Fuel Gauging Systems Terminology
AC SYSTEM: Any capacitance measuring system (fuel, LOX, or level sensing) which uses sensors that are capacitors with no diodes between the variable capacitance of the probe and the signal processing circuit.
AUTO-RANGING: The ability of some testers to automatically change the range of the measuring circuitry, either in capacitance or resistance measurement to include the value being measured. Testers without this capability have RANGE switches to select the range that includes the unknown value.
ANALOG GAUGE: An electromechanical indicator, usually with pointer indicator(s), which contains all the electronics to produce LoZ and Comp LoZ, and to indicate tank contents in pounds or kilograms. These are normally original equipment on older aircraft. They may serve as master (cockpit) gauges or on the fueling panel as "repeater" gauges. Analog technology may also be used as a "totalizer" gauge, summing all tank indications.
BIT: Built-In-Test. This is the ability of an LRU to do self-diagnostics. Usually it is a pass-fail series of internal tests. The LRU will display fault codes, or may simply indicate a failure has occurred.
BITE: The ability of an LRU such as a digital gauge, signal conditioner or processor, or a CPU to analyze the system it is attached to, i.e. aircraft wiring, probes, compensator, level sensors, etc. Normally failure data is stored for future recall in non-volatile memory.
BEAD THERMISTER: A negative-temperature-coefficient device. At higher temperatures it has less resistance, at lower temperatures more resistance. This is opposite of the way a resistor responds. Thermisters are used for low level warning, high and low level fuel transfer shutoff, and to terminate fuel jettison. They are frequently mounted on a fuel probe, but the operation of the thermister is independent of that of the probe.
COMPENSATED SYSTEM: One that corrects for density changes in the fuel due to temperature or fuel type.
COMPENSATOR: A capacitive sensor to correct for fuel density changes. Mayor may not be integral to a fuel probe.
COMP: Refers to the measure and simulate BNC terminals on the tester which attach to the compensator in an AC fuel indicating system. DC systems may be compensated, but not with a dedicated capacitive "compensator".
COMPUTERIZED SYSTEMS: A fuel gauging system based on a central processing unit (CPU), usually in a rack-mounted "computer" or "processor". These are more complex, more accurate, and more capable systems, usually installed on long-range aircraft. The C-130J has a CPU-based system. These include a high level of on-board diagnostics. They usually use the "compensator" only to determine fuel "K-factor", and use a densitometer for the actual density measurement. CPU-based systems can store "failure" information indefinitely in non- volatile memory. These systems usually communicate between the CPU and indicators, and with an On-Board-Maintenance (OMB) computer via a data bus. There may be no dedicated fuel gauges in the cockpit. Fuel gauging may be called up as a menu item on an Engine Indicating and Condition Advisory System (EICAS) computer.
DIGITAL GAUGE: A microprocessor-based indication, usually employing an LCD display. A digital gauge may have both an "analog" indication, and a "digital" readout, both using LCD technology. An analog gauge may conversely have a "digital" readout in the form of a rotating counter-type indication. Digital gauges can do a limited amount of system diagnostics and store "failure" information as long as power remains applied. "Fault codes" are typically displayed as requested by maintenance personnel. A digital gauge will typically "blank" itself if it knows it cannot compute fuel quantity within specification with a known failure of the sensing system.
-DC: Refers to the signal line attached to one of the two diodes or sets of diodes in a DC system. Usually used as the input signal for a single tank.
+DC: Refers to the signal line attached to the diode or diodes opposite the ones used for -DC above. The amount of DC current on the "-DC" side of the diodes is equal to and opposite from the current on the "+DC" side. It is normally used to total the fuel signals from all the tanks on the aircraft.
DTF: Acronym for "Distance-To-Fault". Some testers are capable of reading very high capacitance, up to 39,999 pF. With the right switching, the tester can measure the capacitance of any coaxial line between the center conductor and the shield. It displays this capacitance in pF, pF X 1,000, or pF X 10,000. The measured capacitance can be converted using known conversion factors to distance in feet or meters to an open shield or center conductor. This function is similar to that of a Time-Domain-Reflectometer, only it is intrinsically safe.
DENSITOMETER: A dedicated component of more recent fuel indicating systems that measures the density of the fuel by methodology which is NOT capacitive. A densitometer is more accurate but requires special test equipment to test and evaluate its performance.
DC SYSTEM: Any capacitance measuring system which uses sensors that are capacitors with diodes on the probe to rectify the return signal. An AC system tester will not test a DC system and vice-versa.
FULL-HEIGHT COMPENSATED SYSTEM: One that uses a fixed capacitor in series with the variable probe capacitance to correct for fuel density changes. Can be utilized on either AC or DC fuel indicating systems.
FUEL LEVEL AMPLIFIER: Contains the circuitry to power and monitor the response of one or more bead thermisters. When the thermister is in air, it cannot dissipate heat so well, so it heats up and its resistance goes down. When it is immersed in fuel, it is cooled by the fuel and its resistance goes up. The FLA will operate fuel valves, Low Level Warning lights, or caution lights based on the input is sees from the bead thermister.
FUEL MANAGEMENT SYSTEMS: Computerized fuel systems which not only measure and display fuel quantity, but also perform additional functions such as jettison, center-of-gravity management, and fuel distribution during refueling. Such systems require inputs from the aircraft regarding weight-on-wheels (WOW), aircraft attitude (pitch and roll), wing-sweep, and weapons load. The F -18E fuel indicating system is integrated with a higher-level system called A VICS.
HiZ: Equivalent to "COAX" on older testers. In the context of AC indicating systems, this is the coaxial line between all the probes in the tank and the tester, signal conditioner, signal processor, or indicator. In the context of DC systems, there is a +HiZ and a -HiZ for each probe and for each tank, one connected to each set of diodes.
INTERFACE UNIT: Aircraft-specific interconnecting and switching units, which usually contain the cables required to connect the standard capacitance tester to the aircraft system(s) under test. These typically include the ability to (1) select each probe in a tank for test, and to (2) select each tank on the aircraft for test. They may include simulated empty, added, or full capacitance values to simulate more than one tank simultaneously, and may be have test capability independent of a capacitance tester. For example, an independent thermister test circuit may be built into the interface unit to interrogate and simulate level sensing.
LRU: Line-Replaceable-Unit. Any component that can be replace at the operational level of maintenance.
LoZ: Equivalent to "UNSH" on older testers. This is the drive signal, usually between 400 Hz and 1.6 KHz, and applies equally to AC and DC capacitance systems.
MEASURE: Refers to the ability of the tester to measure any unknown capacitance and display it in picofarads (pF). If it is an AC/DC Capacitance Tester, it can measure either "AC capacitance" or "DC capacitance".
MEGGER: Refers to the ability of the tester to measure insulation resistance, usually in megohms. A DC voltage is applied between the two points to be tested, and the amount of current returning is the reciprocal of resistance. Some testers can read resistance from infinity down to less than one ohm.
ON-BOARD DIAGNOSTICS: A higher level of BITE. A CPU-based indicating system can (1) do megger checks, (2) do the DTF function, (3) measure the capacitance of each probe individually, (4) determine whether one or more probes is out of tolerance based on what it is reading on the other probes in the tank, and (5) "work around" the failure of one or more probes and/or the compensator.
PROBE: The capacitive sensor in the tank. Also referred to as the "tank unit" or "transmitter".
SIMULATE: Refers to the ability of the tester to (1) simulate the empty or full tank capacitance, or (2) add capacitance to the real empty tanks to make them appear to be full. All testers can also simulate "dry" or "wet" compensators, and some testers can simulate an empty tank and a full tank without changing the setting of the tester between tasks (" Aux").
SIGNAL CONDITIONER: Some AC indicating systems, and nearly all DC indicating systems, utilize a signal conditioner to generate the LoZ signal(s), and to convert the resulting HiZ to a DC voltage output to an indicator. In these systems, the indicator is simply a digital voltmeter displaying fuel weight in pounds or kilograms. Signal conditioners are typically found on fighter aircraft and rotary-wing aircraft.
VOLTAGE MEASUREMENT: Some testers are designed to use the LCD to display a DC voltage. The signal conditioner on the Blackhawk helicopter and derivatives of it puts out a DC voltage proportional to fuel quantity. A tester with DCV capability can monitor this voltage for purposes of calibrating the system. Note: the system interface unit or cable must be designed specifically for a tester with this capability.
WET and DRY: Refers to the fact that a compensator or bead thermister is in fuel (wet) or not in fuel (dry).
3-WlRE MEGGER: A circuit for measuring insulation resistance between any two points, say "HiZ" and "Shield" while simultaneously measuring from each of these two points to system ground. It is more accurate than a "2-WlRE" megger, but does not work well if one of the two points is shorted directly to ground or the shield.
2-WlRE MEGGER: A circuit for measuring insulation resistance ONLY between the two points selected, without regard to the resistance between either of them and circuit ground. This method is not as accurate as a 3-WlRE megger, but it is not disabled by either point being shorted directly to ground.
3-WlRE CABLES: Cables designed specifically for a given system on a particular aircraft; they are "aircraft-specific". 3-wire cables are connected either to the MEASURE or to the SIMULATE terminals of the tester. 3-wire cables must there be true "T" cables, with the capability to disconnect the actual tank if required.
6-WlRE CABLES: Also aircraft-specific, but have three wires, HiZ, LoZ, and COMP or COAX, UNSH, and COMP, which connect to the MEASURE terminals of the tester, and another set to connect to the SIMULATE or INDICATOR terminals. Most Navy cables for AC systems currently use 6-wire cables. Most USAF systems use 3-wire designs. Any AC system tester can accept either a 3-wire cable or a 6-wire cable, but the test procedures are necessarily different.
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