It is not unusual to have probe readings that are slightly (+/- 5pf) out of M.M.

This connection must be made properly (screw, washer, terminal, and then probe) and it must be secure.

A low or nonexistent probe capacitance reading is normally caused by a bad connection of one probe terminal.

Fuel can migrate into an older harness and cause shorts in the wires.

Meg checks usually verify problems with the tank harness.

Only a bad probe reading or failed meg check from the spar plug would be cause for accessing the tank. I will exhaust ALL avenues before opening up a fuel tank for troubleshooting.

Fuel quantity indication problems can be located anywhere in the system.

The unit will output the dialed in values and the screen will show dashes or eights.

After the values have been set, the select switch is placed in SIM TU & COMP.

The display should be monitored when setting both tank and comp. The thumbwheel indication does not match actual values.

Tank unit and comp values are dialed in with the course and fine settings of the tester.

Simulation values are set with the mode select switch in MEASURE INT.

LoZ, HiZ, and Comp require correct connections.

All simulations use the INDICATOR co-ax plugs.

The simulation values need to be set up first and then the box switched to simulate mode.

Tank simulation is a two step process.

For calibration purposes, the simulated value is used along with the tank to adjust the “full” value of stand alone indicators. It is also used for calibration of some types of fuel systems.

Simulation of tank values is used for troubleshooting of indication systems.

Minimum values for meg check readings will also be found in the M.M.

Different combinations of shorts can be detected. A major function of the fuel quantity unit is measuring insulation resistance or a “Meg Check”.Total tank and probe capacitance values can be found in the aircraft's Maintenance Manual.The Comp probe is located at the lowest point, if it is still sitting in fuel, the capacitance reading will be high.

All tank and probe readings need to be made with the tank completely empty.Tank or probe capacitance readings will not be correct if the ground leads from the fuel quantity tester to the breakout box and from the breakout box to aircraft structure are not connected.This reduces the chances of incorrect setup (if the LoZ and Comp are swapped, you’ll know it). The fuel quantity unit’s TANK UNIT co-ax (BNC) connections are differentiated by gender.Using the proper breakout box is still the preferred method. Tank or probe readings can be done with meter test leads, but the hookup requires having the correct spar pin-outs.If the breakout box is connected at the spar plug, each individual probe can be measured along with total tank readings.If the test setup is connected at the indicator, only total tank and compensator readings can be made.The co-ax connections are HiZ, LoZ, and Comp (numerous breakouts have only HiZ and LoZ, the Comp is included with the rotary switch selections). Breakout box connection configurations will differ for aircraft type, but all will have the same outputs for the fuel quantity tester. Each aircraft type usually has a breakout box with all the matching plugs to connect to either the spar plug or indicator.Tank capacitance checks are run with the mode selection in MEASURE EXT.The tester shown here is JcAir® “PSD60-2R.

All will have similar switches and functions. The “comp” probe has a unique function, but its output is still read in pf.įuel quantity test boxes are made by numerous manufactures.

Tanks have one probe that measures fuel density.

Capacitance in a parallel circuit is additive.

Fuel capacitance is read in picofarads or pf.

The output of the probes is referred to as HiZ.

The input signal that feeds all the probes is called LoZ.

The higher the fuel, the higher the capacitance. The capacitance value of the probe will change with fuel level changes. These probes are arranged in a parallel circuit. Most common configurations have 6-12 probes. The number of probes used can vary depending on tank size and system design.

Most aircraft fuel tanks have numerous fuel quantity probes installed.

The tester measures tank and probe capacitance, accomplishes resistance checks, and provides simulation for tank quantity indication. Troubleshooting and calibration of these systems is accomplished with the Fuel Quantity Test Unit. Aircraft fuel quantity indications are capacitance based systems.

Fuel quantity indicating system