When Fuel Quantity Goes Up in Flight

What Accurate Fuel Measurement Reveals in the Early Beechcraft Bonanza

Fuel quantity normally has a very predictable relationship with time: start the engine, fly the airplane, and the indicated quantity goes down.

In some early Beechcraft Bonanzas, however, that assumption is not always correct.

The fuel system used in early Bonanza models equipped with the pressure-type carburetor incorporates a return-fuel circuit. Beechcraft documentation for the A35 and B35 states that the carburetor returns approximately three gallons per hour of excess fuel to the left main fuel cell, regardless of which fuel cell is selected.

That creates an interesting operating condition. When the engine is supplied from another tank, fuel is simultaneously removed from the selected tank and returned to the left main.

In other words, the fuel system can actually move measurable fuel from one tank to another while the airplane is in flight.

And with sufficiently accurate fuel quantity measurement, the pilot can watch it happen.

A Fuel System That Does Not Behave Symmetrically

Consider the airplane operating from an auxiliary tank.

Fuel leaves the auxiliary tank and travels through the fuel system toward the engine. The engine consumes only part of that flow. The pressure carburetor returns the excess portion to the left main tank.

The Instrument Can See Fuel Coming Back

The accompanying fuel display provides a real example.

In the first image, the system displays individual quantities for the main and auxiliary tanks. Rather than presenting the pilot with little more than an approximate position of a needle, the instrumentation reports actual measured quantities.

The second image adds another dimension: history.

The trend display records the movement of the fuel quantity over time. As fuel is drawn from the auxiliary supply, its trace moves downward. At the same time, the trace associated with the receiving tank can respond to fuel being returned to the left main.

The important point is not that the fuel indication happens to move upward.

The important point is that the measurement system is sufficiently repeatable to resolve the behavior of the aircraft fuel system itself.

That is a very different standard from simply having a gauge that reads approximately full after fueling and approximately empty near the end of the tank.

Why Sender Accuracy Matters Here

A conventional resistive fuel sender does not measure gallons directly. Like other float-operated systems, it determines fuel level mechanically and allows the aircraft instrument to translate that position into an indicated quantity.

The difficulty with a traditional resistive sender is not the use of a float. The float is an effective way of establishing liquid level. The weakness is the electrical method historically used to determine the float position.

Traditional aircraft senders typically use a mechanical wiper moving across a resistive element. Wear, contact resistance, contamination and mechanical characteristics within that assembly can affect the stability and repeatability of the electrical signal.

Those errors become particularly noticeable when trying to observe relatively small changes in quantity.

If several gallons of fuel gradually return to the left main tank, an imprecise or inconsistent indication may hide much of that change. The pilot knows from the aircraft documentation that return fuel exists, but the fuel gauge may provide little useful confirmation of what is actually occurring.

With accurate, repeatable measurement, the situation changes.

The pilot can see:

• the source tank decreasing;
• the receiving tank responding to returned fuel;
• the rate at which tank quantity is changing;
• and, with compatible instrumentation, the quantity history as a recognizable trend.

The gauge becomes more than an annunciation of FULL, SOMEWHERE IN THE MIDDLE, or EMPTY. It becomes a useful measurement of what the fuel system is actually doing.

CiES Takes a Different Approach

CiES Incorporated fuel senders retain the practical advantages of float-based liquid-level measurement but eliminate the traditional resistive wiper as the position-sensing element.

Instead, CiES uses magnetic-field sensing to determine float position and produce an electronic signal for compatible fuel quantity instrumentation. CiES describes its system as providing accurate and repeatable fuel-level data across the usable measurement range.

That distinction is important.

CiES achieves high-quality electronic fuel measurement while retaining a float-based installation appropriate to aircraft originally designed around float senders.

For early Bonanzas, that means the original fuel-system behavior does not have to change. What changes is the pilot’s ability to see it accurately.

CiES Incorporated supports sender configurations specifically associated with early Bonanza installations; internal application data, for example, includes configurations for the 35 through E35 airplanes with 20-gallon main and center tanks, as well as later 25-gallon-main and 40-gallon-main configurations.

Accuracy Reveals More Than Quantity

There is a broader lesson in this example.

Fuel-quantity accuracy is often discussed primarily in terms of knowing how much fuel remains near the end of a flight. That is certainly important, but it is only part of the benefit.

Accurate measurement also reveals system behavior.

In this Bonanza, the rising left-main indication is not an instrumentation anomaly. It is evidence that the sender and display are observing a real fluid transfer taking place inside the aircraft.

That gives the pilot information that can be compared against expected operation.

If the auxiliary tank is selected, its quantity should decline. If return fuel is being routed to the left main as designed, the left-main indication should respond accordingly. Over time, a trend-capable instrument can make that relationship remarkably clear.

The fuel quantity system is therefore doing something more valuable than displaying a number.

It is allowing the pilot to observe the operation of the aircraft fuel system.

An Old Airplane, Seen with Modern Resolution

Early Bonanzas were sophisticated airplanes when introduced, and their fuel systems contain details that remain operationally relevant decades later.

The return-fuel system is one of them.

Beechcraft designed the airplane knowing that excess fuel would return to the left main tank. The operating instructions account for that behavior by requiring sufficient space in the left main to accept the returned fuel.

What modern fuel quantity instrumentation adds is visibility.

With accurate CiES fuel senders connected to compatible instrumentation, a pilot does not have to regard fuel transfer as an abstract description in the POH. Changes of only a few gallons can become measurable, repeatable events.

In the trend data shown here, the aircraft is effectively drawing its own fuel-system schematic in real time.

The auxiliary tank decreases as it supplies the engine. The return system sends excess fuel back toward the left main. The main-tank indication responds.

Nothing about the underlying Bonanza fuel system has changed.

We are simply measuring it well enough to see what it has been doing all along.