So You Thought that "Capacitive" was the Only Viable Aircraft Fuel Quantity System - Think Again

CiES is offering the highest performance, best value fuel quantity system for aircraft.

Conventional aircraft wisdom suggests that "Capacitive" is the way to go for aircraft fuel quantity of all types.  And for various good reasons that was the case up until a few years ago.   

But that aviation maxim no longer holds water.

First a little history,  

Edward Simmonds brought the idea of aviation capacitive fuel quantity to life after observing a similar system utilized in the process industry.  A light went on, and Mr Simmonds and a Polish engineer worked out how this system could be made to work in Aviation.   This monumental work occurred at the end of WW II.  Mr. Simmonds was a master marketer and the "Pacitor" system soon became the aviation standard.  

The better story is what it took to take a stationary process fluid level system and place it in an aircraft environment.  Mr. Simmonds and more importantly his Polish engineer had a monumental task with many obstacles to reach an acceptable aircraft system. 

The capacitive concept is very simple - you place two metallic plates in the fuel volume and you utilize the fuel and remaining air or more precisely air and fuel vapor (ullage) are a component of the electrical system called the dielectric in the circuit.  This dielectric will be referred to as K factor. This is the space between the inner and outer tube identified as the electric field on the right.

Obstacle 1:    Aircraft transition from warm to cool environments readily - and this K Factor (or Dielectric) changes with temperature of the fuel  so it has to be compensated for.   This temperature shift is significant.   There needs to be compensation for the temperature of the fuel and also of the ullage as each can contribute errors to the fuel volume calculation 

FUEL & ULLAGE TEMPERATURE EFFECTS  

Obstacle 2:  Aircraft fuel is not a static uniform fluid but a combination of different hydrocarbons to meets an ASTM Standard.  Each hydrocarbon component of fuel has different K factor properties and therefore each batch of fuel will need a different fuel quantity calculation .  We need to calculate Permittivity or K factor of the delivered fluid in the tank and compare this fuel in the tank to a reference standard.  This process is necessary to obtain a deviation factor for computing the actual fuel in the tank with a capacitive sender. 

FUEL QUALITY EFFECTS

Obstacle 3:   The fuel in an aircraft especially Jet A can contain quite a bit of mixed air.  This is due to fluid motion and vibration and to a certain extent the loading of fuel into the aircraft.  This entrained air can be up to 14% of fuel volume and will outgas like an opened soda bottle on an increase in altitude.   This changing fluid density effects K factor and therefore a provides a different fuel quantity calculation with a capacitive sender.   We need to calculate the in tank Fuel density with a densimeter and obtain it's K factor and compare this changing fuel density in the tank to a reference standard.  We can now compute actual fuel quantity from the capacitive probe outputs.

FUEL DENSITY EFFECTS

Obstacle 4:  Aircraft ascend and descend and by doing so they introduce moisture into the ullage mixture (air, fuel vapor, humid air) this has multiple effects on fuel measurement.  Quite a bit of moisture is introduced into the fuel itself and will change the fuel quality measurement for a capacitive system.   Water itself is conductive of electricity and this effect on the capacitor circuit  needs to be addressed.   This fuel water mixture produces corrosive components, and by nature the metallic probes cannot be protected.   To account for this you need to add more capacitive probes to cover for corrosion issues that naturally occur in the aircraft fuel tank. 

FUEL AND ULLAGE TEMPERATURE EFFECTS

  FUEL QUALITY EFFECTS 

FUEL WATER LEVEL REJECTION 

FUEL PROBE CORROSION

Obstacle 5: After the TWA 800 Accident the world of fuel quantity changed dramatically.  So while we always had to have a safe system in the fuel tank free from spark energy or lightning effects.  The TWA Accident brought a large spotlight into these issues as the spark from the explosion was found to have come from the fuel quantity system.    Capacitive probes conduct electricity - by nature they are metallic or conductive elements in the fuel volume and they are connected to the aircraft wiring. So you have a direct path into the fuel system through the aircraft wiring.  This introduces a host of issues already covered.  We need to address lightning effects, intrinsic safety (spark energy).  In the case of fuel inerting systems that introduce carbon dioxide into the ullage space you need to calculate this new ullage k factor.

     INTRINSIC SAFETY BARRIER 

LIGHTNING EFFECTS

FUEL INERTING SYSTEM 

WIRING EFFECTS - SFAR 88 & EWIS 

In review - there was a tremendous engineering effort to get "Capacitive"fuel senders to work in aircraft.   

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So Why is CiES Better?

So with a simpler system, we simply didn't have as many obstacles to overcome to provide TSO'd fuel quantity.  Less components, less complexity yields higher reliability.  


CiES utilizes a high reliability position sensor typically found in
automotive systems: drive by wire (brake, throttle & steering), cam
position, and stability control. A non-contact, high reliability, proven
sensor is combined with a simple float, a concept patented by CiES Inc.

The beauty of the CiES system is that our sensor technology allows for better resolution of the fuel volume and is absent the modifications required of an aircraft grade capacitive system.   Fewer obstacles to quality fuel level information. 

So it is immaterial if it is a Beechcraft or Boeing,  simpler is truly better.  Let us show you how we can add value to your aircraft by allowing a CiES FQIS remove weight and complexity from your aircraft fuel quantity system.