Why do Aircraft Fuel Gauges Suck

In the 1950’s when a majority of GA aircraft were designed,  the aircraft manufacturers in the USA turned to automotive fuel senders and equivalent gauges made by the same manufacturer - AC DELCO or in some cases Stewart Warner.

Rebuilt Piper Comanche Sender 

These gauges and senders were produced by the millions for the primary automotive application Buicks and Cadillacs in the mid 1950's.  I get a sense looking at these senders and gauges now 50 or 60 yrs later that there was a sense of pride,  they did last far longer than they should have.  But even 50 yr old automobiles have issues.

The trouble with these automotive senders is that they are: 

• Made of Steel. 
• Only in production for a few years. (more of this later) 

Fuel tanks for both applications automobile and aircraft were very similar in size 

• The sender and gauge face size that were matched. The indication size allowing for  the automotive requirement to provide a warning of impending fuel starvation.

• Interesting note:  in that automotive systems are biased to show a faster fall in fuel level from 1/2 tank to zero - the Big Three were trying to keep car owners from running out of fuel.  That clever manipulation is still being used in automobiles today.

Now because aircraft are aluminum we have introduced dissimilar metal corrosion.  Steel sender body and aluminum aircraft tank or structure.

Water vapor introduced on descent is one of the reasons we sump the tank before flight.  But water vapor also condenses on the steel sender body.  Water increases the rate of galvanic corrosion.

What else is necessary, oh yeah, latent time, when the aircraft tanks sits empty waiting for the next flight. 

What is worse - is that we stuck the resistance measurement method inside this little corrosion lab - it degrades right along side the metal in the sender body and just as quickly.

Fuel senders last longer if they get the regular washing with fuel.


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So now just for fun,  we have introduced corrosion by products into our fuel stream,  rust and rust particles threatening fuel passage.

Now with totalizers, we added a little turbine to measure fuel

We should replace the senders every so often,  yes that would be a good plan - the only issue is that that particular automotive sender is no longer made.

So to accomplish this lofty ideal you would have to wholesale replace the senders and gauges - Cessna Parts Distribution only reports 4 to 5 sales per year for a program like this.  I was told it was a single persons job for 5 years to put together this program.  Somebody thought it was worthwhile.

First for the Stewart Warner System and then for the Rochester Gauges unit (an Aluminum Body finally)

It should have been required - it should have been made into a Mandatory Service Bulletin - Actually it has and actually it is. 

Nobody pays attention to it, very few are aware it exists.  

Piper, Mooney and Beech have similar 

Fuel tanks got larger over time  and only Beech and Cessna in the Twin aircraft   increased the fuel gauge size for better recording accuracy 

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Well where that leaves us is with marginally operational gauges that for a very good functional reason nobody would trust, as they weren’t designed for the job. 

But instead of fixing the issue, and Cessna did try and so did a few others - pilots and aircraft manufacturers  just gave up.  They gave up on new technology, they gave up on new designs. 

Who would want to fix it - if nobody cared or even though they though they didn’t need a functional fuel gauge.



So when a pilot says he trusts his watch or his unapproved  Minimum Equipment List stick or some other fancy but non-required workaround and that they state emphatically that they wouldn’t trust a fuel gauge 

• We are fairly certain they are flying with un-airworthy fuel gauges and senders that haven’t been serviced or calibrated 
• We are dead certain of the lack functional fuel indication capability in an aircraft,  when the pilot emphatically states that fuel gauges don’t have to be accurate or only accurate at zero 

The ridiculous part is that the magazine pundits agree with them, all for a system, that was by it’s very design was going to fail 

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This is the saving grace - CiES now has over 85% of the OEM general aviation fuel sender market - All of Textron Piston and All of Cirrus Piston and Jet  - and a large smattering of others 

The aircraft manufacturers, do not agree with the magazine pundits, and that should be a sobering moment.  Internally in the industry when we have discussions on this topic and the concerns are completely foreign to what is perceived by the pilot community believes.

Aircraft Manufacturers Requirements

• Aluminum Body - that has an additional element of corrosion protection.
• One less thing to introduce rust into the tank environment
• Designed for the harsh aviation environment - DO-160 Testing
• Repeatable and reliable accuracy over the aircraft life.
• Measurement method removed from the fuel volume 
• Intrinsically Safe 
• The ability to effectively stick the tank anytime in level flight
• Digital system to offer the a better owner satisfaction and reduced warranty claims 
• They wanted fuel quantity solution for life of the aircraft 

Aircraft manufacturers know that fuel totalizers, did not and do not fix fuel starvation as Cirrus aircraft in particular had several notable events 

The funny thing is that this new GA fuel quantity system can be retrofitted to older aircraft -

It has the potential to make all aviation fuel level gauges functional - more importantly TRUSTED.