28A Fuel Systems - Monitoring and indicating

 

TTP	B1-L3	ATA 28A
Beech 90 Series	B2-L3	Fuel Systems - Monitoring and indicating

Fuel system controls and indications include gauges, switches, circuit breakers, and indicators on the fuel control and annunciator panels.

 

FUEL CONTROL PANEL

 

The fuel control panel is on the pilot left console

Transfer Pump Switches

 

The transfer pump switches are on the fuel control panel left and right sides. They control operation of respective transfer pumps in the wing center section tanks as follows:

 

•  OVERRIDE    position—Applies    power from the triple-fed bus directly to the fuel transfer pump.

 

•  AUTO position—Allows automatic operation of the fuel transfer system.

 

•  OFF position—Removes power from the fuel transfer system circuits.

 

TRANSFER TEST Switch

 

A three-position TRANSFER TEST switch is on the fuel control panel and is spring-loaded to the center position.

 

•  Left/right  positions—Verifies  the  operation of the  respective  automatic  transfer system.

 

•  Center  position—Switch setting  for normal operation. No test functions are being performed.

 

BOOST PUMP Switch

 

The left and right BOOST PUMP switches are on each side of the fuel control panel.

These switches  operate  the respective  left  and right side nacelle tank boost pumps.

•  ON—Applies power to the boost pump.

 

•  OFF—Removes  power  from  the  boost pump.

 

CROSSFEED Switch

 

The CROSSFEED switch is on the fuel control panel and functions as follows:

 

•  OPEN—In this position, power is applied to the crossfeed valve. This allows cross- feed of fuel from the fuel system on one side to the engine on the opposite side.

 

•  AUTO—This is the normal switch posi- tion. Automatic crossfeed is enabled.

 

• CLOSE—In this position, crossfeed is disabled.

 

FUEL QUANTITY Switch

 

The FUEL QUANTITY switch is on the bottom center of the fuel control panel:

 

•  TOTAL—This position provides an indication of the total fuel in the left and right fuel systems.

 

    • NACELLE—This position provides an indication of fuel only in the left and right nacelles.   

FIREWALL SHUTOFF VALVE Switches

 

Two firewall shutoff switches are on the CB sub- panel of the fuel control panel that control the firewall shutoff valves to their respective engines.

 

• CLOSED—This position is selected by lowering the red switch guard and plac- ing the switch in the down position. In this position, the firewall shutoff valve is closed, stopping fuel flow to the associated engine.

 

• OPEN—Power from the triple-fed bus powers the firewall valve open when the switch is raised. Power to the firewall shut- off valves is NOT provided from the hot bus.

 

ANNUNCIATOR PANEL

 

Caution and warning annunciators on the cockpit annunciator panel illuminate if a problem occurs. Each side of the fuel system has separate annunciators.

 L/R FUEL PRESS—This red warning annunciator indicates loss of fuel pressure in the respective left or right fuel system. Fuel-system pressure loss can be caused by a failed boost pump.

        CAUTION: Engine operation with the FUEL PRESS annunciator illuminated is limited to 10 hours. After this time, the high-pressure, engine-driven pump must be overhauled or replaced.

        CAUTION: When operating with aviation gasoline-based  fuels,  operation  of  the high-pressure, engine-driven pump alone is limited. Operation is permitted up to 8,000 feet for a period not to exceed 10 hours. Operation above 8,000 feet requires boost or crossfeed.

L/R NO FUEL XFR—This amber annunciator illuminates indicating either transfer pressure is lower than 2.5 psi with a transfer pump failure or an empty center wing tank with the 30-second time delay elapsed.

 FUEL CROSSFEED—This is an amber caution annunciator that illuminates when the crossfeed valve is powered and indicates  fuel should be crossfeeding.

FUEL-QUANTITY INDICATING SYSTEM

 

      The fuel-quantity indicating system is a capacitance-type system that compensates for specific gravity and temperature, and reads out in pounds on the right and left fuel gauges. An  electronic  circuit  in  the  system  processes the signals from the fuel-quantity (capacitance) probes in the fuel cells for an accurate readout by the fuel-quantity indicators. 

         When the selector switch between the fuel-quantity indicators on the pilot fuel panel is set to the TOTAL position, the fuel gauges indicate the total quantity of fuel in the left and right fuel systems. When the switch is set to the NACELLE position, the fuel gauges indicate only the quantity of fuel in the nacelles.

        Each side of the airplane has an independent gauging system consisting of five fuel-quantity (capacitance) probes. The probes are distributed as follows:

•  One in the center wing fuel cell

•  One in the nacelle fuel cell

•  One in the inboard aft wing panel fuel cell

 

•  Two in the leading edge wing panel fuel cell

     Five-amp circuit breakers, one for each side, are below the fuel panel and provide power for the indicating system.

     Fuel density and electrical dielectric constant vary with respect to temperature, fuel type, and fuel batch. The capacitance gauging system compensates for these variables. A fuel-quantity probe is simply a variable capacitor with two concentric tubes. The tubes serve as capacitor plates and the level of fuel in the space between the tubes acts as a variable dielectric for the capacitor. The inner tube can be profiled by changing its diameter at different heights to make the capacitance between the inner and outer tube proportional to the tank volume at a specific tank location.              

The capacitance of the fuel-quantity probe varies with a change in the dielectric. As the fuel level between the inner and outer tubes rises, for example as the tank is filled, air with a dielectric constant of one is replaced by fuel with a dielectric constant of approximately two, thus increasing the capacitance of the fuel-quantity probe. As the fuel is used from the tank, the reverse occurs. The fuel with a dielectric constant of approximately two is replaced with air with a dielectric constant of one. This variation in the volume of fuel contained in the fuel cell produces a capacitance variation that is a linear function of that volume. This func- tion is converted to a linear current that actuates the fuel-quantity indicator. The fuel probes are designed to produce a capacitance variation that is linear in relation to variation in weight even through weight is nonlinear with respect to the fuel level. By varying the capacitance per inch, the nonlinear level signal is changed to a linear function. In addition to its capacitance-sensing tubes, each fuel-quantity probe contains a small circuit network that produces an output current whose average value is directly proportional to fuel level, while automatically compensating for fuel-temperature-density variations.

A triangulated VDC waveform, whose characteristics  are insulated  by a regulator from line voltage variations in the incoming +28 VDC, is impressed across the signal out (low Z) side or outer tube, of the fuel-quantity probe. The signal in (SIG) side or inner tube, is the resulting out- put from the capacitor affected by the amount or difference of the air/fuel dielectric. The ensuing signal is further processed by a DC amplifier, which contains a potentiometer for adjusting the “Full” and “Empty” settings. The DC amplifier controls response time and drives a servo movement-type meter.

 The system is accurate to 3% of full-scale indication when used with approved fuels. An integral compensator probe, incorporated into the bottom of the nacelle tank probe, accomplishes adjustments for any variation in the dielectric constant of the fuel caused by temperature changes or variation in the different manufacturer fuels. Since this compensator is always immersed in fuel, changes in its capacitance are caused by the variations in the properties of the fuel. The signal output from the compensator is used as a correction signal for the reference portion of measurement circuit to minimize errors in the indicator. Use of alternate or emergency fuels can affect the accuracy of the system.

TROUBLESHOOTING THE CAPACITANCE GAUGING SYSTEM

 

Troubleshooting problems in the capacitance system can be divided into three areas. The problem is isolated as follows:

 

1.   The gauge assembly

 

2.   The wires and connectors in the system

 

3.   The probes themselves

 

Additionally, combinations of problems may occur. The only truly effective way to troubleshoot is to have the appropriate test equipment. The accepted equipment is manufactured by the Barfield Instrument Corporation. Without the appropriate test equipment, any adjustments or corrections to the  indicating  system attempted may introduce erroneous fuel quantity readings and should be considered dangerous.

 

SYSTEM CHECKS

 

Five tests can be performed on the system:

 

1.   Insulation check

 

2.   Capacitance check

 

3.   Probe test

 

4.   Gauge test/linearity test

 

5.   Calibration of the gauge to the aircraft system

 

INSULATION CHECK

 

The insulation check determines if there are shorts or grounds in the wiring and connectors of the system. Additionally, inductive interference with other aircraft wiring is checked. Three wires are used in this DC-based system. A yellow wire, signal out (LO Z), is the indicator/signal conditioner input to the probes. This input is a triangulated DC waveform that varies from +15 to +17 volts at

15 to 20 KHz. The red wire, signal in (SIG), is the return signal sent back to the indicator, through a diode assembly imbedded in the probe. The green wire, return (RTN), provides a return so as not to leave a charge on the probe. The ground plane of the aircraft could also be considered as another wire in the system although it is not actively used. Any shorts, however minor, between the three wires or the grounding of any to the ground plane adversely effects the capabilities of the system. The probes are all interconnected and are wired in parallel. Either the total system fuel quantity or only nacelle fuel quantity can be monitored. On the C90A/B aircraft, no power to the aircraft is needed to perform the system insulation check.

 

CAPACITANCE CHECK

 

The capacitance check verifies that total capacitance of the probes in the system is in tolerance. Loss of a probe or changes in the values of the probes can be determined. Since the probes are wired in parallel, the total capacitance of the system is a sum of the capacitance values of all the individual probes. The total capacitance value obtained from this test is compared to either historical data from previous tests of the aircraft or values listed in the charts from the Barfield Manual. The Barfield Manual provides minimal, nominal, and maximum chart values to compensate for manufacturing tolerances in the probes. The test must be performed with the fuel tanks either empty, the preferred method, or completely full. On the C90A/B aircraft, the capacitance check can be performed with no power to the aircraft or the fuel system. If the system capacitance checks good, the fault is in the gauge unit; if the system capacitance is inaccurate or out of tolerance, the fault probably lies in the probes.

PROBE TEST

 

If the total capacitance check is out of tolerance, a test of the individual probes is conducted. The test must be performed with the fuel tanks either empty or completely full. The individual probes are disconnected from the system and individually checked with the test equipment. The capacitance value obtained from this test is compared to either historical data from previous tests of the aircraft or values listed in the charts from the Barfield Manual. The Barfield Manual provides minimal, nominal, and maximum chart values to compensate for tolerances in the probes.

INDICATOR TEST/LINEARITY TEST

 

If  the  total  capacitance  check  was  good,  the gauge must be calibrated  and tested. This test is performed with the test equipment connected between the aircraft system and the gauge. The gauge is checked for a zero reading and 1,200 pound reading on the total system and a zero reading on the nacelle tank. If the gauge does not read correctly at these high and low settings, the gauge can be adjusted. If the adjustments are not successful, the gauge must be replaced. Scale linearity is then checked at intermediate values of 300, 600, and 900 pounds. 

        WARNING: The fuel system must be calibrated if any adjustments are made to the                 gauge during the indicator test.

CALIBRATION OF THE GAUGE TO THE AIRCRAFT SYSTEM

If the gauge or any of the probes are replaced or if any adjustments were made during the gauge test, the gauge must be calibrated to the aircraft system. In the gauge test, the gauge was set to test values. However,  the  system  of  probes  in  the  aircraft will be at various values somewhere between the minimum and maximum acceptable values. This calibration of the gauge to the aircraft simply personalizes the gauge to differences in the existing aircraft system. Once again this calibration must be performed with the tanks either empty or full. The preferred procedure is to calibrate with the tanks empty, but it is acceptable to calibrate with the tanks full. With an empty tank, calibration is done with a nominal test set; full tank value is added to a real empty tank condition to achieve a full tank reading. The empty tank reading is an actual reading of an empty situation. When a full tank calibration is done, the full-tank reading is an actual full-tank condition. To achieve a simulated empty-tank condition, a simulated full-tank value is removed from the real full-tank condition to simulate an empty-tank condition.

PTP  Beech 90 Series	B1	LOC	FOT
	B2	LOC	FOT		R/I		TS

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