1. UNIT 4- PARTB FUEL QUANTITY & INDICATING SYSTEM

2. CONTENTS Capacitance type indicating system
Fuel tanks & Indicating systems
Capacitance type indicating system
Float type indicating system
Effects of fuel temperature changes
References

3. Fuel tanks & Indicating system
To know about the fuel quantity indicating system, we mainly need to know the fuel tank locations in the aircraft .
There are two internal tank location
Wing tanks &
Fuselage tanks
Wing tanks - The tanks that are placed on the left and right wing of the aircraft are called as wing tanks.
Fuselage tanks- The tanks placed central body of an aircraft that is designed to accommodate the crew and passengers (or cargo) are called as fuselage tanks.

4. Auxiliary
Left wing
Left engine feed
Main tank
Right wing
Right engine feed

5. Fuel quantity indicator
Fuel quantity gauges(indicators) are necessary so that the pilot may know the quantity of fuel remaining in the tanks during operation of the aircraft.
Fuel quantities are indicated by the fuel quantity indicator located in the lower right corner of the pilot's cockpit (at the pilot's right knee).
Beside the fuel quantity indicator, the system components include a built-in test ( BIT).
A BINGO caution display and an independent FUEL LOW caution light.

6. ABOUT ELECTRICAL capacitance
Whenever a potential difference is applied across two conducting surfaces separated by a non-conducting medium,called dielectric,they have a property of storing electric charge;this property is known as capacitance.
capacitance(electron-holding ability)
C=Q(charge)/V(potential difference)
Unit expressed in terms farads(genarally micro farads)
In the principle of the capacitance to fuel gauge system,picofarad is the standard unit of measurement
FACTORS ON WHICH capacitance DEPENDS
C=E0A/D
C directly proportional area,relative permittivity and
inversely proportional distance between 2 plates

8. when the switch is placed in position 1 a rush of electrons, known as the charging current, takes glace from plate A through the battery to plate B and ceases when the potential difference between the plates is equal to that of the battery.
When the switch is opened, the plates remain positively and negatively charged since the atoms of plate A have lost electrons while those at plate B have a surplus. Thus, electrical energy is stored in the electric field between the plates of the capacitor.
Placing the switch in position 2 causes the plates to be short circuited and the surplus electrons at plate B rush back to plate A until the atoms of both plates are electrically neutral and no potential difference exists between them.

10. capacitance type fuel gauge system
capacitance-type fuel-gauge system consists of
Variable capacitor located in tank
Amplifier and an indicator
The complete circuit forms electrical bridge which is continuously rebalanced as a result of differences between tank capacitor and a reference capacitor
The signal produced is amplified to operate a motor, which positions a pointer to indicate the capacitance change of the tank capacitor and thus the change in fuel quantity .

11. CHANGES IN capacitance DUE TO FUEL AND AIR

12. EQUATION FOR FUEL capacitance
K is given by
K=CT/CA; CA=capacitance of air;
CT=Total capacitance ;
CT= CA+CF (PUTTING CT= KCA)
CF=CA(K-1)
For height L of fuel level and height of tank H
Fuel capacitance is given as
CF=(L/H)CA(K-1)

13. Arrangments of a basic capacitance fuel-gauge system
It has 2 loops(sections).
Loop A, contains tank capacitor Ct and is a sensing loop because it detects current due to changes in capacitance. Vs remains constant.
Loop B, is considered as the balancing loop of bridge, contains a reference capacitor Cr of fixed value is connected to secondary winding of transformer so that voltage Vb is variable
motor drives potentiometer wiper and indicator pointer.
Amplifier is only energized when an unbalanced condition exits in the bridge
Discrimination Stage- discriminates the phase of signal (voltage)

15. Operation of circuit
Initially fuel is full Ib=Is,bridge is in balance no voltage is produced across R
As fuel level drops,Cf decreases in turn Ct decreases hence Is decreases.
A signal voltage IbR is amplified phase detected and given to stepper motor(control phase)
Current in in control-phase lead or lag depends which loop predominating
In this case balancing loop is predominating, control-phase current lags behind that of reference phase causing the motor and wiper of potentiometer to be driven in such a direction as to decrease the balancing Ib.
When Ib=Is, bridge again in balance condition, then
motor stops rotating indicator pointer will come to lower vlaue.

16. Effects of fuel temperature changes
The volume, density & relative permittivity of the fuels are affected when the temperature changes.
During extreme cold, especially at high altitude, the gauge can be checked to determine when fuel temperatures are approaching those at which there may be danger of ice crystals forming in the fuel.
Thus a volumetric gauge system will be subjected to a small error due to variations in fuel temperatures.