C172S fuel gauge drops to zero

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Mikey1719

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mike
I am purchasing a 172S whose fuel gauges drop to zero intermittently and independendantly of each other. Pre-buy inspector thinks it might be the sensors. Anyone have any experience with this issue?
 
Mikey1719 said:
I am purchasing a 172S whose fuel gauges drop to zero intermittently and independendantly of each other. Pre-buy inspector thinks it might be the sensors. Anyone have any experience with this issue?
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Check wiring and connections first.
 
"They all do that"

Really, the Part 141 school I did PPL at had a fleet of five - all had intermittently hinky fuel read outs.
Sensors were replaced on regular basis - to no real effect IMHO.
 
wrbix said:
"They all do that"

Really, the Part 141 school I did PPL at had a fleet of five - all had intermittently hinky fuel read outs.
Sensors were replaced on regular basis - to no real effect IMHO.
Click to expand...

My flight school has problems with them as well. It's very random, sometimes it happens a lot over a short duration and other times it will go hours without happening.

Not sure what the solution is but it wouldn't prevent me from purchasing a particular plane.
 
Very simply put, the sender is a potentiometer, open circuit is empty, zero ohms (closed) is full. the float arm sweeps a winding changing the resistance of the circuit. over time you'll get dead spots on this winding (dirt, lacquer, etc.) causing the gauge to show zero in various spots.

You can buy replacement senders, they'll last a few years (maybe) and end up doing the same thing. http://www.mcfarlaneaviation.com/Products/?ID=71619535&CategoryID=164&

Or you can go to an aftermarket unit : https://www.ciescorp.net/cessna.html more $$ but hopefully a one time deal.

or you can do like every other pilot since Cessna started production... Dip your tanks, know your fuel burn, use a timer, ignore those gauges.
 
Thank you everyone, both 172S planes at my flight school never had this problem which is why I was concerned.

Not sure if this is mixing topics, but anyone have experience with an inaccurate VOR when slaved to a bendix/king OBS? I had to tune it 100 degrees from the true course for it to work properly. VOR works great when slaved to the GPS. Would it be a calibration issue or could the bendix/king be faulty?
 
isn't the 172-S a capacitive system?
 
Welcome to Cessna, best fix is to install a EI and ditch all the engine and fuel instruments.

No matter what, fuel stick and stop watch, the gauges should only confirm what you already know.
 
Pat Elliott said:
Very simply put, the sender is a potentiometer, open circuit is empty, zero ohms (closed) is full. the float arm sweeps a winding changing the resistance of the circuit. over time you'll get dead spots on this winding (dirt, lacquer, etc.) causing the gauge to show zero in various spots.

You can buy replacement senders, they'll last a few years (maybe) and end up doing the same thing. http://www.mcfarlaneaviation.com/Products/?ID=71619535&CategoryID=164&

Or you can go to an aftermarket unit : https://www.ciescorp.net/cessna.html more $$ but hopefully a one time deal.

or you can do like every other pilot since Cessna started production... Dip your tanks, know your fuel burn, use a timer, ignore those gauges.
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This is a really big help. Thank you Pat for taking the time to explain this in detail!!
 
Tom-D said:
isn't the 172-S a capacitive system?
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Nope. Carbon-track potentiometers, as opposed to the old wirewound units. Not really potentiometers, either; just rheostats. Two terminals instead of three.

I have seen the newer ones doing the intermittent-zero thing. It happens sometimes when the tanks are full or nearly so, probably because there's some problem with the moving contact running off the end of the carbon track at the full end. Grrr...modern aircraft-quality components...
 
Isn't the 172S a G1000 equipped bird?

Is this showing up on the range ring on the G1000 or on the fuel gauges?
 
Kelvin said:
Isn't the 172S a G1000 equipped bird?

Is this showing up on the range ring on the G1000 or on the fuel gauges?
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Some of the earlier S models were still six pack, so not all S are G1000. That said, the G1000 I fly does this very randomly as well, but not very often. I still don't trust those gauges at all.
 
The collapse of the range ring on the G1000 was traced to a loose multi-pin connection in the G1000 harness.
 
Well.

The issue with utilizing either wire wound or carbon track Rheostats is that bothe contain two wipers - one from the body of the sensor and the other one on the wire wound track. Lateral motion of fuel disconnects one or both wipers. Cessna never used carbon track according to Cessna engineers, as it was too unreliable and degraded quickly in the aviation environment.

Cessna until this year replaced the rheostat units with a Meggitt TDR system. It is nearly impossible to certify a simple capacitive system like the old penny cap in aircraft due to lightning issues. The TDR system measures fuel level by driving laser light down a glass rod submerged in fuel and measuring the return charachteristic. This solution poses a lightning hazard but it is TSOd . If you price this system in the IPC, sit down first, as this is a $5,000 solution or $$$$$ It reports to the Garmin on CANBUS. Yes-- Lightning on a 172. Yep dems the regs.
Cessna only used this system on the propellor aircraft. Reliability and accuracy were questioned as temperature, fuel density and frothing conspired against accuracy.

New Cessna aircraft are delivered with the CiES system above in the posts and this is retrofittable to 172S aircraft. Our price isn't much beyond McFarland or Airparts of Lockhaven and our solution puts the crappy fuel gauge issue to bed permanently as we have the highest accuracy and reliability ever available for aircraft fuel level. On. CIRRUS aircraft it is 400,000 operational fleet hours and zero unscheduled removals. This is a reliability exceeding any other system on a Cirrus aircraft. New Cirrus aircraft are known for the accuracy of the fuel gauging system.

See use in Flying Magazine

At the OEM level we are replacing capacitive fuel level. Our system is found on the new Cirrus SF50 Jet
 
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Yeah, I've got balky senders in my Navion. The EI ones are going in during the restoration. I've seen them on two other Navions. They are da bomb compared to the original 50's automotive fuel senders these things really are.
 
Tom-D said:
isn't the 172-S a capacitive system?
Click to expand...

We have a 210 with the capacitive senders. The old gauges were never accurate and occasionally dropped to empty. We installed a EDB 900 and eliminated the gauges, fuel display works flawlessly now even with the same senders. Mechanic told us if the capacitive sensors do act up usually it is because of moisture and they can be removed and "dried out". My thoughts are they are more reliable than the potentiometer style, are they?
 
Can't vouch for the long term, but they indeed are better than the pot ones. The pot ones are prone to getting dead spots which is what causes the violent dropping to zero and out. Coupled with an electronic gauge (either a single gauge like EI's FL series or one of the engine clusters like the JPI EDM 900 Primary or the EI MVP-50P) takes a lot of the non-linearity out of the readings if you set it up right. I've flow behind the EI gauge in both the Navion and the 210 and it is slick. I'm upgrading my Navion to have them.
 
You're going to like the EI Magnetic Sensors. I installed them in my Cardinal along with an Aerospace Logic FL252 indicator (could have used the EI, or JPI heads also) and the reliability and accuracy give me great piece of mind. I still dip, and I verify it always matches the FL252. It'll cost about $1300 for the probes and head, but to me, it's worth it vs. the stock gauges.

flyingron said:
Yeah, I've got balky senders in my Navion. The EI ones are going in during the restoration. I've seen them on two other Navions. They are da bomb compared to the original 50's automotive fuel senders these things really are.
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Huh, my Aerospace Logic instrument is teamed up with new MacFarlane resistive transmitters. Kinda impressive to calibrate in one gallon increments for each tank.
 
Stewartb said:
Huh, my Aerospace Logic instrument is teamed up with new MacFarlane resistive transmitters. Kinda impressive to calibrate in one gallon increments for each tank.
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I physically installed and calibrated one of those for a friend, it calibrated in two gallon increments and the software absolutely made calibration a nightmare. It logs the change in millivolts for each two gallons added and if the millivolts doesn't change and you hit the button to record that point it throws and error and you were completely screwed and had to start all over after draining the tank.

Had they printed a couple warnings in the paper instructions, a lot of my frustration wouldn't have happened. But no, so measured a lot of two gallon buckets of fuel and carried them up a ladder, over & over & over.


Despite the calibration being a nightmare I was really impressed with the instrument itself, billet aluminum chassis that just has quality written all over it.
 
The "problem" with my instrument is that the data rate needs to be slowed down. The readout is way too twitchy in flight to be useful. It was so unnerving I added an FS-450 fuel scanner. The only fuel instruments I really trust are a dipstick and a clock. The other instruments serve to validate what I already know.
 
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