Systems Description for CESSNA 172M SKYHAWK

the instrument panel. To apply the parking brake, set the brakes with the rudder pedals, pull the handle aft, and rotate it 90° down. For maximum brake life, keep the brake systems properly maintained, and minimize brake usage during taxi operations and landings. Some of the symptoms of impending brake failure are: gradual de- crease in braking action after brake application, noisy or dragging brakes, soft or spongy pedals, short pedal travel and hard pedal, and excessive travel and weak braking action. If any of these symptoms appear, the brake system is in need of immediate attention. If, during taxi or landing roll, braking action decreases, let up on the pedals and then re-apply the brakes with heavy pressure. If the brakes become spongy or pedal travel increases, pumping the pedals should build braking pressure. If one brake becomes weak or fails, use the other brake sparingly while using opposite rudder, as required, to offset the good brake. ELECTRICAL SYSTEM Electrical energy (see figure 7-7) is supplied by a 14-volt, direct- current system powered by an engine-driven, 60-amp alternator. The 12-volt, 25-amp hour battery is located on the left side of the firewall. Power is supplied to all electrical circuits through a split bus bar, one side containing electronic system circuits and the other side having gen- eral electrical system circuits. Both sides of the bus are on at all times OVER- VOLTAGE WARNING LIGHT REGULATOR ΤΟ PRIMARY ALTERNATOR BUS STARTER CONTACTOR STARTER MASTER SWITCH TO ALT FIELD CIRCUIT BREAKER ALT REVERSE POLARITY CONTACTOR OVER- VOLTAGE SENSOR ΤΟ ALT FIELD CIRCUIT BREAKER AMMETER GROUND SERVICE PLUG RECEPTACLE FLIGHT HOUR RECORDER OIL PRESSURE SWITCH SPLIT BUS CONTACTOR (NORMALLY CLOSED) BATTERY CONTACTOR CLOCK TO INT LT CIRCUIT BREAKER ALT BUS ELECTRONIC BUS PRIMARY BUS TO OVER-VOLTAGE WARNING LIGHT ALT FIELD TO OVER-VOLTAGE SENSOR AND MASTER SWITCH TO AUTOMATIC PILOT AUTO PILOT TO RADIO RADIO 1 TO RADIO RADIO 2 TO RADIO RADIO 3 RADIO 4 TO RADIO OR TRANSPONDER AND ENCODING ALTIMETER TO RADIO RADIO 5 TO AUDIO AMPLIFIER AUD AMP INST TO FUEL QUANTITY INDICATORS TO OIL TEMPERATURE GAGE LTO WING FLAP POSITION INDICATOR FLAP TURN COORD INT LT TO WING FLAP SYSTEM TO TURN COORDINATOR OR TURN AND BANK INDICATOR FROM ALTERNATOR BUS TO DOOR POST MAP LIGHT TO DOME AND COURTESY LIGHTS TO IGNITION SWITCH TO COMPASS, INSTRUMENT, AND POST LIGHTING TO CIGAR LIGHTER (WITH CIRCUIT BREAKER) LAND TO LANDING LIGHT(S) LT TO STROBE LIGHTS STROBE LT BATTERY IGNITION SWITCH TO FLASHING BEACON BCN LT CODE CIRCUIT BREAKER (PUSH-TO-RESET) FUSE DIODE RESISTOR CAPACITOR (NOISE FILTER) NAV LT ☑ R PITOT HT MAGNETOS Figure 7-7. Electrical System TO NAVIGATION LIGHTS AND CONTROL WHEEL MAP LIGHT TO AUDIO MUTING RELAY TO PITOT HEAT SYSTEM 7-24 7-25 SECTION 7 AIRPLANE & SYSTEMS DESCRIPTIONS CESSNA MODEL 172M CESSNA MODEL 172M SECTION 7 AIRPLANE & SYSTEMS DESCRIPTIONS except when either an external power source is connected or the starter switch is turned on; then a power contactor is automatically activated to open the circuit to the electronic bus. Isolating the electronic circuits in this manner prevents harmful transient voltages from damaging the tran- sistors in the electronic equipment. MASTER SWITCH The master switch is a split-rocker type switch labeled MASTER, and is ON in the up position and OFF in the down position. The right half of the switch, labeled BAT, controls all electrical power to the airplane. The left half, labeled ALT, controls the alternator. Normally, both sides of the master switch should be used simulta- neously; however, the BAT side of the switch could be turned ON separate- ly to check equipment while on the ground. The ALT side of the switch, when placed in the OFF position, removes the alternator from the electri- cal system. With this switch in the OFF position, the entire electrical load is placed on the battery. Continued operation with the alternator switch in the OFF position will reduce battery power low enough to open the battery contactor, remove power from the alternator field, and pre- vent alternator restart. AMMETER The ammeter indicates the flow of current, in amperes, from the al- ternator to the battery or from the battery to the airplane electrical sys- tem. When the engine is operating and the master switch is turned on, the ammeter indicates the charging rate applied to the battery. In the event the alternator is not functioning or the electrical load exceeds the output of the alternator, the ammeter indicates the battery discharge rate. OVER-VOLTAGE SENSOR AND WARNING LIGHT The airplane is equipped with an automatic over-voltage protection system consisting of an over-voltage sensor behind the instrument panel and a red warning light, labeled HIGH VOLTAGE, adjacent to the ammeter. In the event an over-voltage condition occurs, the over-voltage sen- sor automatically removes alternator field current and shuts down the al- ternator. The red warning light will then turn on, indicating to the pilot that the alternator is not operating and the battery is supplying all electri- cal power. The over-voltage sensor may be reset by turning the master switch off and back on again. If the warning light does not illuminate, normal alternator charging has resumed; however, if the light does illuminate again, a malfunction has occurred, and the flight should be terminated as soon as practical. The warning light may be tested by momentarily turning off the ALT portion of the master switch and leaving the BAT portion turned on. CIRCUIT BREAKERS AND FUSES Most of the electrical circuits in the airplane are protected by "push- to-reset" circuit breakers mounted on the left side of the instrument panel. Exceptions to this are the battery contactor closing (external power) cir- cuit, clock, and flight hour recorder circuits which have fuses mounted near the battery. The control wheel map light is protected by the NAV LT circuit breaker on the instrument panel, and a fuse behind the panel. The cigar lighter is protected by a manually reset circuit breaker on the back of the lighter, and by the LAND LT circuit breaker. GROUND SERVICE PLUG RECEPTACLE A ground service plug receptacle may be installed to permit the use of an external power source for cold weather starting and during lengthy maintenance work on the airplane electrical system (with the exception of electronic equipment). The receptacle is located behind a door on the left side of the fuselage near the aft edge of the cowling. NOTE Electrical power for the airplane electrical circuits is pro- vided through a split bus bar having all electronic circuits on one side of the bus and other electrical circuits on the other side of the bus. When an external power source is connected, a contactor automatically opens the circuit to the electronic portion of the split bus bar as a protection against damage to the transistors in the electronic equip- ment by transient voltages from the power source. There- fore, the external power source can not be used as a source of power, when checking electronic components. Just before connecting an external power source (generator type or battery cart), the master switch should be turned on. The ground service plug receptacle circuit incorporates a polarity re- versal protection. Power from the external power source will flow only if the ground service plug is correctly connected to the airplane. If the plug is accidentally connected backwards, no power will flow to the elec- trical system, thereby preventing any damage to electrical equipment. 7-26 7-27 SECTION 7 AIRPLANE & SYSTEMS DESCRIPTIONS CESSNA MODEL 172M CESSNA MODEL 172M SECTION 7 AIRPLANE & SYSTEMS DESCRIPTIONS The battery and external power circuits have been designed to com- pletely eliminate the need to "jumper" across the battery contactor to close it for charging a completely "dead" battery. A special fused circuit in the external power system supplies the needed "jumper" across the con- tacts so that with a "dead" battery and an external power source applied, turning on the master switch will close the battery contactor. LIGHTING SYSTEMS EXTERIOR LIGHTING Conventional navigation lights are located on the wing tips and top of the rudder. A single landing light or dual landing/taxi lights are installed in the cowl nose cap, and a flashing beacon is mounted on top of the verti- cal fin. Additional lighting is available and includes a strobe light on each wing tip and two courtesy lights, one under each wing, just outboard of the cabin door. The courtesy lights are operated by the dome light switch on the overhead console. All exterior lights, except the courtesy lights, are controlled by rocker type switches on the left switch and control panel. The switches are ON in the up position and OFF in the down position. The flashing beacon should not be used when flying through clouds or overcast; the flashing light reflected from water droplets or particles in the atmosphere, particularly at night, can produce vertigo and loss of orientation. The two high intensity strobe lights will enhance anti-collision protec- tion. However, the lights should be turned off when taxiing in the vicinity of other aircraft, or during night flight through clouds, fog or haze. INTERIOR LIGHTING Instrument and control panel lighting is provided by flood lighting, in- tegral lighting, and post lighting (if installed). Two concentric rheostat control knobs below the engine controls, labeled PANEL LT and RADIO LT, control intensity of the instrument and control panel lighting. A slide-type switch (if installed) on the overhead console, labeled PANEL LTS, is used to select flood lighting in the FLOOD position, post lighting in the POST position, or a combination of post and flood lighting in the BOTH position. Instrument and control panel flood lighting consists of a single red flood light in the forward part of the overhead console. To use the flood lighting, rotate the PANEL LT rheostat control knob clockwise to the de- sired intensity. The instrument panel may be equipped with post lights which are mounted at the edge of each instrument or control and provide direct lighting. The lights are operated by placing the PANEL LTS selector switch in the POST position and adjusting light intensity with the PANEL LT rheostat control knob. By placing the PANEL LTS selector switch in the BOTH position, the post lights can be used in combination with the standard flood lighting. The engine instruments, fuel quantity indicators, radio equipment, and magnetic compass have integral lighting and operate independently of post or flood lighting. Light intensity of the engine instruments, fuel quantity indicators, and radio lighting is controlled by the RADIO LT rheostat control knob. The integral compass light intensity is controlled by the PANEL LT rheostat control knob. A cabin dome light, in the aft part of the overhead console, is operated by a switch near the light. To turn the light on, move the switch to the right. A control wheel map light is available and is mounted on the bottom of the pilot's control wheel. The light illuminates the lower portion of the cabin just forward of the pilot and is helpful when checking maps and other flight data during night operations. To operate the light, first turn on the NAV LT switch; then adjust the map light's intensity with the knurled disk type rheostat control located at the bottom of the control wheel. A doorpost map light is available, and is near the top of the left for- ward doorpost. It contains both red and white bulbs and may be positioned to illuminate any area desired by the pilot. The light is controlled by a switch, below the light, which is labeled RED, OFF, and WHITE. Placing the switch in the top position will provide a red light. In the bottom posi- tion, standard white lighting is provided. In the center position, the map light is turned off. The most probable cause of a light failure is a burned out bulb; how- ever, in the event any of the lighting systems fail to illuminate when turn- ed on, check the appropriate circuit breaker. If the circuit breaker has opened (white button popped out), and there is no obvious indication of a short circuit (smoke or odor), turn off the light switch of the affected lights, reset the breaker, and turn the switch on again. If the breaker opens again, do not reset it. CABIN HEATING, VENTILATING AND DEFROSTING SYSTEM The temperature and volume of airflow into the cabin can be regulated to any degree desired by manipulation of the push-pull CABIN HT and 7-28 7-29 SECTION 7 AIRPLANE & SYSTEMS DESCRIPTIONS EXHAUST MUFFLER SHROUD FRONT CABIN AIR OUTLET ADJUSTABLE DEFROSTER OUTLET CABIN HEAT CONTROL REAR CABIN AIR OUTLETS ADJUSTABLE VENTILATORS D CESSNA MODEL 172M HEATER VALVE VENTILATING AIR DOOR CABIN AIR CONTROL CODE RAM AIR FLOW VENTILATING AIR HEATED AIR CONDITIONED AIR MECHANICAL CONNECTION Figure 7-8. Cabin Heating, Ventilating, and Defrosting System CESSNA MODEL 172M SECTION 7 AIRPLANE & SYSTEMS DESCRIPTIONS CABIN AIR control knobs (see figure 7-8). For cabin ventilation, pull the CABIN AIR knob out. To raise the air temperature, pull the CABIN HT knob out approximately 1/4 to 1/2 inch for a small amount of cabin heat. Additional heat is available by pulling the knob out farther; maximum heat is available with the CABIN HT knob pulled out and the CABIN AIR knob pushed full in. When no heat is de- sired in the cabin, the CABIN HT knob is pushed full in. Front cabin heat and ventilating air is supplied by outlet holes spaced across a cabin manifold just forward of the pilot's and copilot's feet. Rear cabin heat and air is supplied by two ducts from the manifold, one extending down each side of the cabin to an outlet at the front door post at floor level. Windshield defrost air is also supplied by a duct leading from the cabin manifold. Two knobs control sliding valves in the defroster out- let and permit regulation of defroster airflow. Separate adjustable ventilators supply additional air; one near each upper corner of the windshield supplies air for the pilot and copilot, and two ventilators are available for the rear cabin area to supply air to the rear seat passengers. PITOT-STATIC SYSTEM AND INSTRUMENTS The pitot-static system supplies ram air pressure to the airspeed in- dicator and static pressure to the airspeed indicator, rate-of-climb indi- cator and altimeter. The system is composed of either an unheated or heated pitot tube mounted on the lower surface of the left wing, an exter- nal static port, on the lower left side of the fuselage, and the associated plumbing necessary to connect the instruments to the sources. The heated pitot system consists of a heating element in the pitot tube, a rocker-type switch labeled PITOT HT on the lower left side of the instru- ment panel, a 10-amp circuit breaker on the switch and control panel, and associated wiring. When the pitot heat switch is turned on, the element in the pitot tube is heated electrically to maintain proper operation in possi- ble icing conditions. Pitot heat should be used only as required. A static pressure alternate source valve may be installed adjacent to the throttle for use when the external static source is malfunctioning. This valve supplies static pressure from inside the cabin instead of the external static port. If erroneous instrument readings are suspected due to water or ice in the pressure line going to the standard external static pressure source, the alternate static source valve should be pulled on. 7-30 7-31 SECTION 7 AIRPLANE & SYSTEMS DESCRIPTIONS CESSNA SECTION 7 CESSNA MODEL 172M AIRPLANE & SYSTEMS DESCRIPTIONS MODEL 172M Pressures within the cabin will vary with open cabin ventilators and windows. Refer to Sections 3 and 5 for the effect of varying cabin pres- sures on airspeed and altimeter readings. AIRSPEED INDICATOR The airspeed indicator is calibrated in knots and miles per hour. Limitation and range markings include the white arc (41 to 85 knots), green arc (47 to 128 knots), yellow arc (128 to 160 knots), and a red line (160 knots). If a true airspeed indicator is installed, it is equipped with a rotatable ring which works in conjunction with the airspeed indicator dial in a man- ner similar to the operation of a flight computer. To operate the indicator, first rotate the ring until pressure altitude is aligned with outside air tem- perature in degrees Fahrenheit. Pressure altitude should not be confused with indicated altitude. To obtain pressure altitude, momentarily set the barometric scale on the altimeter to 29.92 and read pressure altitude on the altimeter. Be sure to return the altimeter barometric scale to the orig- inal barometric setting after pressure altitude has been obtained. Having set the ring to correct for altitude and temperature, then read the airspeed shown on the rotatable ring by the indicator pointer. For best accuracy, this indication should be corrected to calibrated airspeed by referring to the Airspeed Calibration chart in Section 5. Knowing the calibrated air- speed, read true airspeed on the ring opposite the calibrated airspeed. RATE-OF-CLIMB INDICATOR The rate-of-climb indicator depicts airplane rate of climb or descent in feet per minute. The pointer is actuated by an atmospheric pressure change supplied by the static source. ALTIMETER Airplane altitude is depicted by a barometric type altimeter. A knob near the lower left portion of the indicator provides adjustment of the in- strument's barometric scale to the proper barometric pressure reading. VACUUM SYSTEM AND INSTRUMENTS An engine-driven vacuum system (see figure 7-9) provides the suction necessary to operate the attitude indicator and directional indicator. The system consists of a vacuum pump mounted on the engine, a vacuum re- lief valve and vacuum system air filter on the aft side of the firewall be- low the instrument panel, and instruments (including a suction gage) on SUCTION GAGE CODE INLET AIR VACUUM DISCHARGE AIR VACUUM PUMP SUCTION ATTITUDE INDICATOR DIRECTIONAL INDICATOR M 24 33 VACUUM 3 E PUSH S REF OVERBOARD VENT LINE VACUUM RELIEF VALVE Figure 7-9. Vacuum System VACUUM SYSTEM AIR FILTER 7-32 7-33 SECTION 7 CESSNA CESSNA MODEL 172M SECTION 7 AIRPLANE & SYSTEMS DESCRIPTIONS AIRPLANE & SYSTEMS DESCRIPTIONS the left side of the instrument panel. ATTITUDE INDICATOR MODEL 172M The attitude indicator gives a visual indication of flight attitude. Bank attitude is presented by a pointer at the top of the indicator relative to the bank scale which is marked in increments of 10°, 20°, 30°, 60°, and 90° either side of the center mark. Pitch attitude is presented by a miniature airplane in relation to the horizon bar. A knob at the bottom of the instru- ment is provided for in-flight adjustment of the miniature airplane to the horizon bar for a more accurate flight attitude indication. DIRECTIONAL INDICATOR A directional indicator displays airplane heading on a compass card in relation to a fixed simulated airplane image and index. The indicator will precess slightly over a period of time. Therefore, the compass card should be set in accordance with the magnetic compass just prior to takeoff, and oc- casionally re-adjusted on extended flights. A knob on the lower left edge of the instrument is used to adjust the compass card to correct for precession. SUCTION GAGE The suction gage is located on the left side of the instrument panel and indicates, in inches of mercury, the amount of suction available for operation of the attitude indicator and directional indicator. The desired suction range is 4.6 to 5.4 inches of mercury. A suction reading below this range may indicate a system malfunction or improper adjustment, and in this case, the indicators should not be considered reliable. STALL WARNING SYSTEM The airplane is equipped with a pneumatic-type stall warning system consisting of an inlet in the leading edge of the left wing, an air-operated horn near the upper left corner of the windshield, and associated plumbing. As the airplane approaches a stall, a low pressure condition is created over the leading edge of the wings. This low pressure creates a differen- tial pressure (vacuum) in the stall warning system which draws air through the warning horn, resulting in an audible warning at 5 to 10 knots above stall in all flight conditions. The stall warning system should be checked during the preflight in- spection by placing a clean handkerchief over the vent opening and apply- ing suction. A sound from the warning horn will confirm that the system 7-34 is operative. AVIONICS SUPPORT EQUIPMENT The airplane may, at the owner's discretion, be equipped with vari- ous types of avionics support equipment such as an audio control panel, microphone-headset, and static dischargers. The following paragraphs discuss these items. AUDIO CONTROL PANEL Operation of radio equipment is covered in Section 9 of this handbook. When one or more radios are installed, a transmitter/audio switching sys- tem is provided (see figure 7-10). The operation of this switching system is described in the following paragraphs. TRANSMITTER SELECTOR SWITCH A rotary type transmitter selector switch, labeled XMTR SEL, is provided to connect the microphone to the transmitter the pilot desires to use. To select a transmitter, rotate the switch to the number corres- ponding to that transmitter. The numbers 1, 2 and 3 above the switch cor- respond to the top, second and third transceivers in the avionics stack. An audio amplifier is required for speaker operation, and is automati- cally selected, along with the transmitter, by the transmitter selector switch. As an example, if the number 1 transmitter is selected, the audio amplifier in the associated NAV/COM receiver is also selected, and func- tions as the amplifier for ALL speaker audio. In the event the audio am- plifier in use fails, as evidenced by loss of all speaker audio, select another transmitter. This should re-establish speaker audio. Headset audio is not affected by audio amplifier operation. AUTOMATIC AUDIO SELECTOR SWITCH A toggle switch, labeled AUTO, can be used to automatically match the appropriate NAV/COM receiver audio to the transmitter being select- ed. To utilize this automatic feature, leave all NAV/COM receiver switches in the OFF (center) position, and place the AUTO selector switch in either the SPEAKER or PHONE position, as desired. Once the AUTO selector switch is positioned, the pilot may then select any transmitter and its associated NAV/COM receiver audio simultaneously with the trans- mitter selector switch. If automatic audio selection is not desired, the AUTO selector switch should be placed in the OFF (center) position. 7-35 SECTION 7 AIRPLANE & SYSTEMS DESCRIPTIONS AUTOMATIC AUDIO SELECTION XMTR SEL TRANSMITTER SELECTOR SWITCH CESSNA MODEL 172M SPEAKER NAV/COM ADF AUTO 1 2 3 1 2 O PHONE AUTOMATIC AUDIO SELECTOR SWITCH AUDIO SELECTOR SWITCH (TYPICAL) As illustrated, the number 1 transmitter is selected, the AUTO selector switch is in the SPEAKER position, and the NAV/COM 1, 2 and 3 and ADF 1 and 2 audio selector switches are in the OFF position. With the switches set as shown, the pilot will transmit on the number 1 transmitter and hear the number 1 NAV/COM re- ceiver through the airplane speaker. INDIVIDUAL AUDIO SELECTION SPEAKER 3 NAV/COM ADF AUTO 1 2 3 2 O PHONE XMTR SEL TRANSMITTER SELECTOR SWITCH AUTOMATIC AUDIO SELECTOR SWITCH AUDIO SELECTOR SWITCH (TYPICAL) As illustrated, the number 1 transmitter is selected, the AUTO selector switch is in the OFF position, the number 1 NAV/COM receiver is in the PHONE position, and the number 1 ADF is in the SPEAKER position. With the switches set as shown, the pilot will transmit on the number 1 transmitter and hear the number 1 NAV/COM receiver on a headset; while the passengers are listening to the ADF audio through the airplane speaker. If another audio selector switch is placed in either the PHONE or SPEAKER position, it will be heard simultaneously with either the number 1 NAV/COM or number 1 ADF respectively. Figure 7-10. Audio Control Panel CESSNA MODEL 172M AUDIO SELECTOR SWITCHES SECTION 7 AIRPLANE & SYSTEMS DESCRIPTIONS The audio selector switches, labeled NAV/COM 1, 2 and 3 and ADF 1 and 2, allow the pilot to initially pre-tune all NAV/COM and ADF re- ceivers, and then individually select and listen to any receiver or com- bination of receivers. To listen to a specific receiver, first check that the AUTO selector switch is in the OFF (center) position, then place the audio selector switch corresponding to that receiver in either the SPEAKER (up) or PHONE (down) position. To turn off the audio of the selected re- ceiver, place that switch in the OFF (center) position. If desired, the audio selector switches can be positioned to permit the pilot to listen to one receiver on a headset while the passengers listen to another receiver on the airplane speaker. The ADF 1 and 2 switches may be used anytime ADF audio is desired. If the pilot wants only ADF audio, for station identification or other rea- sons, the AUTO selector switch (if in use) and all other audio selector switches should be in the OFF position. If simultaneous ADF and NAV/ COM audio is acceptable to the pilot, no change in the existing switch positions is required. Place the ADF 1 or 2 switch in either the SPEAKER or PHONE position and adjust radio volume as desired. NOTE If the NAV/COM audio selector switch corresponding to the selected transmitter is in the PHONE position with the AUTO selector switch in the SPEAKER position, all audio selector switches placed in the PHONE position will automatically be connected to both the airplane speaker and any headsets in use. MICROPHONE-HEADSET The microphone-headset combination consists of the microphone and headset combined in a single unit and a microphone keying switch located on the left side of the pilot's control wheel. The microphone-headset per- mits the pilot to conduct radio communications without interrupting other control operations to handle a hand-held microphone. Also, passengers need not listen to all communications. The microphone and headset jacks are located near the lower left corner of the instrument panel. STATIC DISCHARGERS If frequent IFR flights are planned, installation of wick-type static dis- chargers is recommended to improve radio communications during flight 7-36 7-37 SECTION 7 AIRPLANE & SYSTEMS DESCRIPTIONS CESSNA MODEL 172M through dust or various forms of precipitation (rain, freezing rain, snow or ice crystals). Under these conditions, the build-up and discharge of static electricity from the trailing edges of the wings, rudder, elevator, propeller tips and radio antennas can result in loss of usable radio signals on all communications and navigation radio equipment. Usually the ADF is first to be affected and VHF communication equipment is the last to be affected. Installation of static dischargers reduces interference from precipi- tation static, but it is possible to encounter severe precipitation static conditions which might cause the loss of radio signals, even with static dischargers installed. Whenever possible, avoid known severe precipi- tation areas to prevent loss of dependable radio signals. If avoidance is impractical, minimize airspeed and anticipate temporary loss of radio signals while in these areas. 7-38 MOD MORT TO