Fuel Gauge Inaccurate or Inop?

[Vtec01]

I'm not sure where to post this so I guess I'll just post it here.


So today while pre-flighting my club's 172. I flipped on the master switch to check the fuel gauges. The right fuel gauge, indicated correctly. While the left fuel gauge however, was pegged on empty, even though there was full fuel in the left tank.


I know that fuel gauges should only be accurate when they are empty and under 91.205 which states that an operative fuel gauge(s) indicating the quantity of fuel in each tank has to work for a VFR day flight.


I asked the mechanic and he frowned upon me and gave me a lecture on how "fuel gauges should only read zero when empty blah blah," and that I could still fly the airplane. Weary I went and asked of one of the CFI's in the FBO and he told me If there is fuel in the tank and the gauge indicates zero or empty then its just an inaccurate gauge. If however, there is no fuel in the tank and the gauge indicates fuel, then that is an inoperative guage.

My question is, I am legal to fly? And what are you thoughts on this? I really dont want to be nit-picky about regulations on the airplane that I fly but, that is my name going into that logbook and I just didn't want to risk it.

Thanks!

 

[MikeD]

I know that fuel gauges should only be accurate when they are empty and under 91.205 which states that an operative fuel gauge(s) indicating the quantity of fuel in each tank has to work for a VFR day flight.

You wrote this, so it should answer your question.

A further question could be: What is so critical about flying the particular mission you're seeking to (a pleasure flight), that would require you to go against the above? Especially if something were to happen on that flight that attracted Fed attention, related or even unrelated to the inop gauge? Would the club back you then?

Tell the club to fix their stuff. There's no excuse for that kind of Mx laziness, and allowing it or making excuses for it with the "kick the can down the road" mentality they seemingly have, only perpetuates the problem.

 

[BlueMoon]

I'd say no, till it is either fixed or deactivated in accordance with the MMEL.

The whole "it only has to be accurate at zero" argument to me is moot, It is currently at zero AND it is not accurate.

A broken clock is right twice a day.

 

[murphfly]

Dip your tanks and use those numbers in your calculations. Flt time x Rate of fuel burn = Gallons of fuel needed then add in your fuel reserves. I don't rely on what a fuel gauge in a 172 says.

 

[MikeD]

Dip your tanks and use those numbers in your calculations. Flt time x Rate of fuel burn = Gallons of fuel needed then add in your fuel reserves. I don't rely on what a fuel gauge in a 172 says.

Why bother though? He should be able to rent quality equipment from the club, not give an excuse for Mx to be lazy and not fix things. Where does it end? What other Mx items will be poo-poo'd by Mx then?

Fix the items. They're there. They're part of the onboard equipment. Yes, Cessna gauges aren't necesarily known for their accuracy, but they're still there.

 

[jrh]

Personally, I wouldn't fly it. I certainly don't want to nitpick a plane apart, but the thing is, the fuel gauge isn't doing anything *at all*!

I'd fly a plane that had a gauge reading "some fuel" when there was, indeed, some fuel in the tanks, and reading "empty" when it's empty.

But having a gauge sitting on "empty" 100% of the time isn't acceptable, in my opinion. What's the point in even having the gauge then? Might as well just draw a picture of a gauge on the panel with magic marker, because it would be just as useful.

Ultimately, I'd pull the "would you rent this plane to a fed?" question on them. If they wouldn't be willing to show it to a FSDO inspector, they need to fix it.

 

[Maurus]

(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. In addition:
(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 §23.959(a);

Notice where the "zero" comes from. It is a guideline for when the fuel gauge must read zero. This is to keep manufacturers from using zero "total" fuel when calibrating the gauge instead of "usable". Other than that there is no mention of gauge "accuracy" in the reg.

Also.

Powered civil aircraft with standard category U.S. airworthiness certificates: Instrument and equipment requirements.

(a) General. Except as provided in paragraphs (c)(3) and (e) of this section, no person may operate a powered civil aircraft with a standard category U.S. airworthiness certificate in any operation described in paragraphs (b) through (f) of this section unless that aircraft contains the instruments and equipment specified in those paragraphs (or FAA-approved equivalents) for that type of operation, and those instruments and items of equipment are in operable condition.

(b) Visual-flight rules (day). For VFR flight during the day, the following instruments and equipment are required:

(9) Fuel gauge indicating the quantity
of fuel in each tank.

Notice the word "operable" in the reg?

Is the gauge in an "operable" condition if it is always reading zero? No.

Is the gauge in an "operable" condition when reading inaccurately? No.

 

[Bernoulli Fan]

A lot of 172s, especially newer R and SP models, will indicate zero fuel when they are filled to the top. Usually this fixes itself during the taxi as the fuel level decreases a bit.

I'm not advocating breaking regs, but it is a known issue and usually resolves itself quickly. As far as I've heard, there is little mx can do to fix it.

 

[drunkenbeagle]

Why bother though? He should be able to rent quality equipment from the club, not give an excuse for Mx to be lazy and not fix things. Where does it end? What other Mx items will be poo-poo'd by Mx then?

Fix the items. They're there. They're part of the onboard equipment. Yes, Cessna gauges aren't necesarily known for their accuracy, but they're still there.

Daftarian - sounds great in theory, but I have not yet seen an accurate Cessna fuel gauge. Ever. A fairly expensive AD would be the only thing that would ever change that.

MX could replace the gauge and the sender for it, but it would probably be just as bad as it is now.

I personally take it as a reminder of why it is necessary to visually inspect the fuel condition.

 

[Maurus]

A lot of 172s, especially newer R and SP models, will indicate zero fuel when they are filled to the top. Usually this fixes itself during the taxi as the fuel level decreases a bit.

I'm not advocating breaking regs, but it is a known issue and usually resolves itself quickly. As far as I've heard, there is little mx can do to fix it.

No one says the gauge has to work in order to taxi

From my experience by the time the runup is complete the fuel level has gotten to the point where the gauge reads properly and is in working order.

 

[MikeD]

Daftarian - sounds great in theory, but I have not yet seen an accurate Cessna fuel gauge. Ever. A fairly expensive AD would be the only thing that would ever change that.

MX could replace the gauge and the sender for it, but it would probably be just as bad as it is now.

I personally take it as a reminder of why it is necessary to visually inspect the fuel condition.

One should always visually inspect fuel tanks if at all possible (some planes it isn't). Still, I take issue with Mx poo-pooing things because they don't even want to look into it. Im not naive or saying the plane has to be perfect, but the gauge should at least indicate something close to what it's supposed to. I've flown Cessna's long enough to know all about their gauge accuracy, but there comes a point where some clubs/FBOs, etc, get Mx lazy. I like to hold Mx to at least some standard.

 

[Maurus]

Cessna Fuel Gauges are inaccurate. Mostly a problem with using floats instead of some other type of sensor. It should be relatively accurate though. If you have a topped off tank and it shows 1/3 a tank, there is something very wrong with the gauge.

Also don't forget that there are some C-172s out there were certificated before Part 23 was created and certification of the aircraft was done under a different set of regs.

 

[Krieger]

Dip your tanks and use those numbers in your calculations. Flt time x Rate of fuel burn = Gallons of fuel needed then add in your fuel reserves. I don't rely on what a fuel gauge in a 172 says.

@murphfly Senario -
Ever have a fuel cap fall off? Are you 100% sure you put it on tight enough? You can't see it on that high wing... If you are losing fuel at a faster rate than you calculated, how would you know?

If the fuel gauge worked at the beginning of your flight and then shortly after you took off showed Half or Empty what would you do?

I rely on my fuel gauges to work properly. They don't have to be super accurate, but they do serve a purpose.

Just something to think about

 

[Vtec01]

Ultimately, I'd pull the "would you rent this plane to a fed?" question on them. If they wouldn't be willing to show it to a FSDO inspector, they need to fix it.

Ahh Should've thought about that one!

No one says the gauge has to work in order to taxi

From my experience by the time the runup is complete the fuel level has gotten to the point where the gauge reads properly and is in working order.

I've talked to the maintenance officer for the club and he said he will check it out tomorrow. But I'll definately crank her over and taxi her next time if it ever happens again.

Thanks! For all your guys input, it really helps a lot. I knew it wasn't legal, but yet for some strange reason the mechanics and CFI's at the FBO had a million ways to explain and get around it.

 

[Vtec01]

I actually have never relied on any fuel gauge on any airplanes that I have flown. I've always calculated everything out, fuel burn wise. I just use my fuel gauges as reference during my flights, and the fact that it was indicating zero while full was a red flag in my book.

 

[Maurus]

If it is an older 172 give it a shake from the affected side's wing tip. Sometimes the float gets stuck in the fuel tank. Had that happen once in an N model.

I have also seen a mirror the size of a license plate pulled out of a C-182 tank. That was a fun one.

 

[Vtec01]

It's a 69 172K model, so that might be the problem...Whoops might've called maintenance and the whole brigade for what could be a fluke LOL. Oh well, you learn something new everyday.

 

[murphfly]

@murphfly Senario -
Ever have a fuel cap fall off? Are you 100% sure you put it on tight enough? You can't see it on that high wing... If you are losing fuel at a faster rate than you calculated, how would you know?

If the fuel gauge worked at the beginning of your flight and then shortly after you took off showed Half or Empty what would you do?

I rely on my fuel gauges to work properly. They don't have to be super accurate, but they do serve a purpose.

Just something to think about

I understand what you're saying but really??

 

[Roger Roger]

You gotta know the difference between "broken" and "el cheapo 40 year old crap adapted from automotive parts". A fuel gauge bouncing back and forth 1/4 tank either side of something that averages to be in the general vicinity of the actual amount of fuel on board is the latter, what you experienced is the former. If you're feeling particularly motivated, give the wing a good shake and see if that helps, if not, write it up.