Aircraft Fuel Totalizers - A Self Fuel Filling Prophesy

One of the more difficult things to measure is liquid fuel on a moving vehicle.  

This is true of aircraft, boats and heavy equipment.  

By it's very nature, liquid fuel refuses to stay still.

So to address these issues with fuel level sending technology,  we have turned to fuel flow instruments and fuel totalizers.  

These instruments measure the fuel flow to the engine and remove that fuel volume from an owner/operator entered starting fuel value.  They can give you a  knots per gallon indication. 

This can be done by several methods - we can count injection pulses and duration or we can get a sensor to measure fuel on the way to the engine,  and in injected engines fuel on the way back to the tank.   There are several sensor types that will perform this function

Some of our cars do the same thing - only they reset when we fill the tank or in other words when the fuel level reaches full.

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Many owners will swear by the fuel totalizer and it's uncanny accuracy.

I filled the tank and then burned 35 gallons of fuel.   When I refilled - I put 35 gallons back into the tank.  

Sounds good - and a justification for the expense of having this equipment installed - However an aircraft tank filled in the same manner and in the same location - the fuel totalizer is a self - fuel filling device.

• I will never run out of fuel with this equipment installed.

• I don't need fuel gauges 

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So what does the FAA say about fuel totalizers 

Digital fuel flow computer systems have a fuel flow transducer that directly measures the fuel being fed to the engine.

The fuel flow transducer may be a small paddle wheel, an impeller, or spring-loaded movable vanes. 

Digital displays with a fuel computer also allow these instruments to display total fuel consumed, total fuel remaining, and time remaining at the present fuel
flow rate for fuel management. Overall accuracy for fuel remaining and time
remaining readings depends on the transducer processing unit and display.

The largest possible error is the initial fuel supply, which is entered by the pilot at the start of each flight. Errors in the initial fuel supply may be caused
by an uneven ramp, unusual loading, volume changes of the fuel because of
temperature variations, malfunctions in the fuel system such as leaks,
siphoning actions, collapsed bladders, and other factors.

So, total fuel remaining should be verified with the fuel quantity indicator. According to § 23.1337(b)(1), fuel quantity indicators are required to be calibrated to read "zero" during level flight when the quantity of fuel remaining in the tank is equal to the unusable fuel supply. Therefore, fuel quantity indicators should be used as the primary fuel-remaining instruments.

Fuel quantity indicators that are inaccurate should be periodically calibrated, repaired, or replaced, as necessary, to ensure reliable readings.  

Taken from  FAA AC 23-17C

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The only issue with the FAA message above is that it was written for aircraft engineers, certifying new aircraft or modifications -  

It really should be shared with the pilot and aviation maintenance community.   

Why not?

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Overnight - The world of aviation fuel level measurement changed

Just like the classic instrument six pack.

Seemingly while you weren't looking  -   The days of simple capacitive and resistive fuel senders disappeared and were relegated to history. 

 You need only to look at the parts manuals for the latest Cirrus SR20, SR22 or Cessna 172, 182 & 206 single engine aircraft.     Take a real close look and  you will see terms like AMR and TDR  (Anisotropic Magneto Resistive & Time Domain Reflectometry).  And instead of a voltage or resistance output to be checked in the maintenance manual - you will see terms like CANBUS and Digital Frequency.  

So what changed ...... well quite a lot ...... the digital age hit fuel level.

• First ---- The FAA requires there to be working fuel gauges in aircraft.  

Fuel gauges that are accurate throughout the range.   Yes the FAA really wants accurate gauges in the aircraft not just at "Empty"

• The FAA in their regulations makes that clear -

• The pilot is required by Part 91.205 to have working fuel gauges
• The definition of working is:   Provide accurate fuel level through the  tank range.

• The aircraft manufacturer is required  by Parts 23.963, 23.1305, 23.1337, 23.1555 to provide working fuel gauges.

• These regulations very clearly indicate that the installed fuel gauges must give accurate fuel level through the whole tank range.
• Note:  Some of these requirements were recently revised.

• Second ----- The rate of fuel starvation events remained unchanged.
•  2 to 3 per week in the US 

For a long time,  excuses have been made for fuel level reporting systems in General Aviation aircraft.   

General aviation fuel gauges were (insert your own adjective) bad, pilots didn't trust them and pilots did not rely on the information they provided.   Plain and Simple 

Uncharacteristically,  the FAA turned a blind eye to the fuel level regulations they provided, tacitly acknowledging in their own documents and publications, that the state of the art was not up to the task.  

Pilots sought and effectively utilized "work arounds" - pilots relied on wristwatches only and aircraft manufacturers and avionic shops installed or incorporated fuel totalizer equipment in the aircraft.  

Iphone App showing fuel level 

Contrary to popular belief totalizers are not required equipment. but they will give pilots confidence in their fuel level.   They have proven to be accurate while in flight and provide a reliable cross check of the wristwatch fuel calculations  at any time.   You will find a fuel totalizer or range graph is on virtually every piece of modern avionic equipment.

Fuel leaking from an aircraft fuel pump

The posting on the web (pictured right) of illustrates succinctly the common complacency of having accurate gauges in the cockpit.   The particular pilot shows his "trusted" iPhone App illustrating a half tank - this app standing in stark contrast to the values depicted on the aircraft fuel gauge.

So why did the FAA really want working fuel gauges in the aircraft.  

Nothing but an actual working fuel gauge .... can tell you exactly what you actually have onboard the aircraft.   

Case in point --- Fuel leaks at a pump are one of the hazards a working gauge will detect.

You would believe that somewhere in the digital age and the era of MFD avionics, the simple fuel gauge would be relegated to the trash heap.   

However 

--- No Pilot Calculation -- 

--- No Totalizer ----

--- No Stop Watch ----

--- No Range Map ----

Only a working aircraft fuel gauge will let you know what you have in your tank and modern technology can give you that information .  

Welcome to the digital age in fuel level reporting