Type Rating Knowledge — Cessna 172S NAV III
Fuel system
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Two integral tanks, and one number that is not the useful one
Fuel is carried in two vented INTEGRAL tanks, one in each wing — the tank is the sealed wing structure itself. Each holds 28.0 gallons, so total capacity is 56.0 gallons. But the number that matters for planning is USABLE fuel: 53.0 gallons in all flight conditions, because 1.5 gallons in each tank — 3.0 in total — is unusable. Note the phrase "in all flight conditions": the usable figure is the amount the aeroplane guarantees regardless of attitude, which is why it is lower than a level-flight measurement would give. Everything above the reservoir tank in the diagram works by GRAVITY: the tanks are in the wings, above the engine, and no pump is needed to get fuel down to the reservoir.
- L TANK 28 GAL — Left integral wing tank — 28.0 gallons total, 26.5 usable, 1.5 unusable. Carries the check valve equipped overboard vent.
- VENT LINE — Interconnecting vent line between the tanks, with the overboard vent under the left wing. Complete blockage decreases fuel flow and eventually stops the engine.
- R TANK 28 GAL — Right integral wing tank — 28.0 gallons total, 26.5 usable, 1.5 unusable.
- SELECTOR BOTH/L/R — Three-position fuel selector — BOTH for takeoff, climb, landing and any prolonged slip or skid; LEFT or RIGHT is for level cruise only.
- RESERVOIR TANK — Fuel reservoir tank — everything above it is fed by gravity alone; it also receives the fuel return line.
- AUX FUEL PUMP — Electrically driven auxiliary fuel pump — priming, vapour suppression, and enough flow for maximum continuous power if the engine driven pump fails.
- SHUTOFF VALVE — Fuel shutoff valve, downstream of the auxiliary pump.
- FUEL STRAINER — Fuel strainer — one of the five sampling points drained before flight.
- ENG DRIVEN PUMP — Engine driven fuel pump — the normal supply for the injection system, needing no electrical power at all.
- FUEL/AIR CONTROL — Fuel/air control unit (fuel servo) under the engine — meters fuel in proportion to induction air flow. The mixture control acts here.
- FLOW SENSOR — Turbine type fuel flow transducer between the servo and the flow divider — the source of FFLOW GPH on the G1000 EIS.
- FLOW DIVIDER — Fuel distribution valve (flow divider) on top of the engine — spring tension on a diaphragm and valve distributes fuel evenly to the nozzles.
- INJECTOR NOZZLES — Air-bleed type injector nozzle in the intake valve chamber of each of the four cylinders.