Pilot's Operating Handbook

Figure 1-4 FUEL SYSTEM. Fuel is supplied to the engine from two tanks, one in each wing. With the fuel selector valve on BOTH, the total usable fuel for all flight condi- tions is 38 gallons for the standard tanks and 48 gallons for the optional long range tanks. Fuel from each wing tank flows by gravity to a selector valve. De- pending upon the setting of the selector valve, fuel from the left, right, or both tanks flows through a fuel strainer and carburetor to the engine induction system. The fuel selector valve should be in the BOTH position for take-off, climb, landing, and maneuvers that involve prolonged slips or skids. Operation from either LEFT or RIGHT tank is reserved for cruising flight. NOTE With low fuel (1/8th tank or less), a prolonged steep de- scent (1500 feet or more) with partial power, full flaps, and 80 MPH or greater should be avoided due to the pos- ⚫sibility of the fuel tank outlets being uncovered, causing temporary fuel starvation. If starvation occurs, leveling the nose should restore power within 20 seconds. NOTE When the fuel selector valve handle is in the BOTH po- sition in cruising flight, unequal fuel flow from each Reims/Cessna F172 Flight Manual Page: 1-8 tank may occur if the wings are not maintained exactly level. Resulting wing heaviness can be alleviated gradually by turning the selector valve handle to the tank in the "heavy" wing. NOTE It is not practical to measure the time required to con- sume all of the fuel in one tank, and, after switching to the opposite tank, expect an equal duration from the remaining fuel. The airspace in both fuel tanks is in- terconnected by a vent line (figure 1-3) and, therefore, some sloshing of fuel between tanks can be expected when the tanks are nearly full and the wings are not level. For fuel system servicing information, refer to Lubrication and Servicing Procedures in Section VI. LONG RANGE FUEL TANKS (OPT.) Special wings with long range fuel tanks are available to replace the standard wings and fuel tanks for greater endurance and range. When these tanks are installed, the total usable fuel for all flight conditions is 48 gallons. ELECTRICAL SYSTEM. Electrical energy is supplied by a 14-volt, direct-current system powered by an engine-driven alternator (see figure1-5). A 12-volt bat- tery is located on the left-hand forward portion of the firewall. Power is supplied to all electrical circuits through a split bus bar, one side con- taining electronic systems and the other side having general electrical systems. Both sides of the bus are on at all times except when either an external power source is connected or the ignition/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 pre- vents harmful transient voltages from damaging the transistors 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- Reims/Cessna F172 Flight Manual Page: 1-9 ELECTRICAL SYSTEM SCHEMATIC REGULATOR ALTERNATOR OVER- VOLTAGE 95 WARNING LIGHT A MASTER SWITCH 10 ALT FIELD CIRCUIT BREAKER OVER- 10 PRIMARY BUS ALT CIGAR LIGHTER VOLTAGE SENSOR TO (WITH CIRCUIT BREAKER) ALT HELD CIRCUIT BRIAKER STARTER CONTACTOR REVERSE POLARITY CONTACTOR STARTER BATTERY CONTACTOR H BATTERY 117 شتند H 144 FLIGHT HOUR RECORDER (OPT) OIL -PRESSURE SWITCH (OPT) CLOCK (OPT) H CODE O CIRCUIT BREAKER IPUSH TO RESET FUSE DIODE RESISTOR + CAPACITOR INQISE FILTERI AMMETER GROUND SERVICE PLUG RECEPTACLE (OPT) ALT FIELD 10 OVER-VOLTAGE WARNING LIGHT TO OVER-VOLTAGE SENSOR AND MASTER SWITCH TO AUTOMATIC PILOT (OPT) ALT BUS ELECTRONIC DES AUTO PILOT TO RADIO (OPT) RADIO 1 TO RADIO (OPT) RADIO 2 TO RADIO (OPT) RADIO 3 TO RADIO (OPT) RADIO 4 -TO RADIO (OPT) RADIO S AUD AMP SPLIT BUS CONTACTOR (NORMALLY CLOSED) LAND LT TO NAVIGATION LIGHT CIRCUIT BREAKER IGNITION STARTER SWITCH MAGNETOS Figure 1-5. PRIMARY BUS TO AUDIO AMPLIFIER (OPT) FROM ALTERNATOR BUS -TO LANDING LIGHT (OPT) TO NAVIGATION LIGHTS AND OPTIONAL CONTROL WHEEL MAP LIGHT NAV LT TO TRANSMITTER RELAY (OPT) TO IGNITION-STARTER SWITCH TO FLASHING BEACON (OPT) BCN LT TO DOOR POST MAP LIGHT (OPT) INT LI TO DOME AND OPTIONAL COURTESY LIGHTS TO COMPASS AND INSTRUMENT LIGHTS INST TO FUEL QUANTITY INDICATORS TO WING FLAP POSITION INDICATOR TO WING FLAP SYSTEM FLAP STROBE LT -10 STROBE LIGHTS (OPT) TO PITOT HEAT SYSTEM (OPT) PITOT HT TURN COORD 10 TURN COORDINATOR OR OPTIONAL TURN AND BANK INDICATOR