillate(s) at speeds above the synchronizing value, determine that the total oscillation does not exceed the allowable tolerance. 10-81 Pointer oscillation can also occur with a mechanical indication system if the flexible drive is permitted to whip.

The drive shaft should be secured at frequent intervals to prevent it from whipping. When installing mechanical type indicators, be sure that the flexible drive has adequate clearance behind the panel. Any bends necessary to route the drive should not cause strain on the instrument when it is secured to the panel. Avoid sharp bends in the drive. An improperly installed drive can cause the indicator to fail to read or to read incorrectly. Magnetic Compass Maintenance & Compensation The magnetic compass is a simple instrument that does not require setting or a source of power. A minimum of maintenance is necessary, but the instrument is delicate and should be handled carefully during inspection. The following items are usually included in an inspection: 1. The compass indicator should be checked for correct readings on various cardinal headings and re- compensated if necessary.

2. Moving parts of the compass should work easily. 3. The compass bowl should be correctly suspended on an antivibration device and should not touch any part of the metal container. 4. The compass bowl should be filled with liquid. The liquid should not contain any bubbles or have any discoloration. Airframe mechanics cannot refill the fluid of a whiskey compass. 5. The scale should be readable and be well lit. Compass magnetic deviation is caused by electromagnetic interference from ferrous materials and operating electrical components in the flight deck. Deviation can be reduced by swinging the compass and adjusting its compensating magnets. An example of how to perform this calibration process is given below. The results are recorded on a compass correction card which is placed near the compass in the flight deck. [Figure 10-139] There are various ways to swing a compass. The following is meant as a representative method. Follow the aircraft manufacturer’s instructions for method and frequency of swinging the magnetic compass. This is usually accomplished at flight hour or calendar intervals. Compass calibration is also performed when a new electric component is added to the flight deck, such as a new radio. A complete list of conditions requiring a compass swing and procedure can be found in FAA Advisory Circular (AC) 43.13-1 (as revised), Chapter 12, Section 3, paragraph 12-37.

To swing a compass, a compass rose is required. Most airports have one painted on the tarmac in a low-traffic area where maintenance personnel can work. One can also be made with chalk and a good compass. The area where the compass rose is laid out should be far from any possible electromagnetic disturbances, including those underground, and should remain clear of any ferrous vehicles or large equipment while the procedure takes place. [Figure 10-140] The aircraft should be in level flight attitude for the compass swing procedure. Tail draggers need to have the aft end of the fuselage propped up, preferably with wood, aluminum, or some other nonferrous material. The aircraft interior and baggage compartments should be free from miscellaneous items that might interfere with the compass. All normal equipment should be on board and turned on to simulate a flight condition. The engine(s) should be running.

The basic idea when swinging a compass is to note the deviation along the north-south radial and the east-west radial. Then, adjust the compensating magnets of the compass to eliminate as much deviation as possible. Begin 10-82 by centering or zeroing the compass’ compensating magnets with a non-ferrous screw driver. Align the longitudinal axis of the aircraft on the N-S radial facing north. Adjust the N-S compensating screw so the indication is 0°. Next, align the longitudinal axis of the aircraft on the E-W radial facing east. Adjust the E-W compensating screw so that the compass indicates 90°. Now, move the aircraft to be aligned with the N-S radial facing south. If the compass indicates 180°, there is no deviation while the aircraft is heading due north or due south. However, this is unlikely. Whatever the south- facing indication is, adjust the N-S compensating screw to eliminate half of the deviation from 180°. Continue around to face the aircraft west on the E-W radial and use the E-W compensating screw to eliminate half of the west-facing deviation from 270°.

Once this is done, return the aircraft to alignment with the N-S radial facing north and record the indication. Up to 10° deviation is allowed. If the deviation cannot be corrected to within 10° , the compass should be replaced if further inspection reveals that there is no other reasons for this deviation. Align the aircraft with the radials every 30° around the compass rose and record each indication on the compass compensation card. Date and sign the card and place it in full view of the pilot near the compass in the flight deck. Vacuum System Maintenance Errors in the indication presented on a vacuum gyroscopic instrument could be the result of any factor that prevents the vacuum system from operating within the design suction limits. Errors can also be caused by problems within the instrument, such as friction, worn parts, or broken parts. Any source that disturbs the free rotation of the gyro at design card attached, on which the results of swinging the compass should be recorded.

speed is undesirable resulting in excessive precession and failure of the instruments to maintain accurate indication. The aircraft technician is responsible for the prevention or correction of vacuum system malfunctions. Usually this consists of cleaning or replacing filters, checking and correcting insufficient vacuum, or removing and replacing the vacuum pump or instruments. A list of the most common malfunctions, together with their correction, is included in Autopilot System Maintenance The information in this section does not apply to any particular autopilot system but gives general information that relates to all autopilot systems. Maintenance of an autopilot system consists of visual inspection, replacement of components, cleaning, lubrication, and an operational checkout of the system. Consult the manufacturer’s maintenance manual for all of these procedures.

With the autopilot disengaged, the flight controls should function smoothly. The resistance offered by the autopilot servos should not affect the control of the aircraft. The interconnecting mechanisms between the autopilot system and the flight control system should be correctly aligned and smooth in operation. When applicable, the operating cables should be checked for tension. An operational check is important to assure that every circuit is functioning properly. An autopilot operational check should be performed on new installations, after replacement of an autopilot component, or whenever a malfunction in the autopilot is suspected.

After the aircraft’s main power switch has been turned on, allow the gyros to come up to speed and the amplifier to warm up before engaging the autopilot. Some systems are designed with safeguards that prevent premature autopilot to swing an aircraft magnetic compass. 10-83 engagement. While holding the control column in the normal flight position, engage the autopilot system using the switch on the autopilot controller. After the system is engaged, perform the operational checks specified for the particular aircraft. In general, the checks are as follows: 1. Rotate the turn knob to the left; the left rudder pedal should move forward, and the control column wheel should move to the left and slightly aft.

2. Rotate the turn knob to the right; the right rudder pedal should move forward, and the control column wheel should move to the right and slightly aft. Return the turn knob to the center position; the flight controls should return to the level-flight position. 3. Rotate the pitch-trim knob forward; the control column should move forward. 4. Rotate the pitch-trim knob aft; the control column should move aft. If the aircraft has a pitch-trim system installed, it should function to add down-trim as the control column moves forward and add up-trim as the column moves aft. Many pitch-trim systems have an automatic and a manual mode of operation. The above action occurs only in the automatic mode.

Check to see if it is possible to manually override or overpower the autopilot system in all control positions. Center all the controls when the operational checks have been completed. Disengage the autopilot system and check for freedom of the control surfaces by moving the control columns and rudder pedals. Then, reengage the system and check the emergency disconnect release circuit. The autopilot should disengage each time the release button on the control yoke is actuated. When performing maintenance and operational checks on a specific autopilot system, always follow the procedure recommended by the aircraft or equipment manufacturer.

LCD Display Screens Electronic and digital instrument systems utilizing LCD technology may have special considerations for the care of the display screens. Antireflective coatings are sometimes used to reduce glare and make the displays more visible. These treatments can be degraded by human skin oils and certain cleaning agents, such as those containing ammonia. It is very important to clean the display lens using a clean, lint-free cloth and a cleaner that is specified as safe for antireflective coatings, preferable one recommended by the aircraft manufacturer. 1. No vacuum pressure or insufficient pressure 2. Excessive vacuum 3. Gyro horizon bar fails to respond 4. Turn-and-bank indicator fails to respond 5. Turn-and-bank pointer vibrates Problem and Potential Causes Isolation Procedure Correction Defective vacuum gauge Check opposite engine system on the gauge Replace faulty vacuum gauge Vacuum relief valve incorrectly adjusted Change valve adjustment Make final adjustment to correct setting Vacuum relief valve installed backward Visually inspect Install lines properly Broken lines Visually inspect Replace line Lines crossed Visually inspect Install lines properly Obstruction in vacuum line Check for collapsed line Clean & test line; replace defective part(s) Vacuum pump failure Remove and inspect Replace faulty pump Vacuum regulator valve incorrectly adjusted Make valve adjustment and note pressure Adjust to proper pressure Vacuum relief valve dirty Clean and adjust relief valve Replace valve if adjustment fails Relief valve improperly adjusted Adjust relief valve to proper setting Inaccurate vacuum gauge Check calibration of gauge Replace faulty gauge Instrument caged Visually inspect Uncage instrument Instrument filter dirty Check filter Replace or clean as necessary Insufficient vacuum Check vacuum setting Adjust relief valve to proper setting Instrument assembly worn or dirty Replace instrument No vacuum supplied to instrument Check lines and vacuum system Clean and replace lines and components Instrument filter clogged Visually inspect Replace filter Defective instrument Test with properly functioning instrument Replace faulty instrument Defective instrument Test with properly functioning instrument Replace defective instrument 10-84