Aviation Fuel Level

Traditional Aircraft Fuel Sender 

Why don't my fuel gauges work?

Let us cover a few basics  

• Aviation industry has utilized level senders from other applications
• Aviation is not the largest market or the most financially rewarding for a dedicated fuel level sensing solution.

• Aircraft fuel tanks are larger than those for most other applications. 
• An aircraft carries more fuel.

• An aircraft has three degrees of motion freedom 
• The liquid fuel is free to move about.

There are other factors.

• Why not design a new system 
• Fuel Level Intellectual Property is hotly contested territory (a patent minefield) 
• A new aviation design must avoid other patents 
• The market share is small
• The perceived liability - Top 5 aircraft incident issue

• Fuel Level in Aircraft needs to be "Intrisically Safe".
• Explosion Proof 
• Limited Spark or Heat Energy in the fuel tank. 
• Non - Contact "The best solution" for Intrinsic safety is the hottest IP in Fuel Level
• Required for multi fuel and hybrid vehicles
• Required for vehicles with long term fuel storage - i.e. Boats
• Required in LNG / LPG

------------------------------------------------------------------------------------------------------------Fuel sending works in my car.

• Automotive fuel tanks are smaller
• What would be adequate for a resolving 16 gallons of fuel has difficulty with 3 times the volume.   
• If you have a boat - this is also an issue as you are well aware
• The variable resistor technology used in automotive applications doesn't scale well
• The variable resistor cards used,  allow 60 degrees of travel  - a good resistive card for aircraft or boats would require minimum. 180 degrees or more.

• Relatively stable fluid motion 
• The automotive sensor is exposed to some fluid motion but overall less. 
• Aircraft fuel can remain quite stable - however turbulence will creates concentrated fluid motion.
• This motion wears the resistive trace - so inadequate resolution of the variable resistor is now faced with increased wear.  From bad to worse.

• The basic requirements 
• Standard aircraft fuel gauges are calibrated only for the full and empty position.
• Caveat - new automotive systems are calibrated to the tank - great improvement
• Caveat - new aircraft MFD's allow tank calibration - Aerospace Logic, JPI, Garmin

• Almost daily use  
• The automotive sensor is continually washed with new solvent at every filling and fuel fills are more frequent. 
• Aircraft fuel can remain in the tank for long periods - it is designed to do so - however impurity buildup on the variable resistor are common at a fraction of usage time of the automotive application.
•  it is quite common to have your aircraft senders ultrasonically cleaned

---------------------------------------------------------------------------------------------

Propane Level Gauge

New requirements for intrinsic safety for all aviation fuel level systems 

• The general aircraft industry found that they could utilize a commercial off the shelf, patented float sending unit from the propane industry
• This met the new safety requirements and directly replace a float level legacy system.

• A Non- Contact solution - the resistance card was removed from the tank environment.
• Shock inputs from turbulent air can however de-couple the matched magnetic drive and the the gauge will take a set but in-accurate reading or drift back to the correct reading.
• Suffice to say - Propane Tanks were not meant to fly

----------------------------------------------------------------------------------------------------------------------------------

Aircraft Capacitive Sender Ad - 1948

Alternatives  - 

Capacitive Senders -

Conceived in the Late 1940's this aircraft fuel sending system set about to address the two most common issues presented above - fluid motion and resolution of the larger fuel quantity.   

These are the most widely used in aircraft above 12,500 lb 

Capacitive Sensor Benefits 

• Immunity from fluid motion 
• Greater resolution of the large fuel volume 
• Shapable to the fluid volume - tailored to the tank volume.

Capacitive Sensor Issues

• Difficult to Prove Intrinsic Safety
• The definition of the sensor is to utilize two metallic or conductive elements in the tank 
• The intrinsic safety requirement must be met with increasing the complexity of the system.
• It is a spark in the capacitive sensor that initiated the explosion in TWA 800 fuel tank. 

• Moisture Rejection
• An aircraft fuel tank breathes on ascending and descending - moisture in the air is constantly brought in contact with the fuel.  Sealing the tank is not an option.
• As the capacitive utilizes the fuel contents as an integral element of the level reading - this variable is very difficult to manage.

• What's in the tank - matters a great deal 
• Fuel must be:
• uniform 
• moisture-free
• stable and uniform temperature
• contaminant free
•  to give a consistent and accurate fuel level indication 

When you review the system above - the capacitive sender solved several issues with fuel level indication, however it brought a few of it's own limitations along for the ride.

--------------------------------------------------------------------------------------------

Ideal Aircraft Solution Characteristics

• Non Contact - New FAA Requirement
• Intrinsically Safe - 
• The alternative is proven safe electronics in the fuel
• Immune to fuel contamination

• Unaffected by Fluid motion 
• No wear items related to fuel level reporting

• High Resolution 
• Address the larger quantity of fuel in aviation systems.

• Simplicity 
• The best aviation solutions eschew complexity where a simple system provides equivalent benefit.

----------------------------------------------------------------------------------------------------------------------------------

CIES Inc AMR Fuel Level 

Float Based Level Sensing - Retrofits or directly replaces most units in the field

• Non Contact 
• No - Safety Issues 
• No - Fluid Contamination
• No - No moisture or fuel quality issues

• Immune to Fluid motion
• No wear of fuel level reporting system

• Resolution 
• Digital output
• More fuel level information 
• Integrity of the level signal
• Interfaces to modern avionic solutions
• Ability to output fuel temperature - Lbs fuel. 

• Simplicity




• No complexity required
• On the platforms we have been applied - exceed it's requirements

CIES Inc AMR  A sensor that was designed for and patented with the aviation industry in mind, but suitable for all high value applications.

Float Based Sensors - Accuracy Quality Reliability

Float Based Sensors 

These sensors have been in use for a long period of time.  They have a few advantages that are difficult to overcome.   If they have a less than stellar reputation it is more about the electronics used, than the method  of finding a fluid level.    We will assume that quality and good practice

• The float will find the surface 

• Provides a reliable value for full and empty
• Provides a stable output, even under fluid agitation.

• Simple mechanism and controlled motion about a pivot.

• Used everyday in the most critical applications 

• Independent of fluid type or composition

• The singular most popular method for measuring liquid level 

• A float and pivot arm can be maintained for millions of operational cycles 

The typical methodology for finding level is resistance - either in a wound wire or a resistance grid.  The output is simply a distinct resistance value.  However this mechanical method for obtaining an electrical value for fluid level has a few problems.

• This practice introduces a mechanical wear item into the system.  It is this wear on the resistance surface that draws the most criticism.

• The CIES Inc float sensor design has only a pivot to wear - and we can utilize a lighter float as there is limited friction to overcome.

• Resistance needs electrical modification to correlate with volume 

• The CIES Inc. float sensor design is digital output and can be programmed to tank volume 

• Resistance circuits immersed in fluids are susceptible to corrosion from alcohol in gasoline or de sulfered diesel or potentially to the 100LL replacement.

• The CIES Inc. float sensor design sensing element is outside of the fuel compartment no chance for corrosion 

CIES Inc answered the quest  - What if we could combine the simple and effective elements of a pivoted float and replaced the resistance element and its problems from the fuel tank sending system

Only CIES Inc has a patent to combine a sophisticated, reliable and stable sensor technology to a simple float mechanism.

CIES FUELRITE Level Sensor 

Utilizing the patented Anisotropic Magneto Resistive technology allows CIES Inc in an intrinsically safe non contact system.

CIES Inc  effectively married the simplicity and effectiveness of the float based system with a non contact level sensing technology with the following characteristics:

• Effective 
• Accurate
• Sensitive - Measures down to 0.02 Degrees 
• Immune to nearby magnetic fields
• No hysterisis -
• No drift over time or temperature - stable 
• Intrinsically Safe 

A proven system backed by exhaustive testing. 

Let us know your unique application

Contact Us