The news of Raisbeck and CiES improving the fuel quantity indicating system of the King Air fleet is welcome news indeed. The factory indicating systems have been the bane of many an operator and maintenance shop.
There’s really nothing technically wrong with the current capacitive systems, but they can be sensitive to the environment that we operate our aircraft in and present vulnerabilities.
Capacitive level detection technology has been around since the 1940s or so and has been used in innumerable aircraft types, as well as many non-aviation applications. Where the system becomes vulnerable for us is with the tiny signals in use to prevent any ignition sources for fuel vapors. Terms like picofarads and nanosiemens become part of your vocabulary during diagnostic testing.
I won’t dive too deeply into the theory, but some basics will help you understand the use of the principal piece of test gear for the King Airs, the Barfield DC400/A.
The test set allows for performing two critical measurements used in troubleshooting the aircraft’s system: capacitance and insulation. Let’s delve into those items and how they are pertinent to our indicating system.
Now’s not the time to get into the electrical weeds. The editors draw a line on my column length, so I don’t fill up the magazine! Suffice it to say you can measure capacitance in picofarads or insulation in nanosiemens without a thorough understanding of the theory, much like techs can measure volts or ohms for pass/fail uses without being able to describe the specifics.
Essentially, each of the fuel probes strategically located in the King Air wings is a capacitor. Two tubes, no electrical contact, separated by air or a level of fuel. As the level of fuel increases between the tube walls, the electrical property of capacitance increases proportionally. Capacitance allows a changing electrical signal (like AC) to be transferred from one leg of the capacitor to the other. How strong that returning signal is provides a simple way to think of how the system works and how high the quantity indicator will rise. For my fellow geeks, the excitation signal sent out to the probes is a sawtooth waveform at 5 volts peak, at 16 kHz. The return is at a millivolt level. In a nutshell, more capacitance from more fuel equals more gauge indication.
With that in mind, the values (voltage or capacitance) are miniscule and tiny signals are subject to small system issues resulting in big problems. Anything that degrades these small signals involved can cause low indication problems. I’ll return to digesting these measured levels of capacitance a bit later.
Small signals can “leak” away with any problems that may compromise high-quality insulation. That’s why our test equipment includes the function to test the system insulation, and what you should be checking first during any troubleshooting event. If you consider that there are three wires involved with each probe, and the wires interconnecting them, these signals need to be very effectively isolated from each other, as well as isolated from aircraft ground. To look at the possible combinations, we need to check: Yellow (LO-Z) to ground (GND), Yellow to Green (RTN), Red (SIG) to Ground, Red to Green, Green to Red (more in a bit), and Green to Ground (see Figure 1).
In all cases, the amount of resistance should be very high (> 20 megohms). The test set measures using the inverse of that called conductance. High resistance = low conductance. Get it? The magic number in King Airs is less than 50nS (nanosiemens, our unit of measure). That equates to greater than 20 megohms. The reason for not using a Megger here is that Meggers typically send a high voltage in an attempt to break down insulation. That’s not a great idea for a fuel vapor environment.
The physical insulation itself is less frequently the problem with these tests than corrosion built up in system interconnections.
So why do the insulation test before checking the system capacitance? Because the insulation test can be performed regardless of the fuel state in any of the aircraft’s tanks. If you can identify a wiring issue with insulation testing, you may be able to correct it with no need to defuel or consider calibration whatsoever. You can save a ton of time and effort by testing insulation first and it can be started at the back of the indicators.
That less than 50nS value applies to all the combinations provided by the test set switch, except for the SIG/RTN position, which will return an “out of range” indication of “1.” That’s because the test uses DC for insulation tests and that setting has the plus (+) on SIG and the minus (-) on return. The probes contain
low-voltage crystal diodes and in that setting you’re reading through both. That’s lots of conductance and it should be.
Any readings too high on the other settings become a case of “find the short.” It may not be an actual out-of-range “1,” but the need to start isolating the system to make that conductance disappear is much the same mentality as trying to isolate a shorted wire. There are a few connectors that you can use as isolation initially. One set is behind and slightly forward of the indicator panel and unfortunately below the storm window. This location is always worth a look, since water intrusion is not our friend. These two connectors are circular black plastic and are quite common in King Airs and other Beech products. These connectors are not likely the cause of insulation test failure, but they are easy access and if your problem reading goes away when disconnected, you’ll be sure you need to head out to the wing.
The other two points with connectors for system isolation are the wing break (inside the fairing that runs down the line from wing to nacelle). Access it from the top, but if you have a wing locker it will need to be off. Splitting that wing break will remove or confirm whether the problem is inboard (wing center section) or outboard of that point.
The last point worth trying before getting out your plastic red and white extraction tool is found above the panel near the wing integral fuel cell on King Air 200s and 300s. The panel to remove to gain access is the first panel inboard and in line with the five (flush) wet wing fuel panels on the wing underside. Again, it’s not likely that the insulation problem is out there, but for the want of a few screws it’s certainly worth isolating the two probes that reside out in that wet wing.
Beyond those disconnections, I’m afraid it’s time to start de-pinning individual probes to continue isolating the system looking for that insulation test failure. You’ll be de-pinning M81714 splices and matrix blocks. Be sure to use a plastic red and white extraction tool. These are not the same as the metal version. The plastic tool has a thicker wall to displace the contact locks properly. The white end is your extractor.
The interconnections, particularly the junction (matrix) blocks, are common places to find connection or insulation issues. These are great junctions for many uses in avionics and such, but Beech’s choice to use these out where they get wet wasn’t their finest hour. Look closely at the blue portion of the block. If it appears to be swollen at all, don’t waste your extraction tool or time on it. Go for replacement since the swelling is due to internal corrosion and absorbing moisture. Intermittent indicating issues can often be located by misting water on the matrix blocks and interconnections. Only if there’s a problem inside the junctions should this disturb your readings.
Since we’re on the matrix blocks, many operators have improved the reliability of their systems by changing one matrix block to three 4-way splices instead. Without the black plastic section, the blue rubber splices are more resistant to water. In any case, be sure the unused holes are filled with a red plastic MS27844-20-1 (or 2) sealing plug.
If you have corrected any insulation issues with just wiring repairs, there’s no need to recalibrate the system. Recalibration is only required after replacing probes or an indicator or performing the indicator tests found in the manuals.
During insulation testing it’s important to realize that all the Yellow (LO-Z) wires are tied together electrically, regardless of the main and auxiliary systems. That means a LO-Z/RTN or LO-Z/GND failure can happen anywhere and show up during both main and aux testing. The same is true for Green (RTN) wiring. Only the Red (SIG) wiring stays independent for main/aux systems. On C90-series aircraft, the same is true for the NAC/TOTAL wiring. On older 90/A/B/E90s, check into the serial-specific wiring. Some predate the capacitive system or have unique wiring.
Give Service Bulletin 2037 from 1985/1995 a look. That’s of particular importance when you feel like you’ve disconnected everything while troubleshooting an insulation issue but still have a problem. Many aircraft were made with a conduit from the inboard aux probe that comes into the pressure vessel carrying those aux wires. The inside ends of these were factory sealed to reduce cabin pressure leakage and that creates a trap for water where the wires can reside for years without ever getting the chance to dry out. In this case, the problem truly is saturated wire insulation and they’ll need to be replaced. Bottom line: Before pulling in the new wires, be sure you have a way for the conduit to drain (including drilling a hole).
In this issue, we’ve covered the initial testing you should consider when troubleshooting the fuel quantity indicating system. In the next issue, I’ll delve into the capacitive testing to ensure you have a healthy collection of wing probes and we’ll take a closer look at crew squawks versus how those observations happen. Prevention of future problems will also be covered.
In another future issue, I have detailed tips in mind for dealing with connectors, pins, sockets, extractors and crimping. Those subjects have been hoarded too long by our avionics brethren but can easily be accomplished by any A&P with a mind to learn.
As always, reach out with any questions or suggestions for future columns.
Keep them flying!