Your Senders Are Too Accurate

One of the more unusual comments we occasionally hear about CiES fuel senders is that they can seem “too accurate.”

At first, that sounds like a contradiction. How could a fuel quantity system be too accurate?

Usually, what the pilot is actually noticing is not an accuracy problem at all. It is the difference between a system that can resolve small changes in fuel level and one that tends to hide them.

Fuel Does Not Sit Still in Flight

Fuel inside an aircraft tank is a moving liquid.

As the aircraft pitches, rolls, accelerates, decelerates, or encounters turbulence, the surface of the fuel moves with it. Tank geometry can amplify that movement, particularly when fuel levels are lower or when the tank has irregular internal contours.

A float-type fuel sender follows that liquid surface.

Unfortunately, the fuel did not get the memo that pilots prefer steady numbers.

CiES senders still use a float and float arm, but unlike a traditional resistive sender, the position of the float arm is measured using non-contact sensing. There is no resistive wiper dragging across a track to determine position.

That allows the sender to respond consistently to relatively small changes in float position.

So when a pilot notices the indicated fuel quantity moving slightly during flight, the first question should not necessarily be:

“Why is the sender moving around?”

A better question may be:

“What is the fuel actually doing inside the tank?”

In many cases, the sender is simply reporting that movement.

A Stable Gauge Is Not Necessarily an Accurate Gauge

Pilots naturally associate a steady indication with a good indication.

That makes intuitive sense. If the number on the display remains stable, the system appears confident. Stability and accuracy are not the same thing.

A fuel quantity system can appear extremely stable while still being several gallons away from the actual quantity in the tank. Conversely, a highly accurate system may show small short-term variations while remaining much closer to the true fuel quantity.

There are several different characteristics at work:

• Accuracy describes how closely the indicated quantity matches the actual quantity.
• Resolution describes how small a change the system is capable of detecting.
• Repeatability describes how consistently the system reports the same condition.
• Damping or filtering determines how much short-term movement is allowed to influence the displayed indication.

Those characteristics should not be confused with one another.

A gauge that hardly moves may simply have lower resolution, mechanical friction, aggressive damping, or some combination of all three.

Smooth does not automatically mean accurate.

Why Older Fuel Quantity Systems May Look Smoother

Traditional resistive fuel senders generally determine float position through a mechanical contact moving across a resistive element.

That architecture introduces several characteristics that can make the indication appear less active.

Mechanical friction can resist very small float movements. Wear in the linkage or resistive element can affect repeatability. Electrical characteristics of the system may further limit how finely changes are displayed. Aircraft and instrument manufacturers may also incorporate damping to keep the needle or digital indication from reacting excessively to short-term movement.

The result can be a fuel indication that looks calm; some of that calmness may simply be information that has been lost.

A CiES sender approaches the measurement differently. The float remains mechanical and it still follows the fuel surface, but the actual position sensing is non-contact.

That allows small float movements to be measured without relying on a wiper moving across a resistive track. Sometimes that means the pilot sees the fuel moving.

Think of It Like a More Precise Scale

Consider these two digital scales.

Two digital bathroom scales displaying similar values. One has more resolution.

The second scale may appear less stable because the number displayed after the decimal place changes.

But that does not mean it is less accurate; although, I am sure we all hope the bathroom scale will be stuck at a lower number.

It means it is capable of resolving changes the first scale simply cannot show.

Fuel quantity measurement works in much the same way.

If the liquid surface moves and the sender can detect that movement, the measured value may change.

The challenge is not necessarily to eliminate that information, but rather, deciding how that information should be presented to the pilot.

Measurement and Display Are Different Jobs

This distinction is important. The fuel sender’s primary job is to measure the position of the fuel level.

The indicating system’s job is to turn that measurement into useful information for the pilot.

Those functions should be considered separately.

The sender should begin with the most accurate and repeatable measurement practical within its approved design. The complete fuel quantity system can then determine how much filtering, damping, averaging, or annunciation logic is appropriate for the aircraft and display.

That leads to a simple philosophy:

Measure accurately. Display intelligently.

Suppressing information at the measurement source solely to make the indication appear smoother can come at a cost. Once measurement detail has been removed, the rest of the system cannot recover it.

Starting with better data gives the indicating system more options.

It can average readings, apply damping, account for tank geometry, or control how quickly a change is presented to the pilot.

The specific implementation depends on the approved system architecture, but all of those decisions benefit from beginning with a reliable measurement.

Not Every Movement Is Important to the Pilot

None of this means that a fuel quantity display should constantly bounce around the cockpit.

Good human-machine interface design matters.

A pilot does not necessarily need to see every small movement of the liquid surface caused by a bump in turbulence. Some damping is appropriate.

The important distinction is between a sender that cannot detect small changes and a system that detects those changes and then intentionally manages how they are presented.

The second approach preserves measurement capability.

That becomes particularly important as modern avionics provide increasingly precise digital indications. Installing a sophisticated new display does not automatically improve the quality of the measurement feeding it.

The display can only work with the information it receives.

Low Fuel Is Where This Can Become Especially Noticeable

The behavior can become more apparent as the fuel quantity approaches the bottom of the tank.

At low fuel levels, relatively small movements of the liquid surface can result in more noticeable changes in float position. Depending on the geometry of the tank and the location of the sender, turbulence, pitch, roll, or acceleration can move the sensed level above and below a particular quantity repeatedly.

That is usually just visible as movement on the gauge.

But there is another consequence when the aircraft’s avionics use fuel quantity to trigger a warning. Imagine that a low-fuel annunciation is configured to activate at 5 gallons.

As the aircraft moves, the measured quantity might momentarily represent:

4.8 gallons
5.1 gallons
4.9 gallons
5.2 gallons

If the warning logic simply turns the annunciation on below 5 gallons and immediately clears it above 5 gallons, the warning can flash on and off as the fuel surface moves through that threshold. The sender is not creating the movement.

The fuel is moving across the threshold, and the measurement system has enough resolution to see it. That distinction is important.

The Warning Light Is a Different Problem Than the Measurement

When a low-fuel warning repeatedly activates and clears, the natural reaction may be to make the sender less responsive.

But the sender, the displayed quantity, and the warning logic are three separate parts of the system.

The sender measures the fuel level. The indicating system converts that measurement into a quantity. The warning logic determines what happens when that quantity crosses a particular threshold.

On many digital displays, that warning may be tied directly to the indicated quantity. If the measured fuel level is hovering near the programmed threshold, normal movement of the fuel can cause the indication to cross above and below that value.

For example:

Warning ON below 5.0 gallons
Warning OFF above 5.0 gallons

If the sensed quantity moves between 4.9 and 5.1 gallons as the fuel moves in the tank, the warning may activate and clear repeatedly.

The annunciator is not panicking. It is simply doing exactly what it was told to do—albeit possibly a little too enthusiastically.

That behavior can be frustrating to the pilot, but it is important not to confuse it with inaccurate measurement. The sender is reporting changes in float position, while the warning system is reacting to those changes according to the logic built into the display.

The exact warning behavior depends on the aircraft and avionics installation. The broader point is straightforward: a warning that toggles near a threshold does not necessarily mean the underlying fuel quantity measurement is unstable or incorrect.

Why Not Just Make the Sender Much Slower?

This leads to one of the most reasonable questions:

If the sender is seeing fuel movement that the pilot does not necessarily need to see, why not simply slow the sender down until the indication becomes perfectly steady?

There are several reasons, but one of the most important comes directly from certification.

CiES CC-Series and CD-Series fuel senders are certified to FAA TSO-C55a and the associated SAE AS405C fuel and oil quantity instrument standard.

Speed of response is part of the minimum performance requirements associated with this certification basis.

The applicable requirement specifies that, under ambient room conditions, the indication must travel from empty to full, or full to empty, in less than 30 seconds but more than 5 seconds. At environmental extremes, the permitted response time is also tied to the response demonstrated at ambient conditions.

That is worth emphasizing:

A certified fuel quantity system cannot simply be made arbitrarily slow in pursuit of a perfectly steady indication. Response time is itself a performance characteristic.

Certification Requires a Balance

That 5-to-30-second response window illustrates an important part of fuel quantity system design.

The indication should not respond instantaneously to every transient movement of the fuel.

But it also cannot be damped so heavily that a meaningful change in quantity takes an excessive amount of time to be reflected.

Certification therefore creates a balance between two competing objectives:

Stability and responsiveness.

It would be relatively easy to make an indication appear exceptionally calm by applying enough filtering.

But damping is not free.

Every time additional filtering is introduced, the system becomes slower to respond to a real change.

Eventually, a system can become so smooth that it is no longer adequately representing what is actually happening in the tank.

That is precisely why response time matters.

What a 30-Second Maximum Really Means

Consider what would happen if an extremely aggressive filter caused the indication to take several minutes to respond to a major change in sensed fuel level.

The indication might look wonderfully stable.

But it would be displaying old information.

That is not the objective of an aircraft fuel quantity system.

The certification requirement establishes that the indicating system must be capable of responding within a defined period.

For CiES, that means we cannot solve every complaint about visible fuel movement simply by continuing to increase damping at the sender.

At some point, doing so would change a certified performance characteristic.

And that changes the nature of the conversation.

The question is no longer simply:

“How smooth do we want the gauge to look?”

It becomes:

“How do we maintain an accurate, responsive, certified measurement while presenting that information to the pilot in a useful way?”

That is a much better engineering question. We could make the indication look wonderfully calm by slowing everything down enough. We could also make a watch look very stable by removing the second hand.

The Fuel Is Moving Whether the Gauge Shows It or Not

This is perhaps the simplest way to look at the entire issue.

When an aircraft encounters turbulence and the fuel moves inside the tank, two different fuel quantity systems may behave differently.

One indication barely changes. The other moves slightly with the liquid surface.

It is tempting to assume that the first system is behaving better.

But the fuel moved in both airplanes. One system simply told you about it.

The same applies near a warning threshold.

If the liquid level repeatedly moves above and below the threshold, making the measurement less responsive does not stop the fuel from moving.

It only changes how much of that movement reaches the rest of the system.

Modern Avionics Make Better Data More Visible

This behavior is also becoming more noticeable because today’s avionics displays are capable of showing fuel quantity with considerably more resolution than many of the analog gauges pilots grew up with.

That can expose information that was previously hidden by the complete legacy system.

The assumption can then become:

“The old gauge never did this, so the new sender must be causing it.”

But the better question is whether the old system was capable of showing it in the first place.

A mechanical resistive sender, analog gauge, friction in the mechanism, coarse markings, electrical damping, and display damping can collectively remove a great deal of short-term information.

Replace part of that architecture with a high-resolution non-contact measurement source and a modern digital display, and the pilot can suddenly see more of what is happening inside the tank.

That does not automatically make every visible change useful.

It does mean we should be careful about treating visibility as error.

“Too Accurate” Is a Problem We Are Comfortable Having

When pilots tell us that a CiES-equipped aircraft shows small changes in fuel quantity that they did not notice with the original system, we understand why that initially seems unusual.

For decades, pilots have become accustomed to fuel gauges that move slowly, show relatively coarse changes, or remain in one position for extended periods.

Modern sensing changes that expectation.

A CiES sender is designed to provide high-quality position information from the float throughout the operating range of the tank.

When the fuel moves, the float can move.

When the float moves, the sensor can see it.

And when the rest of the indicating system has enough resolution, the pilot may see it too.

At low fuel levels, that same measurement capability can also expose shortcomings in how a warning threshold is filtered or qualified.

The answer is not necessarily to make the measurement less accurate.

And for a certified fuel quantity sender, simply making the response slower and slower is not an unlimited option.

The sender has to operate within an approved performance envelope that includes response time.

That leaves us with what we believe is the right philosophy for the complete system:

Measure accurately. Respond appropriately. Display intelligently. Warn intelligently.

Because when it comes to determining how much usable fuel is actually aboard an aircraft, the starting point should always be the best measurement possible.

A perfectly steady fuel indication can be reassuring. Of course, so can a broken bathroom scale.

CiES — Accuracy Before Empty.