In the April issue we started our dive into the fuel quantity indicating system by focusing on insulation testing the system. That should always be your first step because if you can locate insulation or wiring issues (even if you need to replace wires or splices), there’s no need to be concerned with system calibration until you need to replace probes or an indicator. To continue, Part 2 will cover capacitance measuring, indicator testing and problem prevention.
Capacitance measuring
The capacitance of a probe or the system will vary based on the level of fuel versus air at any given system probe(s). When capacitance increases, the indication increases proportionally. The system probes are all wired together in parallel, creating a system total that is what’s measured by our system gauges. The total capacitance of a King Air wing is unique due to the manufacturing min/max values of individual probes. Capacitance of probes, both individually and in total, is measured in a unit called picofarads. That’s the second function of our Barfield DC-400A test set, essentially a capacitance meter.
Losing any of the probes on a system due to probe failure or lost connections will always result in a lower-than-normal system reading. Often, the loss of even a single probe in a system of five or six can result in an indication of less than zero, particularly when the tanks are less than full. Low readings after the crew has loaded the airplane with fuel are the most frequently reported indicating squawks on King Airs.
After the insulation tests have proven our wiring is in good shape, the next step will be testing the total capacitance of the discrepant wing. The min/max expectations are published in the appropriate aircraft maintenance manual (AMM) or the King Air test supplement provided by Barfield.
The first thing we need to do is decide whether to defuel the aircraft or top off the fuel to full. If you don’t have defuel facilities then the full option may be a place to start troubleshooting, but keep in mind that even if you identify a bad probe or gauge, you’ll be stuck deferring the repair until the system can be drained for parts replacement and calibrations. That’s worth considering if you’re the pilot or owner reading this: Ensure the facility has defuel capability and a Barfield DC-400A and bring it to maintenance as light on fuel as safely possible.
Let’s try an example using the numbers for a B300. You’ve defueled the left main system that was the target from your pilot reporting. There’s no need to have the gauge attached, since the Barfield’s batteries will test the main system with no need for aircraft power. So, after following the switchology described in the manual, you get a reading of 173pF. You consult the chart (see Figure 1) for empty mains only to find that the value is comfortably between the min and max values. The listed nominal value is meaningless at this point, so we still have some troubleshooting to do since capacitance didn’t point to a problem.
However, if the reading is lower than minimum, then I like to call this “Find the missing picofarads.” The nominal value may help save some steps when heading out to find the faulty probe or connection. Individual probe values will be the key. There’s a partial chart in Figure 2. If your value is more than 50pF below nominal, you should head directly to the nacelle tank cover. If the value missing is less than 20pF, then the inboard aft is most likely.
Keep in mind the uniqueness of a wing. Your wing under test won’t really be “nominal,” so there’s a limited value to this measurement observation unless you or someone else took the time to capture the empty values when the system was last repaired or drained when working properly. The AMM has always made that recommendation, but no one is going to search maintenance records to see if that recording happened. I’m fond of a sticker located behind the fuel gauges. With an accurate start point for an empty wing, you can go straight to the problem probe by employing a little subtraction.
With the probe harness provided with the King Air adapter for the DC-400A, testing individual probes either in the aircraft (just isolate the probe) or on the bench is a cinch. That applies to insulation tests as well, if the probe itself seems to be your problem.
Indicator testing
If all’s well in the wing system, it’s time to address the performance of the gauge. This will be the first time we’ll need to connect the Barfield set to the gauge, and we’ll need either aircraft power or a 28VDC source if on the bench. Note that as soon as we start adjusting during a gauge test, we will have disrupted any calibration that may have existed. Recalibration will be required. Gauge swapping for problem evaluation is very rarely of use, since just the uniqueness of the wing system could show a difference in excess of 200 pounds of fuel.
During this procedure, the third function of our test set comes into play. The Barfield has what they like to call a capacitance simulator or CAP SIM (see Figure 3). By using that to dial up a specific capacitance, we can test the gauge for response and linearity, as well as to be sure that the range of control of the ZERO adjustments will accommodate any King Air that the gauge is installed into. After properly adjusting empty and full per AMM instructions, adding 21pF successively to the gauge results in an additional 300-pound indication on the gauge. Note that although you’ll feel a bit of resistance in the 0-10 knob on the CAP SIM, you can keep turning it to get the value you need. That’s not a hard stop that you’ll feel.
I have one caution for anyone using CAP SIM CAL while testing a gauge or calibrating the system. Twice, I have been performing or observing these operations and the instructions were being followed perfectly, but during the selections made on the function knob when an indicator was attached, we watched the indicator slam to the fully deflected position, and then it never worked properly again. That was from CAP SIM CAL to IND AMP (see Figure 4). Something in the test set switching sent a strong signal out that damaged the indicator under test. The first time, we sent out the Barfield as defective. The second time (another location, a different DC-400A) I realized there was an inherent danger in the switching. I strongly recommend that before rotating the function selector during any testing with the indicator attached to the test set, the power switch should be placed in the OFF position.
Recalibrating the system after troubleshooting and/or parts replacement will include zeroing the gauge with empty tanks and then simulating fuel by adding capacitance and adjusting the full control to indicate a specified amount.
For decades, the only source of the troubleshooting procedures for King Air fuel quantity indicating was to be found in the Barfield instruction supplement that comes with the adapter that allows for the DC-400A test set’s use on King Airs. In more recent years, Textron Aviation has incorporated troubleshooting procedures and test set instructions into the AMM for the 200s and B300s. The straight 300s only refer to a rather ancient test set, but you can certainly use the DC-400A instead. Unfortunately, the AMMs for 90 models have no instructions to speak of and refer you to the Barfield instructions. If your test set still has an old paper version of the manual, be sure to download an up-to-date PDF version. The manual was updated several times and is much more understandable than the original versions.
Prevention
Finally, much of what we’ve been discussing is not inevitable. A lot of the problems described are preventable with 90% or more of the failures tracing back to water intrusion. I’ve opened many a wing panel to find a veritable pond that I’ve always said could float a rubber ducky. So, let’s talk about prevention.
I understand the shop environment where management or customers don’t necessarily understand why it’s taking so long to close panels after troubleshooting or inspection. By explaining how attention to detail upfront could save big on parts and labor to fix this system later, maybe customers would not only agree to the time but insist that the extra time is allotted. Prevention is always less expensive than waiting for a system failure.
Specifically, I’m talking about creating and maintaining Fay seals on the panels to keep the water out. Fay sealing wasn’t always in Chapter 20 Standard Practices, but it certainly is now. Properly performed, the wing bays under the panels can be kept completely dry, and the probes and junctions could last indefinitely. But, when those panels are reopened during later inspections, a good look for condition, and taking the time to repair those seals as necessary is critical.
Another trick that I’ve seen is the fashioning of a rain bonnet for the back of the fuel gauge panel and over the connectors for the panel. Nothing fancy needed, just water deflection for when that dang storm window leaks. Be sure to select sheeting material suitable for aircraft use. Kapton film meets mil-spec and is flame-retardant.
Lastly, and I mentioned it in Part 1, is to substitute 4-way M81714/12-20D-1 (MWS20E-2) splices for the black and blue matrix blocks. You’ll need three of those for each of the matrices you intend to replace, and you should fill the extra hole with a red plug (MS27488-20-2). These have proven to be a lot more resistant to water issues and are in use in the system inside the fuselage.
Anyone who becomes proficient at troubleshooting these systems is an asset to their organization. Taking the time to really understand what you’re testing and looking for is always worthwhile. I hope these tips have helped you with your fuel quantity diagnostic missions.
As always, reach out if I can help, and keep them flying!