Trouble shoot Fuel Guage

Southpaw

Southpaw

Line Up and Wait
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Joined
Aug 4, 2019
Messages
565
Location
Montana
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Display name:
Ernest Taft
The fuel gauges on my 1959 172 are always off . Some times close others not even close.
Look for ground problems , continuity ?
Have a good VOM where to start ?
 
Make sure you have ground continuity from sending unit to fuselage. My C195 sending units were missing a ground wire . After adding ground wires, they are fine. Good luck.
 
I used to own a Cherokee 140 that had a fuel gauge inop for a fair amount of time. I supplemented a lot with the ‘known fuel quantity & timing’ thingy. I realize we like working fuel gages, but not much beats known amount & timing. Yes, round up for the fudge factor.
 
But.... you can’t fly with an INOP fuel gauge. ATOMATOFLAMES and such. Required day VFR required equipment.
 
Southpaw said:
The fuel gauges on my 1959 172 are always off . Some times close others not even close.
Look for ground problems , continuity ?
Have a good VOM where to start ?
Click to expand...
Original transmitters? Probably time to change them.
 
I think every 172 I’ve flown had inaccurate fuel gauges except when full or empty..lol


Sent from my iPhone using Tapatalk
 
Most are upgrading to digital senders, which are very accurate over a long period of time. If memory serves the original resistive senders when new were only required to be accurate for empty for the type certificate.
 
NordicDave said:
Most are upgrading to digital senders, which are very accurate over a long period of time. If memory serves the original resistive senders when new were only required to be accurate for empty for the type certificate.
Click to expand...

That is a common misconception.

§ 23.1337 Powerplant instruments installation.

(b) Fuel quantity indication. There must be a means to indicate to the flightcrew members the quantity of usable fuel in each tank during flight. An indicator calibrated in appropriate units and clearly marked to indicate those units must be used.
 
smv said:
That is a common misconception.

§ 23.1337 Powerplant instruments installation.

(b) Fuel quantity indication. There must be a means to indicate to the flightcrew members the quantity of usable fuel in each tank during flight. An indicator calibrated in appropriate units and clearly marked to indicate those units must be used.
Click to expand...

That's Part 23. The vast majority of GA airplanes fall under CAR3, 3.672, which in essence is close to the same wording.
 
Stewartb said:
Original transmitters? Probably time to change them.
Click to expand...
Probably. They wear out. They're being worn even when the airplane is tied down. The wind rocks the airplane, moves the fuel a bit, which moves the float and rheostat wiper, which wears out the coil of resistance wire inside the sender.

But a check of grounding between the sender and airframe is first. The OEM grounding was already a bit sketchy. Age and oxidation and residues from seeping fuel all make it worse.

Could also be loose connections at the back of the gauge assembly.
 
smv said:
That is a common misconception.

§ 23.1337 Powerplant instruments installation.

(b) Fuel quantity indication. There must be a means to indicate to the flightcrew members the quantity of usable fuel in each tank during flight. An indicator calibrated in appropriate units and clearly marked to indicate those units must be used.
Click to expand...

I don't think the two statements are incongruous to each other. They are more accurate on the lean side of a tank rather than more full. These were originally automotive gages back in the day, and cars using the same type of probe show more full than actual and are much more accurate closer to empty.
 
Thank you all for the advice and ideas. When the weather warms up I will go have a look see.
It's not been above 0 for a week. Currently -7 at 11AM.
 
NordicDave said:
I don't think the two statements are incongruous to each other. They are more accurate on the lean side of a tank rather than more full. These were originally automotive gages back in the day, and cars using the same type of probe show more full than actual and are much more accurate closer to empty.
Click to expand...

The next sentence in that FAR 23.1337 says this:

(1) Each fuel quantity indicator must be calibrated to read "zero" during level flight when the quantity of fuel remaining in the tank is equal to the unusable fuel supply determined under [Sec. 23.959(a);]

The misconception comes from that there. The requirement to show usable fuel in flight is still there, though no details are given as to accuracy.
 
A long long time ago I had a flight instructor tell me to “never believe the fuel gauges unless they were reading empty” ,,,, I have never forgot that statement. I always check my tanks before flight and my watch during .
 

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