Fuel Pressure, Temperature and Flow

Fuel Temperature and Pressure of the low pressure fuel supply are electrically transmitted to their respective indicators. They are similar in operation to those for oil temperature and pressure, so no further description will be given in these notes.

Fuel Flow

Although the amount of fuel consumed during a given flight may vary slightly between engines of the same type, fuel flow does provide a useful indication of the satisfactory operation of the engine and of the amount of fuel being consumed during the flight. A typical system consists of a fuel flow transmitter, which is fitted into the low pressure fuel system, and an indicator, which shows the rate of fuel flow and the total fuel used in gallons, pounds or

As stated fuel flow indicating systems have two main units.

These are the fuel flow sender or transmitter and the indicator itself. The systems are included in aircraft to measure the rate of fuel flow to the engines. In order to operate successfully the following criteria must be met:

1. They must be able to indicate the rate of fuel flow accurately.

2. The transmitter must not impede the flow of fuel.

3. If a mechanical breakdown occurs, then the maximum rate of fuel flow to the engine should be provided.

4. They must include compensation for changes in fuel temperature.

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Types of Fuel Flow Systems

The following notes describe three types of fuel flow meters.

Rotating Vane Fuel Flow System Transmitter

The transmitter has 4 sections:

1. The rotating vane, whose shaft determines the electrical output.

2. The damping section, with a fuel filled compartment containing a damping vane to remove oscillations of the moving vane, the damping vane also acts as a counter-balance to the moving vane, this section also houses the calibration spring.

3. Information transmission section, - a ring magnet is attached to the moving vane shaft and transmits shaft movement to a bar magnet in the electrical section. The use of a ring and bar magnet eliminates the risk of fuel in the electrical section.

4. The electrical section houses a bar magnet, whose movement varies the output from a potentiometer and therefore the output to the indicator.

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Indicator

The indicator is of the integrated type that indicates the fuel flow rate and the total fuel consumed. It contains an inverter circuit, which provides various signals to the circuit:

1. Motor reference phase.

2. Tacho - generator feed-back reference phase.

3. Amplifier input.

4. Fuel flow transmitter potentiometer supply.

5. An anti-phase signal for the resetting of the total fuel consumed counters.

The indicator consists mainly of a low inertia two-phase induction motor, which provides 2 integrated outputs (the instantaneous flow rate and the total fuel consumed). The motor also drives a feedback tacho-generator, which provides damping through a negative feedback to the amplifier proportional to the motor speed. The fuel flow pointer is driven via a magnetic drag cup assembly. The total fuel consumed counters are operated by a mechanical drive via a gearbox.

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Fuel entering the metering chamber is straightened before it impinges on the vane, which rotates against the tension of the calibration spring. The chamber is non-linear in shape (involute) to produce a linear vane shaft movement, which is conveyed to a potentiometer via ring and bar magnets.

The potentiometer output is fed to the amplifier to drive an induction motor coupled to a gearbox producing 3 outputs:

1. A drive to a drag/disc assembly to operate the flow rate pointer.

2. A mechanical drive to operate the total fuel consumed counters.

3. A drive to a tacho-generator, which produces a negative feedback signal proportional to the rate of fuel flow.

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Impeller Fuel Flow System

This type of flow meter is designed to continually display the rate of fuel flow, and the total fuel quantity consumed, in terms of mass units. The function of the flow meter transmitter depends upon the volumetric rate of fuel flow and therefore if the system is measuring fuel mass, a correction will have to be made for the density and temperature of the fuel.

The transmitter consists of a light alloy casting with guide vanes and an electrical ‘pick-off’ coil. Inside the casting there is a helical vane rotor which has a magnet embedded in it. When the impeller rotates (due to the fuel flow) a sinusoidal signal, at a frequency proportional to the speed of the rotor and hence the rate of fuel flow, will be induced in the pick-off coil.

The output of the transmitter is fed to an integrator within the indicator, to be amplified and shaped for the operation of the fuel flow rate and total fuel consumed indicators.

Temperature Correction

For a decrease in temperature, the fuel becomes denser, giving a lower flow rate. This results in a decrease in the signal frequency and therefore the indicator would under- read. A temperature sensor output is fed to the indicator and applied to the computed outputs from the transmitter to give a temperature corrected indication.

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As alternatives to the basic counter and milliammeter the signals can be fully digitised and then fed to electronic digital display indicators. Further modifications to the signal are carried out by temperature and density compensators.

Impeller/Turbine Fuel Flow Indicating System

This type of fuel flow transmitter has 4 sections:

1. A static frequency controller, which will maintain its output to close tolerances - i.e. ± 0.3% frequency.

2. An impeller, which is driven through reduction gearing by a motor using the frequency controller output - this keeps the impeller at a constant speed.

3. A turbine that contains fuel-straightening vanes. The turbine is mechanically independent of the impeller, but is restrained by a spring.

4. An electrical transmitter, whose output is controlled by the position of the turbine.

The picture at on the following page shows an integrating flow meter system as used in the Boeing 737 aircraft. The constant speed impeller imparts an angular momentum to the fuel, proportional to the rate of fuel flow. This angular momentum of the fuel is applied to the straightening vanes in the turbine, causing it to rotate until the calibrated restraining springs balance the force due to the momentum of the fuel. The deflection of the turbine shaft positions the LVDT to a position corresponding to the fuel flow in the line. A signal voltage (up to 5V at maximum fuel flow) is induced in the secondary of the LVDT and is supplied to the indicator servomotor via the closed contact of the reset switch and amplifier.

The servomotor rotates at a rate proportional to the flow rate, driving the Flow Rate Pointer via a magnetic drag cup and the Fuel Used counters via a mechanical gearbox.

The Reset Switch is located on a panel in the cockpit and, when pressed, energises the reset relay, whose contacts supply 115 V a.c. to the servo amplifier and motor, causing it to drive the fuel used counters rapidly to zero. The decoupling disc prevents any hydraulic coupling between the impeller and turbine at low flow rates.

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