C172S Fuel System

How is the fuel system distributed

Fuel flows by gravity from the two wing tanks to a three-position fuel selector valve, (BOTH, RIGHT and LEFT), then to the fuel reservoir tank. From the fuel reservoir tank, fuel flows through the electrically-driven auxiliary fuel pump, through the fuel shutoff valve, the fuel strainer, and to the engine-driven fuel pump. From the engine driven fuel pump, fuel is delivered to the fuel/air control unit on the bottom of the engine. The fuel/air control unit (fuel servo) meters fuel flow in proportion to induction air flow. After passing through the control unit, metered fuel goes to a fuel distribution valve (flow divider) located on the top of the engine. From the fuel distribution valve, individual fuel lines are routed to air bleed type injector nozzles located in the intake chamber of each cylinder

What type of tanks do we have

Vented integral fuel tanks (AKA wet wing)

What does integral fuel tank mean

Means that the wing itself is used as the fuel tank. Instead of having a seperate metal or bladder tank installed, the internal structure of the wing is sealed so that fuel is stored directly inside it.

How many gallons of fuel remain when an empty tank is displayed

1.5 gallons of unusable fuel

What is the limit for sensor measurement range for the fuel quantity indicator

approximately 24 gallons and is indicated by the maximum limit of the green band.

What does LOW FUEL L (left) and/or LOW FUEL R (right) displayed in amber on the PFD mean

The fuel quantity indicators detect less than 5 gallons indicated (and remains less than this level for more than 60 seconds) and incorrect sensor outputs.

What does the LOW FUEL L and/or LOW FUEL R remaining amber and the indicator pointer(s) and label change to flashing red.

When fuel quantity reaches the calibrated usable fuel empty level

How is fuel flow measured

By use of a turbine type transducer mounted on top of the engine between the fuel/air control unit and the fuel distribution unit

What is the purpose of the fuel/boost pump

Priming the engine, vapour suppression on hot days,

What is the purpose of the fuel return system

improve engine operation during extended idle operation in hot weather environments. It's designed to return a metered amount of fuel/vapor back to the fuel reservoir tank. The increased fuel flow, due to the fuel return system, results in lower fuel operating temperatures at the engine inlet, which minimizes the amount of fuel vapor generated in the fuel lines during hot weather operations

What are the components of the fuel return system

the fuel-air control unit (fuel servo), fuel return line, with check valve, and a fuel reservoir tank.

What are the components of the fuel venting system

Consists of an interconnecting vent line between the fuel tanks and a check valve equipped overboard vent in the left fuel tank assembly, The overboard vent protrudes from the bottom surface of the left wing.

How does the fuel venting system work

The fuel filler caps are vacuum vented; the fuel filler cap vents will open and allow air to enter the fuel tanks in case the overboard vents become blocked.

What can happen when fuel tanks are 1/4 or less with prolonged uncoordinated flight

E.g. slips and skids can uncover fuel tanks outlets (where fuel leaves the tank) causing fuel starvation and engine stoppage. Do not allow the airplane to remain in uncoordinated flight for more than 30 seconds

How does the fuel injection system work?

Fuel is delivered by the ED fuel pump to the F/A control unit. The F/A control unit proportions the fuel flow to the induction air flow. After passing through the control unit, induction air is delivered to the cylinders through the intake manifold tubes, and metered fuel is delivered to a fuel manifold (flow divider). The fuel manifold, through spring tension on a diaphragm and valve evenly distributes the fuel to an air-bleed type injector nozzle in the intake valve chamber of each cylinder. Turbine-type fuel flow transducer mounted between the F/A control unit and fuel distribution unit produces a digital signal that displays fuel flow on the EIS pages.