What it Looks Like - Aircraft Fuel Sender - Mark Twombly AOPA

What It Looks Like Fuel sender - Variable Potentiometer  By Mark Twombly How do you know with confidence how much fuel is in the tanks when you taxi for takeoff? The same way you do in your car: Check the fuel quantity indicators. As every new pilot eventually comes to realize, however, fuel quantity indicators in the typical light aircraft are notoriously inaccurate except, perhaps, when they read Empty. Fuel quantity needles bounce in turbulence, often lag in indicating a depleting supply of fuel, and then seem to speed up their descent as they near the big E. This is why we visually check the fuel quantity as part of our preflight inspection. A big reason for the temperamental behavior of analog fuel quantity gauges is the fuel sender, the mechanism buried within the fuel tank that measures fuel level. Although there are several different types of analog and digital fuel senders, the type found in most light single-engine aircraft is based on a simple float system. The lightweight float is attached to one end of a thin metal arm, the other end of which pivots inside a sealed housing. The housing, which is submerged in the fuel, contains a resistance element and contact system. The float arm contacts a coil or winding inside the housing that is energized with current sent from the fuel quantity indicator on the instrument panel. Different positions on the winding represent different resistance levels. As fuel is consumed and the float descends in the tank, the float arm contact slowly moves along the winding, gradually changing the resistance in the current read by the gauge. The current reading is translated into an analog needle indication. So what happens if the electrical current somehow shorts and "leaks" into the fuel tank? No worries -- the current is too weak to ignite the fuel or even fuel vapors. One manufacturer of resistance type float fuel senders says the design life is 100,000 empty-full-empty cycles but includes this important caveat: "Actual service life may be less because of wear caused by vibration and fuel movement." Add to that the ravages of time and infrequent use that no doubt accelerate the aging process of a fuel sender . So, along with learning to take off with sufficient fuel in the tanks to complete the intended mission plus some, we soon learn to treat fuel gauges with a healthy skepticism. The best bet is to back up the fuel quantity indicators with another simple measuring device: a watch.

Tilting @ Propellers

Illus. Chris Demerest http://chrisdemarest.net 

Today I took a call,  the pilot on the line was enthused, he really wanted to get our new technology fuel senders on his aircraft.  

Further he engaged us in a dialogue wanting to let us know how significant this product was to aviation, and to his passion, you know the type,  a typical Mooney pilot.

He told me about the AOPA statistics on fuel mismanagement and starvation, and that it wasn't good press for general aviation or general aviation safety  to have a fuel starved aircraft parked along  a busy highway.

He was adamant to tell us about the significance of redundant and confirming cockpit information.  Being able to cross-check the two independent fuel quantity indications in his cockpit, he felt, would virtually eliminate fuel starvation in general aviation aircraft.

He talked about the level of quality he saw in fuel senders used in aviation, and that he just hoped he wouldn't have to overhaul his original set for the umpteenth time, and then have them fail hours later in flight, as they had on the past two occasions.

He related to me that as a multi type rated and experienced pilot he had never flown behind a good aviation fuel gauge in his entire life.

I didn't say a word  I sat back and listened to the sentiment that put us on the path to start a company around this product and idea.

The idea that we will build a good fuel level sensor, a quality fuel level sensor,  around robust patented technology using aerospace materials and standards.   We will prove our commitment by qualifying this fuel sensor to the most ridgid FAA standards for manufacturing and design.

My dream is that pilots will get this and install or demand this technology on their aircraft.  If that happens, we will make a lasting contribution to the aviation industry.

So many times in this aviation business,  you feel like you are setting your train along the wrong track,  why are you building a company in aviation,  when successful people all around you question the market size, vitality and rational.   You question your own sanity on a daily basis.

I guess we do this,  because our passion makes us tilt toward that propeller.  some days it's nice to know we aren't the only ones.

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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Aircraft Fuel Management - Down Under - Civil Aviation Authority

Sometimes it's helpful to look at how other countries address a similar problem.  The below excerpts come from the New Zealand Civil Aviation Authority.

Determining Fuel Available

Accurately determining what quantity of fuel you have on board is important. Assuming the fuel required for the flight has been calculated correctly, it should be a simple matter of uplifting it and completing the flight with a comfortable margin to spare.

It is good practice to check the fuel available before flight by at least two separate methods (in Australia, this is a legal requirement). We can do this by referring to the fuel gauge(s), loading a known quantity and, in many aircraft, by dipping the tanks. There are a number of considerations that should be borne in mind when determining the fuel available.

Unusable/Usable Fuel

Understanding the difference between the terms usable and unusable fuel is important in determining the fuel available for flight.

The unusable fuel is the quantity of fuel that cannot be used in level flight. It is the quantity remaining in each tank after the tank outlet becomes uncovered in level and balanced flight. The amount of unusable fuel can vary considerably from aircraft type to aircraft type – refer to your aircraft Flight Manual for specific figures. The fuel tank outlets on some aircraft types are very susceptible to becoming un-ported during prolonged unbalanced flight, which eventually leads to fuel starvation and engine failure.

Extreme care must be taken to ensure that the unusable fuel quantity is not included in the fuel available, as it can equate to as much as 20 minutes extra flying time that you don’t actually have.

It follows that the usable fuel is the quantity of fuel available for flight planning purposes. This is the only figure that should be used when calculating fuel endurance. Most dipsticks are calibrated to read the total fuel quantity in the tank, which means that the unusable fuel must be subtracted to determine the fuel available for flight. Care must be taken when converting between litres, and US or imperial gallons. Calculations should always be double-checked.

Fuel Gauges

Most fuel gauges read reasonably accurately, and if they don’t, they must be fixed. Gauge accuracy can easily be checked before the flight by dipping the tanks (if that is possible) and comparing the figures with the actual gauge readings. Any discrepancies must be allowed for until the problem can be fixed.

Be aware that fuel gauges can stick or fail in flight, sometimes in a subtle way, so don’t rely on higher-than-expected readings which seem at odds with expected consumption as the flight progresses. Also, in some common aircraft types, fuel gauge indications will vary widely according to the direction and degree of any slip or skid.

Some aircraft have tank designs where a dipstick reading can’t be obtained at certain fuel levels, so the use and accuracy of the fuel gauges becomes even more important to the pilot.

The "Zero Fuel" Myth - You know, Fuel Gauges only Need to be Accurate when Empty

You don't really need to know what happened after this true Accident Narrative - It is evident from the beginning.

Picture Draw Jerel Draw,  jereldye.com

Prior to the incident, I received an instructional ride and completed my C-172 checkout. I then flew with a passenger and solo, three flights, for a total of 5.3 hours in the bird.

In all three of those flights.

• I observed erroneous fuel quantity indications,
• Intermittent cycling of the fuel gauges to zero.
• LH Fuel Low Level Warning light coming on intermittently. 

An [instructor] told me this condition was well  known, typical for this aircraft and not uncommon for general aviation aircraft.

I discussed the erroneous fuel quantity indications with the Chief Pilot, and asked
him if I should write them up. 

He said no, that the indications were within the spec, which requires only that they
read accurately when empty.

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The statement that fuel gauges only read accurately when empty is repeated so many times in the aviation dialog.

 You would almost have to believe it is true.

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Actually far from it.

• It is a comment you hear repeated by many pilots, the magazines & AOPA / FAA Safety Briefs.  

• It is not uncommon to have bad fuel gauges in aviation, it is frighteningly almost the rule.  

What is true: 

• Fuel gauges are "Required" aircraft instruments for powered aircraft.

• They are required to be functional by design and in operation on the aircraft   -  Specifically:

• Read fuel level from FULL to EMPTY.  
• EMPTY needs to be calibrated at "Zero Usable Fuel"
• The Zero Usable Fuel is the fuel level,  where in the worst condition - fuel cannot be drawn from the tank.

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Designing, Maintaining or Flying an aircraft with bad or in-operative fuel gauges is illegal.  

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Why is it surprising that so many GA pilots run out of fuel.

I think the answer is obvious.

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Selecting your Fate - Choosing the Proper Tank.

We read so many aviation fuel starvation accident reports and the published responses from AOPA and others  

"On March 12, 2005, a 24,611-hour ATP made a forced landing in an open field after a total loss of engine power while on a visual approach to Runway 13 at Lancaster Airport in Lancaster, Texas. He and the one passenger were not injured.
The pilot did not visually check the fuel tanks prior to takeoff and could not recall "what the fuel gauges indicated"  during the flight.
He thought both auxiliary fuel tanks were full, and both main fuel tanks were almost full.  While descending for the approach, the pilot moved the fuel selector valve from the left main tank to the right main tank" 

Here is where we believe accurate or at least functional fuel level comes into play - The pilot did not look at his gauges - if he had,  and they were operating as designed - He probably would not have switched to an empty fuel tank.

Very shortly after, the engine quit. The pilot unsuccessfully attempted to restart the engine by switching the fuel selector valve back to the left tank and cycling the throttle.  

It is interesting that the neither the FAA nor NTSB tested this aircraft's fuel gauges for operation.  They only tested the engine. 

   We have all become complacent that aircraft fuel gauges can offer little or no assistance to fuel starvation in our mode of transportation.

One aircraft manufacturer has looked at this issue and did something about it - why guess if the tank you are switching to holds fuel.   

Cirrus Aircraft has incorporated accurate fuel level - and with accurate fuel level you may eliminate switching to an empty tank.   

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Let's Elevate the Discussion about Fuel Exhaustion so it won't Bring us Down

I have been engaging with an active Aircraft Owners Pilot group about Fuel Exhaustion and it's potential causes and solutions.  As Fuel Exhaustion and Starvation span the  GA spectrum I thought I might share the dialogue.