The fuel quantity system is a important part of the aircraft.
Because weight is a important factor in aviation we don't just 'fill her up' before flying. The amount of fuel that is tanked onboard an aircraft depends on how far the aircraft is flying and on how much fuel costs on the airfield that the aircraft is flying to.
It may be provitable for an airline to tank up for the return flight also if the fuel cost at the destination airfield is way higher than the fuel on homebase (large airlines buy fuel in bulk so at homebase fuel is usually cheaper).
Then there is the emergency fuel, at any point in the flight the aircraft may be told that the destination airport is closed or not suitable to land for whatever reason. The aircraft must then find another airport to land at and must have sufficient fuel onboard to reach the diversion airport without any problems.
During the flight, the fuel quantity system is used by the flight crew to be able to select the center tanks off, keeping an eye on the fuel throughout the flight and to ballance the fuel between the wing tanks to keep the aircraft's weight ballanced. If for example one wing is heavier, the ailerons will have to counter the imballance to keep the aircraft flying level wich causes more drag on the aircraft and this drag costs more fuel and is therefore more expensive.
The fuel quantity system also feeds into the FMS (Flight Management System) wich displays the total fuel on the CDU's (control display units) by means of the fuel summation unit (wich adds up the fuel in all tanks to get the total fuel value).
On some aircraft the total fuel quantity is also used for calculating the stall warning pitch (stickshaker) and calculating yaw damper corrections to compensate for the fuel weight.
FUEL QUANTITY BASICSHere's an aircraft with three fuel tanks, a L/H wing tank a center tank and a R/H wing tank.
The wing tanks are fueled first and when they are full and more fuel is needed only then will the center tank be filled.
All aircraft use center tank fuel first and the main tanks (wing tanks) to limit the structural load on the wing roots.
If the aircraft were to use the wing tanks first then this is what the forces would be.
To reduce these stresses, aircraft with centertanks try to keep the fuselage weight as low as possible with the wings relatively as heavy as possible.
If the flight crew suspects that the fuel quantity indication in the flightdeck is incorrect the maintenance staff on the ground can manually check the fuel quantity in the tanks by a so called 'dripstick' or a 'dipstick' check.
A dipstick is a stick with a float on top wich maintenance staff can unlock and drop down at the bottom of the tank. A dripstick uses the same principle but has no float, the maintenance staff knows when the fuel is reached as soon as it starts to come out.
The float will come down until it meets the fuel and floats on top of it. the part that sticks out the bottom has a scale wich we can use to read of a quantity code. Then we need to measure the aircraft attitude with a plumb bob or a bubble glass and go to the fuel quantity correction table in the maintenance manual to read the fuel quantity for that specific aircraft attitude.
U can see that when the tank is full, the two most inboard dipsticks don't give a valid reading because the fuel level is so high that the floats are submerged with fuel.
If the fuel level drops lower however, the outboard dipstick's float will drop to the bottom whilst the two inboard will indicate the fuel quantity.
So how does an aircraft measure the fuel?
Throughout the tanks there are capacitance measuring fuel probes, placed in such an order to attempt to create a more or less constant fuel quantity measurement at all aircraft attitudes.
At the bottom of the fuel tank is a compensator and in some aircraft also a densitometer.
The fuel probes and compensator are the main source for fuel qty measurement. The densitometer is a way of measuring the quantity more accurately.
The idea behind this is that when the aircraft fuels up the densitometer measures the fuel quantity accurately and compares that to the measurement from the fuel probes and compensator. The system then corrects the measurement from the fuel probes and compensator with the correction factor.
This correction data from the densitometer is then used to correct for that 'batch' of fuel that is in the tanks. This is why it is only used during refuelling, when the system has the correction data it has all the information for that fuel batch it needs. It then relies on the fuel probes, the compensator and the correction factor from the densitometer (the correction factor is taken from the last received data from the densitometer so this is at the end of the refuelling). This densitometer helps the compensator to get the fuel quantity's accuracy to within 1 percent deviation.
(The 737 present generation do not have densitometers).
The compensator measures the specific gravity of the fuel by capacitance measurement.
A certain volume of fuel expands when the temperature increases but the potential energy contained in the fuel doesn't increase.
To make sure that we know how many kg's of fuel with a certain potential energy is in the fuel tank we need the compensator to calculate how much fuel regardless of the temperature is actually in the fuel tank.
On a hot day, a full fuel tank indicates less fuel than on a cold day, the volume is still the same but the energy contained is less and therefore the quantity in kg's indicated in the flightdeck is less.
In our company we use kilograms but it is also possible to indicate fuel in lbs.
The fuel quantity calculation effectively takes the fuel quantity probes measurement and multiplies this value with the compensator value to reach the fuel quantity in kg's or lbs.
The reason for the compensator to be at the bottom of the tank is to keep compensator submerged in fuel because even if the tank is almost empty we still need to measure the fuel specific gravity for the quantity calculation.
The fuel probes receive a low impedance excitation signal from the fuel quantity processor called a LO-Z.
The fuel probes send back a high impedance signal called HI-Z. The cumulative ammount of HI-Z's from the fuel probes represents the volume of fuel in the tank.
A compensator works similarly, it receives a LO-Z and sends a HI-Z back to the processor (the same HI-Z line as the fuel probes use) but this value only changes for the specific gravity of the fuel (difference in the fuel dielectric).
Here's a schematic of a wing (main) fuel tank
The fuel probes and the compensator have seperate LO-Z wires but a common HI-Z wire wich is the total fuel quantity value for this tank.
The connector on the fuel tank has the complete fuel tank tank units and compensator wire harness connected to it but these wires are all tied together to their respective TU LO-Z, COMP LO-Z and cumulative HI-Z wires to get only three wires that are fed to the fuel quantity processor.
This is done with a so called 'bussing plug'.
FUEL QUANTITY COMPONENTS
This is a 'round type' 737 bussing plug that gets connected to the tank connector.
The bussing plug connects on the tank connector wich has the fuel quantity wire harness connected to it.
The compensator LO-Z being the middle one, the Tank Units LO-Z are the pins around that and the HI-Z are the coaxial contacts on the outside.
They are tied together to get the three wires that go to the fuel quantity processor.
Some 737 aircraft use rectangular type bussing plugs, these are easier to remove than the round ones.
The one on the right is saturated with fuel by a leaking tank receptacle.
If u put a grinder on a bussing plug to see whats inside this is how it looks.
If u take the fuel quantity wire harness out of the tank, this is how that looks.
Note that the fuel probes and compensator are connected with terminals and that the tank connector is sealed up, fuel should not get through the tank connector into the bussing plug because that will create indication problems for the fuel quantity indication.
Before the wires go to the fuel quantity processor, on our company's aircraft they first go to a IFQT (Integrated Fuel Quantity Transmitter).
The IFQT stops unwanted electricity to enter the tanks. The IFQT only allows the fuel quantity signals to pass.
Each tank has it's own IFQT, in our B737 pg they are all installed behind the aft cargo panel in the forward cargo compartment.
In a 737-300 the IFQT's are installed above the mix manifold.
In the longer 737-400, the IFQT's are installed here behind the left recirculation fan.
This is what the IFQT's look like.
The signals from the IFQT's then go to the flightdeck where they connect to a DCTU (Digital Calibration Trim Unit), this unit calculates the fuel quantity by using set calibration data that we can enter by setting the DCTU to 'CAL' (calibrate). More about that later because this calibrating of the DCTU is part of the adjustment test.
INSPECT SHIELDED CONTACT ON DCTUIf the aircraft suffers from fluctuating fuel quantity indications it is not a bad idea to have a look at the coax connector at the DCTU behind the indicator (the HI-Z). This connector has been known to cause problems.
To do this, remove the strain relief from the connector.
Then u have the bare connector.
Take the proper size front release extract tool. (M81969/19-03, DRK56-8A, ATF2252 or ATML1903B).
Push out the coax connector.
And take a good look for damage, this one looks ok.
Sometimes we find part of the shielding broken or entirely loose and intermittently making contact.
After that u can push it back into the connector using a insert tool. (M81969/17-06, DAK55-8A, ATF1260, ATF1256 or ATML1706B).
SYSTEM ADJUSTMENTNow let's have a look at the testset.
As we have seen earlier in this demo, the center tank is allways used up first to keep the upward force from the wings at the wing roots to a minimum.
For this demonstration I will do the adjustment test for the center tank of a 737 pg but it is the same for the main tanks.
There are different types of fuel quantity systems and different types of fuel testsets. For this demonstration I will use the Goodridge PSD60-2R testset on a aircraft configured to our customers fuel quantity system.
First we make sure that the center tank is empty wich is a requirement to measure the probes.
Our defect is in the center tank wich is allready empty but we still need to drain the remaining fuel out of it. This last bit of fuel is sometimes called 'unuseable fuel' because the pumps can't pump this out.
We use the so called 'sump drain' to drain this last bit of fuel out.
The center tank fuel is drained by opening the sump drain hatch and pull on a hook.
On the main tanks a sump drain hose should be connected to get rid of the last bit of fuel in the tank.
When the center tank is completely empty we pull the CB's and connect the testset to the bussing plug and ground the testset to the aircraft.
At this time we only connect the bussing plug and not the IFQT wire harness because we want to measure the empty TU and COMP capacitance values. The adapter cable p/n is PSD60-504
Grounding the testset is very important. not grounding the testset causes large misreadings.
Use the 400 Hertz setting because this is what the aircraft uses.
Once again, notice that at this point we do not connect the IFQT wire harness.
We can now measure the empty tank units capacitance by measuring between the TU LO-Z and the HI-Z by setting the function switch to 'measure ext' and the select switch to 'TU'.
We measure 164.29 pico Farads, for the center tank we must have 164.2 with a tolerance of + or - 2.7 pF so this is well within limits.
Do not forget to note the value and then we measure the compensator by setting the select switch to 'Comp'.
Now we measure 39.72 pF, this value must be 40.2 with a tolerance of + 3.0 or - 1.2 pF so once again, well within limits.
If either of these two values were incorrect the manual refers u to do a test of all individual probes by taking the bussing plug off and measuring the wiring insulation by meggering and the capacitance of the compensator and tank units seperately.
Once again, note the value, we will need these later.
Our values were within limits so we continue with the adjustment test.
The maintenance manual says at this point that the qty indicator and DCTU must be removed from the pilots panel, for some people this can be confusing, it is not intended to disconnect the connector at this point, we need to pull the indicator and DCTU forward so that we can access the CAL/RUN jumper and the PLUS/MINUS switches on the DCTU.
We then connect both the IFQT wire harness and the tank connector to the testset because we want to simulate a full fuel tank by adding a certain full tank capacitance value to the measured empty tank value.
We leave it connected like this but we will not yet enter the full fuel tank values, we will first look at the callibration values in the DCTU.
Because we don't want to insert the full fuel tank value just yet we set the function select switch to 'aircraft only', this will effectively remove the testset out of the aircraft wiring as if it weren't there.
It is also important to swap the coax connectors over to the indicator side, this step is sometimes forgotten to mention in the manuals and this can rob u of precious time.
Now we go to the flightdeck, push the CB's back in and let the system warm up for a minimum of 5 minutes and remove the cover from the DCTU to get access to the CAL jumper and switches.
We change the jumpers position from 'run' to 'cal'.
Then we open and close the circuit breakers again (resetting the system in the CAL mode).
U can see that the DCTU is now in calibration mode.
On the DCTU are 2 pushbuttons, a plus and a minus. Pressing plus increases a value, pressing minus decreases a value and pressing both at the same time jumps the DCTU into the next option.
We press both buttons at the same time to jump to the first DCTU option P01 wich is the Lbs or Kg's select.
Kg's was allready selected so we press both switches again to bring the DCTU to option P02 wich is the full tank value.
The full tank value is given in the maintenance manual and should be 7600 kg's + or - 80 kg's.
The correct value was allready in the DCTU so we leave this at 7600 and press both switches again to bring the DCTU to option P03 wich represents the dry tank units value in pF's.
This value should be the same as we've measured earlier but we've measured 164.2 pF for the dry tank units capacitance so this one is 2.5 pF's different.
Note that the decimal point is not shown on the indicator, the value displayed here is 166.7 pF's.
We change this value to 164.2 pF's. Do not forget to note all these values, we need to write them on the callibration placard later.
Then we go to P04 wich represents the wet tank unit capacitance. We take the value that we've measured and we add 190.5 pF's for the center tank (this 190.5 pF's value comes from the maintenance manual and represents the added tank capacitance for a full tank).
We measured 164.2, we add 190.5 to that so the value should be 354.7 pF's.
We press the two pushbuttons again to go to P05 wich is the dry compensator capacitance. 39.7 pF's, this is exactly what we've measured earlier.
Next is P06 wich is the wet compensator capacitance, once again we take the measured value and add a certain wet value to it.
In this case it's 39.7 + 42.6 = 82.3 pF's.
After this we go to P07 and P08 wich both should read zero.
After P08, the DCTU shows CAL again.
Now that we have set these values in the DCTU we go back to the testset and select the Function switch to 'MEASURE INT', this means that the testset measures a internal capacitance value wich can be added to the capacitance values in the tank (when capacitors are connected parallel the effective capacitance can be added up). This internal capacitance value can be changed by using the course plus or minus switches on the testset or the fine vernier knob.
First we will set the full (submerged) compensator value by setting the function measure switch to 'COMP', the maintenance manual states that the added capacitance for a submerged compensator is 42.6 pF.
We adjust the switches for the course and the vernier knob for the fine setting to reach 42.6 pF.
After that we set the function select switch to 'TU' to change the test unit's internal TU capacitance to the added full fuel value capacitance stated in the maintenance manual. For us this should be 190.5 pF
The values are now set so we select the function select switch to 'SIM TU & COMP', this sets the test unit's internal compensator capacitance parallel with the compensator capacitance in the fuel tank and the test unit's TU capacitance value parallel with the tank probes capacitance in the fuel tank.
This will make the fuel quantity system see a full fuel tank.
We go back to the flightdeck and set the DCTU from CAL back to RUN.
And we should read the full tank value on the fuel quantity indicator wich for the center tank is 7600 kg's + or - 80 kg's.
If these values are out of limite there is a adjustment procedure to calculate a 'scaling factor' that can be added or subtracted from P04 in the DCTU to get the values accurate.
Full difference value (FDV) = Displayed indicator value - Nominal full value
For example, if 7400 was displayd, it would be FDV = 7400 - 7600 = -200
Then the adjustment value of P04 should be the scaling factor (wich is given in the maintenance manual and for the center tank this is 0.02507) times the Full difference value (-200).
So 0.02507 times -200 = -5.014
We know the value at P04 was 3547 but if it would have displayed 7400 kg's here we should change the P04 value to 3547 -5 = 3542.
Now we need to see if the empty tank value is correct so we set the testset's function switch to 'aircraft only' and we read 0 kg's on the indicator.
Like the full tank value, if this value were to be incorrect we could adjust the P03 value.
For this calculation we take the indicated value times the same scaling factor used for the full tank adjustment and add the figure to P03.
After this, continue with the maintenance manual instructions and check if all indications are within limits (don't forget to check the repeater on the refuelling panel).
Thats the adjustment test done. With this testset it is possible to measure the tank components and the wiring.
First we measure the tank components, for this we pull the CB's again and connect the testset to the bussingplug only.
Set the Function switch to 'measure ext', do not forget to move the coax connectors back from 'indicator' to 'tank units'.
Now we set the measure select switch to the different megger selections and make sure that the resistances measured are within the maintenance manual limits for the FQIS (Fuel Quantity Indicating System) resistance check.
Note that the LoZ-HiZ and the COMP-HiZ have to have a much larger resistance value than the other selections on all parts of the wiring from the DCTU to the tank components, at the time I am doing this test the maintenance manual states that these must have a minimum of 250 megaohms.
Or means infinite resistance (outside of the testsets measuring range).
The tank components measurement was all well within limits, next is the IFQT (integrated fuel quantity transmitter) wire harness, for this we need to disconnect the J2 connector from the IFQT. For our 737-300 airplanes these IFQT's are located behind the aft panel in the forward cargo compartment behind the airconditioning manifold.
Now that the IFQT wire harness is isolated between the IFQT and the bussing plug we can now measure this length of wire with the testset.
This wire is also within maintenance manual limits so now we remove the J1 connector at the IFQT (to disconnect the wiring between the DCTU and the IFQT). Do not forget to disconnect the repeater indicator on the refuelling panel also.
We disconnect the connector at the DCTU and connect the testset to the DCTU connector in the flight deck using a DCTU connector adapter (p/n PSD60-503).
We measure this wire and also this is within maintenance manual limits.
Now all the wiring and the tank components have been measured. We reconnect all connectors and we do a operational test to make sure there are no faults in the system.
We also check the repeaters on the fuel control panel for the fuel truck driver to use. They may not differ more than 20 kg's from the indication in the flightdeck.
I put some fuel in the centertank again so I can check the fuel pump pressure switches.
To transfer fuel between tanks first I open the 'defuel valve'
Then I open the applicable fill valve (electrically open, the valve will only open when it is both electrically selected open and fuel pressure is on it).
For fuel pressure I need to select the main tank fuel pumps on in this case. Depending on wich tank u want to fill with wich tank u may want to open the fuel isolation valve.
As soon as I have enough fuel in the center tank I pump the fuel from the center tank to the main tanks and see at what value the 'low pressure' lights come on for the center tank.
If a low pressure light doesn't come on, the pressure switches are located on the forward side of the wing and are easy to replace.
If the fuel quantity system suffers from a soft fault, the fuel quantity indication will be present but a 'err' symbol is in view in the indicator.
On the 737 there are different error codes that can be logged into the memory of the indicator.
When the ground staff performs a operational test, first 'err' is displayd and a '0' at wich the engineer releases the test switch.
If the testswitch is held for too long, a error code '4' can be stored.
The normal accuracy of a 737pg is roughly +/- 2.5 percent error.
With a soft fault the fuel quantity system can still measure the fuel quantity but with a certain degradation of the normal accuracy, the degradation is roughly an extra 3 percent.
When the fuel quantity system suffers from a hard fault, the 'err' symbol will be displayed and the fuel quantity indication will be 0.
Note that this zero is not a fault code, it simply states that the fuel quantity system has a hard fault and can not calculate the fuel quantity.
In order to get the error code the maintenance staff needs to perform the bite test.
Let's look at the error codes.
Error code 1: (soft fault)
Indicates that the compensator LO-Z (the signal to the compensator) suffers from a short circuit or a open loop.
When troubleshooting this error codeit may be wise to megger the wiring and tank components for a short and to measure the continuity of the wiring for a open loop.
Error code 2: (hard fault)
Indicates a short circuit between the HI-Z and the compensator LO-Z.
It may help to use the system tester to megger the tank components and wiring for COMP-HiZ. (minimum 250 megaohms).
Error code 3: (soft fault)
This is the most common one, this indicates that there is a electrical leak in the compensator unit, in most cases this is caused by water in the fuel tank.
In this case a good sump draining is in order to get the moisture out. When presented with this problem I'd say drain plenty of fuel out of the tank, roughly 40 kg's would do the trick. leave the fuel untouched in the tank for at least 45 minutes to let the moisture collect in the bottom of the tank before draining.
Error code 4: (hard fault)
This indicates a short circuit or a open loop in the LO-Z to the fuel tank probes. This error code can also be caused by keeping the test switch engaged for too long and some mod stats fuel quantity systems allways display error code 4.
Error code 5: (hard fault)
This indicates a short circuit between the tank units LO-Z and the HI-Z. Like error code 2 but here the short is between the TU LoZ and the HiZ, the megger value minimum should be 250 megaohms on LoZ-HiZ.
Error code 6: (soft fault)
This indicates that there is a electrical leak in a tank unit, like error code 3 this is usually caused by water in the fuel tank.
Again, a good sump draining is in order to get rid of the water.
Error code 7: (soft fault in flight, hard fault on power up)
This one indicates that the DCTU does not operate correctly. Replacing the DCTU should solve this.
Error code 8: (hard fault)
This indicates a error in the DCTU data. Reprogramming the DCTU should solve this.
Error code 9: (soft fault in flight, hard fault on power up)
This indicates that there is a problem with the indicator memory, replacing the indicator should solve this.
Error code 10: (hard fault)
This indicates a short circuit or a open loop in the HI-Z line. Inspecting the wiring, connections and the shielding of the HI-Z wiring. Note that the shield right behind the DCTU is often damaged.
When the fuel quantity indication is not correct. (The drip stick measured quantity differs from the indicated quantity, the filled fuel value from the tank truck does not conform with the fuel quantity or the fuel overfill switch engages shutting of the fill valve with less than full indication etc. etc.).
A relatively quick and easy thing to do to check the quantity indication is to note the fuel quantity and pump the fuel out of the suspected tank into a tank with a good quantity indication and check to see if the total fuel between these two tanks remains the same.
If for example u have 1000 kg's left and a 1000 kg's right and u tank the 1000 kg's from left into the right tank, u should have 2000 kg's in the right tank. If u end up with 1600 kg's, the fuel quantity system from the left tank has a higher fuel quantity indication than actual fuel in the tank (400 kg's less), this can be caused by a incorrect full capacitance value setting in the callibration of the DCTU, check to see if the values in the DCTU are still the same as on the calibration label.
If the total fuel turns out to be 2600 kg's for example, then there was more fuel in the left tank than what the quantity indication measured (600 kg's more), this can be caused by a faulty fuel probe, a broken tank wire or most commonly a faulty connection in the bussing plug.
Here's a view of a rectangular bussing plug that has suffered exactly this problem, it lost the HI-Z connection to probe 9 resulting in losing one tank probe capacitance and therefore underindicating from the point where the fuel contacts the probe (at above approximately 4000 kg's).
The one on the right has been saturated with fuel by a leaking tank receptacle. U can see the rubber seperator extruding from the contacts.