Digital Fuel Sensors: Installation-Critical

CiES digital fuel sender retrofits yield highly accurate fuel quantity measurements. But shortcuts in the installation will shortchange performance.

Venture on a primary engine instrumentation system retrofit (which often includes a fuel quantity display) and you’ll have some critical decisions to make. Primarily, will you use the existing OEM analog fuel tank sensors? This is where a lot of otherwise good installations go off the rails.

That’s because in an effort to save money, it’s tempting to use the existing sensors as they sit or have them repaired or overhauled in hopes of tweaking out some accuracy and reliability. As many discover, this isn’t a guarantee that the new digital fuel quantity display will read any more accurately than the mechanical quantity gauge.

For real accuracy, shops have been using CiES digital fuel senders, which have proven accuracy and reliability over most analog OEM senders. CiES digital senders are used by plenty of OEMs, including Cirrus, Piper and Textron. Even Garmin suggests using them when installing the EIS (Engine Indication System), which has a fuel quantity function to replace the OEM gauges. 

But with so many potential traps, we thought it would be a good idea to talk with CiES founder Scott Philiben, who offered sound, take-it-to-the-shop advice for getting the installation right the first time.

PLENTY OF BENEFITS

We’ve covered the topic before, but a brief review is in order. At this point we can easily recommend switching from old analog fuel senders to new digital ones from Oregon-based CiES Inc., which are compatible with a wide variety of third-party systems including Garmin, Dynon, JPI, Electronics International and others. It’s all about accuracy (although it’s not true that all OEM fuel gauges are inaccurate) and reliability—not the case with many aging systems when the wiring and components are disturbed for one reason or another. 

Consider the shortcomings of the average mechanical float-style sender, which in many applications was born from typical automotive senders. You might have grown accustomed to questionable performance, and there’s truth to the old saw that aircraft fuel gauges are accurate when the tank is out of fuel. That’s actually true, as many fuel gauges are calibrated for the empty and full position. Automotive resistance-type ceramic sensors that are reconfigured for use in aircraft often fall short in performance due to the large quantity of fuel that’s stored in an aircraft fuel tank. You’ve probably witnessed a signature trait of a fluctuation in displayed fuel quantity as the sender’s resistance wiper moves. Plus, floats and senders live hard lives because unlike automotive applications, the same fuel often remains in the tank for long periods of time, allowing impurities to build up on the measuring resistors. Over time, this can have a negative effect on the resistors’ value and overall accuracy. 

To contrast, the CiES digital sensor is magnetoresistive or specifically, AMR (for anisotropic magnetoresistive). By now this magnetic fuel sensor is proven to work we’ll in a variety of temperatures and conditions that aircraft fuel tanks are exposed to, including the extreme sloshing of fuel within the tank (especially in turbulence). It’s also compatible with alternative fuels. The design of the sensor has a safety advantage, too, since there are no wires or traces of resistance inside of the fuel tank. Instead, the CiES sensor utilizes a magnetic pickoff located on the float arm, which is detected by the non-contact sensor on the outside of the tank. 

Last, the value of a digital output really shines because the sender converts the changes in fuel quantity (which the internal sensor sees as a change in voltage or resistance) to a digital signal. Computed fuel level is transmitted through a simple digital databus, cleanly transferring multiple pieces of fuel level data to the display. If there’s a rub, it might be a complicated installation, particularly in complex aircraft with multiple fuel tanks. 

DOWN THE RABBIT HOLE

That’s a new CiES digital sender at the top, and a stock analog fuel sender it replaces deep in the tanks of an earlier-gen Cirrus, middle. The bottom image shows lasers being used to align the installation of the float sensor inside the main tip tank of a Cessna 340 twin.

For less experienced shops installing digital engine and fuel displays, there’s generally a tendency to keep the old fuel senders in place and simply try them when installing a new fuel quantity system. We say go all in and replace the senders, too. The trouble is the old fuel senders might not even allow the new instrument’s software to pass the calibration portion of the setup—which is a critical step in the installation. 

For the avionics shop dealing with a new fuel quantity interface, there’s a learning curve with almost every installation. Given the wide variety of aircraft that come into the hangar, there’s a wide variety of fuel sensors, too. Many of the earlier legacy systems used variable resistance to produce a value of measurement, with components scattered about the airframe. This means sizable disassembly to access the components and working in tight spaces. It isn’t pleasant work.

Step one is picking the right shop. That means one that is experienced with the aircraft’s stock fuel system, and one that knows how to calibrate the new system once it’s installed. Face it, the shop needs to understand that customers expect highly accurate fuel measurements after spending thousands on new digital senders. Almost every installation could bring a learning curve because as Scott Philiben at CiES pointed out, there could be differences in fuel systems among similar aircraft models that vary by year or even by serial number. The fact that many of these aircraft are 40-plus years old doesn’t make it easier.

“I tell my customer support people that every aircraft is unique—even ones separated by one serial number, in some cases,” Philiben told us. We’ve dealt with it ourselves on the shop level. Mods, mechanics and repairs (standard and nonstandard) tend to leave a unique mark on an aircraft, and in many cases it deviates from the service manual without any documentation to support it. There are also variations in the routing of wiring bundles—or even the presence of old dead-ended wiring that runs out to the fuel tanks. 

The other challenge is that all of these aircraft—especially complex twins—have a myriad of different configurations. That’s because fuel quantity was an active option list. From wing locker fuel tanks, aux tanks and even extended aux tanks, there could be sizable differences among seemingly the same model. Using the heavier Cessna twins as an example, Philiben pointed out that there could be close to 18 different configurations.

ALWAYS LEARNING

With over 120,000 digital fuel senders (and counting) in the field, Philiben said there’s an ongoing effort to revise the installation drawings as CiES get more information from the field. Worth mentioning is that for certified aircraft, CiES fuel senders are installed via an AML-STC using an approved STC installation manual. Still, installers oftentimes need to think outside the box because of differences.   

There isn’t an official dealer network for CiES fuel components and we wish there was because anyone can buy these fuel senders and install them. However, Philiben said he tries to maintain close working relationships with active Garmin dealerships. “There are shops highly specialized in updating a wide variety of aircraft with Garmin equipment, and because Garmin recommends CiES senders with every one of its EIS retrofits, there’s a good chance the install will be a success the first time,” Philiben pointed out. 

One rookie mistake is installing a new set of senders and not checking the wiring before adding fuel to the tanks to commence the calibration process. The last thing an installer wants to deal with is having to drain the tanks (again) to fix a problem that could have been remedied earlier. And the wiring and components are easy enough to check with a decent digital multimeter. CiES has built in some smart troubleshooting by nature of the digital circuitry. For example, if an outboard fuel sender were to fail in a two-sender system, there would be an attention-getting radical change in output (pegging the needle in order to provide an obvious clue that something isn’t right).

   Like any good avionics manufacture, CiES is focused on providing field training for installers. Maybe it’s a virtual course or live training at trade gatherings like Aircraft Electronics Association conventions, but we think there needs to be more of it for better outcomes. 

As for installation effort, basic installations might require five hours of shop labor, per sender. Complex installs will take substantially longer.

OEM – QUALITY ADVANTAGE

You don’t need an engine/fuel monitor to display CiES digital sender data. That’s an Aerospace Logics FL202G smart digital fuel gauge that replaces a stock Cirrus fuel gauge in the top photo. Fuel range rings like the ones on the Garmin GTN 750, bottom photo, rely on fuel flow—not fuel quantity—for primary computations. See the sidebar on page 7 for more.

Because CiES has a such a large OEM business (it ships out over 1500 fuel senders to aircraft OEMs per month), Philiben says the company goes to great length to make sure every unit that’s shipped out is proven functional per the specs.  

“While we aren’t perfect, we’ll know almost immediately if there’s an issue with a batch because there are so many OEM senders getting installed,” Philiben told us. Worth mentioning is that a version of the digital senders is used in Cirrus (and other) TKS deicing systems. He also points out that it’s becoming relatively easy to help shops diagnose problems with all the OEM units in service, especially with the quality control that’s required before any senders go out the door. “When a shop tech calls and says he has four faulty CiES senders, I know there’s a pretty low probability that the senders are the problem in the retrofit installation,” he pointed out. Shops we talked with that do a lot of CiES work tell us that the quality of the hardware is indeed excellent. So is compatibility, especially with Garmin products. There’s a reason for that. 

Interestingly, one of the first CiES interfaces was on a Garmin OEM integrated avionics platform because there was a need to bring fuel quantity data in as a digital input simply because all of the analog inputs were used up. The fact that CiES could interface to a digital interface in the Garmin software world early on set the stage for more seamless and efficient interfacing on the Garmin retrofit level. Plus, the benefits of a robust communication chain were proven early on because the sender’s 5-volt square wave variable frequency is quite immune to electrical noise and interference. It’s also easy to troubleshoot the digital circuitry without advanced test equipment. 

And because the CiES senders are microprocessor equipped, the devices can also output an analog signal, making them compatible with more displays, including Dynon and others. But don’t try to interface these digital senders with typical vintage mechanical fuel gauges (later-model non-G1000 Cessna models are among a few exceptions); Philiben suggests avoiding it.

“Technically, with the analog outputs we can connect with some mechanical gauges, but the difficulty involved makes it a fool’s errand,” he told us. We say try the Aerospace Logics digital fuel control head, instead. It has smart features including fuel unbalance warnings, low-fuel alerts and a bright high-res display.  

Fuel Metering: Belt And Suspender Backup

Long before digital fuel tank sensors, the hot retrofit was fuel totalizers—aka fuel computers. Whether displayed on a dedicated fuel computer, an engine monitor or more recently on a GPS navigator like a Garmin GTN or Avidyne IFD (using a remote fuel computer), there is still huge value in measuring just how much or how little fuel is being burned. 

The backbone of a fuel totalizer is the electromechanical fuel flow transducer. It converts the energy of the spinning turbine wheel to electronic signal pulses, which are passed along to a microprocessor and then displayed on the control head or GPS/PFD. A misconception is that fuel totalizers measure fuel quantity. While that may ultimately be the end result (you first have to enter the amount of fuel that is in the tanks), a fuel totalizer meters the amount of fuel that flows to the engine. Think in terms of fuel endurance (based on flow) rather than fuel quantity. Fuel metering is done with a fuel flow transducer installed in-line and prior to the fuel distributor or carburetor. The fuel passes through a small axial turbine wheel and past an optical sensor located inside the transducer body, which spins faster at higher fuel flows and slower at lower ones. A higher capacity transducer may be required for larger engines, including turbines and radials. But these transducers are generally provided with the system at the time of installation and are matched for the application. 

We’ve covered fuel totalizers over the years in Aviation Consumer (and installed our share of them) and generally favor models from Shadin, including the AIS-380 RemoteFlo. This is essentially a remote box connected to a fuel transducer, with serial data and ARINC 429 signal outputs for connecting to a GPS or compatible flight display. Models from Electronics International and JP Instruments (that’s a classic JPI FS-450 pictured here) have proven reliability and good compatibility, with liberal third-party interface potential. Last, if you’re planning a budget engine monitor install, ask the shop if it will have fuel flow capabilities. It’s generally an option.  Larry Anglisano

 

FRESH FROM THE HANGAR

As luck would have it, when we were in the middle of the research for this article, Frank Bowlin (Editor of sister publication IFR Magazine) had just retrieved his Cessna 340 twin from the shop after a major avionics upgrade, which included CiES fuel senders for display on a Garmin EIS. To close, Bowlin weighs in on the experience:

“Let me skip to the end: Like some upgrades, the process and cost were daunting, but the final result is quite an improvement from which I will benefit long after I feel the sting of the difficulty and cost of getting there. 

“That said, the process of getting there wasn’t for the faint of heart. The instructions to install the CiES senders inside the tip tanks on my Cessna 340 were insufficient, which appears to have been an oversight. I understand that better instructions are now available thanks to CiES keeping close tabs on retrofits and modifying manuals accordingly, so my advice to any shop contemplating the install is to first check with CiES to find out if the latest instructions or even hints and tips are available. Next, during installation, if you find that your installation is requiring some custom fitting or engineering, check first with CiES as they might have some unpublished assistance to offer. 

“Last, don’t be reluctant to check with CiES for recommendations on additional hardware, electrical connectors, sealant or sealant remover. They’ve been there and done that and have probably already solved most any problems you might encounter. Bottom line: Yes, it was difficult and quite expensive to get there, but the end result is extremely accurate and confidence-inspiring. It’s a real safety enhancement to know exactly how much fuel is in the airplane.”

Visit www.ciescorp.com. 

Larry Anglisano
Editor in Chief Larry Anglisano has been a staple at Aviation Consumer since 1995. An active land, sea and glider pilot, Larry has over 30 years’ experience as an avionics repairman and flight test pilot. He’s the editorial director overseeing sister publications Aviation Safety magazine, IFR magazine and is a regular contributor to KITPLANES magazine with his Avionics Bootcamp column.