how much confidence do you have in fuel gauges?

[MauleSkinner]

It’s a requirement that cannot be verified definitely, then, but verified only as a matter of opinion. I had to write, review, edit, and interpret technical requirements throughout my career in aerospace. This one doesn’t hold up.

Throughout my career in aviation I’ve seen enough definitely verifiable requirements that were arbitrary enough to be useless that I think useful holds up better than verifiable.

 

[MooneyDriver78]

I thought they only had to be accurate when empty? Or is that an urban myth?

 

[Omalley1537]

Somewhere between full and none.


Someone had to make the groan-worthy post…might as well have been me…

 

[MauleSkinner]

I thought they only had to be accurate when empty? Or is that an urban myth?

That’s part of the story about how Paul Bunyan and Babe the blue ox invented the airplane.

 

[frfly172]

I fly by time, gauges seem to be accurate. However I very seldom let the tanks go below half before stopping to refuel.

 

[Dan Thomas]

Okay, what is the accuracy level required for a given airplane? Pick a model and tell me, showing where that number is documented. TCDS, AFM,... I’ve never found a number for my Beech.

If there isn’t one, I can pick any error band I like and arbitrarily say it’s “good enough.” That’s not a requirement, it’s a wild-assed guess.

Imposed by Cessna in 2003:







I have done that job many times.

Now, there's debate as to whether this is a legal requirement. There's this in FAR43:

 

[Dana]

My fuel gauge reading is only a matter of opinion...

 

[mryan75]

Is this advice for all aircraft or one particular airplane.??

He was talking about legacy GA airplanes. And given that at least one bonehead a week runs out of fuel in a plane in this country I think it’s pretty good advice.

And clearly enough people here have me on block so as not to see that I answered the question everyone is arguing about.

 

[Half Fast]

I thought they only had to be accurate when empty? Or is that an urban myth?

 

[Dan Thomas]

And clearly enough people here have me on block so as not to see that I answered the question everyone is arguing about.

You said:

I was taught by an instructor in his 70s who had been teaching for 50 years. He gave me the best piece of advice I’ve gotten about fuel gauges: if they read more than half, you’re good. If they read less than half, it’s time to land for fuel.

The typical system uses two rheostats in a Wheatstone bridge circuit to vary the current through the gauge's electromagnets.

Now, if the wire to the sender shorts to ground, the gauge reads full all the time.

If the wire to the sender breaks, the gauge reads empty all the time.

Either is possible, but the first is the deadlier, assuring the pilot that he has lots of fuel. The second creates complacency, since the pilot knows it's dead and ignores it and has no way of knowing if he has a leak or uneven tank flow. He might even believe that myth about it being legal because it reads empty when the tank is empty.

Suppose the gauges are good and they read a bit less than half. Are you going to make a habit of landing for fuel at half-tanks, thus shortening your range and carrying extra weight for no good reason?

Advice to accept defects in an airplane is never wise advice. This stuff is fixable.

 

[MauleSkinner]

And clearly enough people here have me on block so as not to see that I answered the question everyone is arguing about.

I presume you’re talking about either post #25 or post #27, neither of which answer the question.

 

[Tantalum]

Have a 172 with float style. How much "trust" would you put in these? Or basically how low would you actually feel comfortable running them in the real world?

I trust whatever is lower, my calculations or the fuel gauge

If the fuel gauge says a quarter but my calculation should say a half, I start worrying. If my calculations see a quarter but the fuel gauge is a half, I assume my calculations are correct

 

[Albany Tom]

I think I have a similar take to most people here. I use the gauges in the planes I fly as a double check to a watch, and I'll take a look at them as part of preflight, after manually making sure there's fuel.

I don't know if you're asking this question in an implied way, but I visually/manually check the fuel tanks every time before I fly, no exception. But I fly PA-28s and cubs, and those are easy to check on the ground w/o a ladder. Would I trust the fuel gauge as a double check of the fuel guy telling me he filled both tanks with fuel? I wouldn't, but I could understand others taking that risk if they saw or knew the fuel guy, with the gauge reading being a 2nd check.

 

[Stewartb]

Make a dipstick. Wear a watch. Fuel quantity gauges are for the FAA. If you want the real
story but a fuel flow instrument.

 

[MooneyDriver78]

Make a dipstick. Wear a watch. Fuel quantity gauges are for the FAA. If you want the real
story but a fuel flow instrument.

None of the above will tell you if your fuel tanks sprung a leak, except for the fuel quantity gauges. So they’re not just for the FAA.

 

[Pinecone]

I was taught by an instructor in his 70s who had been teaching for 50 years. He gave me the best piece of advice I’ve gotten about fuel gauges: if they read more than half, you’re good. If they read less than half, it’s time to land for fuel.

So. My airplane has 104 usable. So I should land at 50 gallons in the tanks????

 

[Pinecone]

None of the above will tell you if your fuel tanks sprung a leak, except for the fuel quantity gauges. So they’re not just for the FAA.

Exactly. A guy on another forum reported one tank leaking about 1 gallon per hour.

 

[Dan Thomas]

Just leaving a fuel cap off can lose a bunch of fuel real quick, and you'd want to know about it.

Even worse, doing that in an airplane with bladder tanks can cause the gauge to read full. The suction pulls all the fuel out of the tank and sucks the air out of the bladder, pulling it up so it holds the sender float at full. Cessna had an SB on that, warning owners to maintain the cap seals.

 

[MooneyDriver78]

So. My airplane has 104 usable. So I should land at 50 gallons in the tanks????

Like many instructors, their experience is limited to aircraft typically used as trainers.

If I listen to all their advice I probably would have crashed my Mooney by now.

For many, the simplistic advice is probably the best.

 

[mryan75]

I presume you’re talking about either post #25 or post #27, neither of which answer the question.

27 most certainly does answer the question halffast and most of this thread have been arguing about.

 

[mryan75]

So. My airplane has 104 usable. So I should land at 50 gallons in the tanks????

You should do whatever you want.

 

[mryan75]

You said:


The typical system uses two rheostats in a Wheatstone bridge circuit to vary the current through the gauge's electromagnets.

Now, if the wire to the sender shorts to ground, the gauge reads full all the time.

If the wire to the sender breaks, the gauge reads empty all the time.

Either is possible, but the first is the deadlier, assuring the pilot that he has lots of fuel. The second creates complacency, since the pilot knows it's dead and ignores it and has no way of knowing if he has a leak or uneven tank flow. He might even believe that myth about it being legal because it reads empty when the tank is empty.

Suppose the gauges are good and they read a bit less than half. Are you going to make a habit of landing for fuel at half-tanks, thus shortening your range and carrying extra weight for no good reason?

Advice to accept defects in an airplane is never wise advice. This stuff is fixable.

I made two posts earlier, you quoted one and gave an answer to the other. All good. I fully expected everyone to tell me I was wrong. And I’m fine with that.

 

[mryan75]

Like many instructors, their experience is limited to aircraft typically used as trainers.

Not this guy, though as I stated earlier he was referring to legacy GA aircraft in this context.

 

[Stewartb]

None of the above will tell you if your fuel tanks sprung a leak, except for the fuel quantity gauges. So they’re not just for the FAA.

Years ago I changed my fuel instrument in the Cessna to new digital gauges with new transmitters. The indication is so up and down with any wing movement it’s impossible to know how much fuel is in there, but I’ll admit that it helps to know there’s something there. I cross reference that to fuel remaining on my FS-450. Also years ago There was a local fuel exhaustion accident where the pilot was flying by his FS-450 and was unaware of a line leak upstream of his transducer. Lesson learned.

FWIW, I can set my FS total quantity using my dipstick. That simple piece of wood that I scribed lines into by adding 5 at a time is my most trusted fuel gauge.

 

[Tarheelpilot]

Not this guy, though as I stated earlier he was referring to legacy GA aircraft in this context.

Regardless of that guys experience it was still terrible advice.

 

[mryan75]

Regardless of that guys experience it was still terrible advice.

Okay!

 

[jbarrass]

To me it's like a weather report, very useful, but...

As long as it's consistent with my watch and fuel flow I feel good. If it diverges, I'll likely pull over and check it out.

 

[MauleSkinner]

27 most certainly does answer the question halffast and most of this thread have been arguing about.

I must be reading this wrong…I didn’t realize they were asking for partial answers about transport category airplanes.

 

[mryan75]

I must be reading this wrong…I didn’t realize they were asking for partial answers about transport category airplanes.

Touché. Part 23 had the exact same wording prior to June 2019, and still applies to aircraft certified prior to that time. The point stands, although I do realize there’s nothing to be gained by pointing this out.

 

[Stewartb]

Part 23 does not govern Car 3 airplanes. From Car 3-

3.672 Fuel quantity indicator. Means shall be provided to indicate to the flight personnel the quantity of fuel in each tank during flight. Tanks, the outlets and air spaces of which are interconnected, may be considered as one tank and need not be provided with separate indicators. Exposed sight gauges shall be so installed and guarded as to preclude the possibility of breakage or damage. Fuel quantity indicators shall be calibrated to read zero during level flight when the quantity of fuel remaining in the tank is equal to the unusable fuel supply as defined by § 3.437.

 

[mryan75]

Part 23 does not govern Car 3 airplanes. From Car 3-

Which is exactly the same thing as part 23 said when it replaced CAR 3. Fuel gauges have to read empty in level flight with only unusable fuel remaining.

 

[MauleSkinner]

Touché. Part 23 had the exact same wording prior to June 2019, and still applies to aircraft certified prior to that time. The point stands, although I do realize there’s nothing to be gained by pointing this out.

What about part of the regulation(s) that specifically address the other 99+% of the usable fuel quantity indication? What about the tolerances @Half Fast asked about, since you specifically mentioned him?

 

[Dan Thomas]

Years ago I changed my fuel instrument in the Cessna to new digital gauges with new transmitters. The indication is so up and down with any wing movement it’s impossible to know how much

That's the problem with undamped systems. The original Cessna gauges had tiny electromagnets in the gauges that were made with very fine wire, lots of it, that added up to a lot of inductive reactance. Slow to move the needle. It would not be difficult to damp modern systems, and I don't know why they wouldn't. Damped enough to show average fuel level.

 

[Pinecone]

Just leaving a fuel cap off can lose a bunch of fuel real quick, and you'd want to know about it.

Even worse, doing that in an airplane with bladder tanks can cause the gauge to read full. The suction pulls all the fuel out of the tank and sucks the air out of the bladder, pulling it up so it holds the sender float at full. Cessna had an SB on that, warning owners to maintain the cap seals.

One advantage to low wing. You can SEE the fuel streaming over the wing.

And it really does get sucked out at a pretty good rate. BT, DT.

 

[Pinecone]

You should do whatever you want.

So you subscribe to landing at half tanks for ALL aircraft. Hmmmm, makes for carrying a lot of excess fuel on a 787.

 

[Pinecone]

Not this guy, though as I stated earlier he was referring to legacy GA aircraft in this context.

What do you mean by legacy aircraft? Aircraft built in the 60s have fairly large fuel capacities.

Basically that is an overly simplistic, pretty bad bit of advice.

 

[Gary Ward]

Have a 172 with float style. How much "trust" would you put in these? Or basically how low would you actually feel comfortable running them in the real world?

What model 172 do you have?

Surprising the fuel gauges work very well in my 1980 N model 172. Both sides are always within a 1/2-1 gal consistently when I stick the tanks. I have run them really low and stuck the tanks and they are right in line with what the gauge shows.
I have had both sending unit out replacing the gaskets and replaced one gas tank but reused the original senders and they still work good. I use them every flight. And when my right fuel tank started leaking I could clearly see the level going down quicker than it should on the gauge and smell it.

Every time I refuel I can confirm my gauges are still working accurately.

I have read many posts about 172 fuel gauges being off and not working but not mine. Is it an earlier 172 problem?
The lowest I have run them was 3-4 gallons in each tank giving me 45 mins of reserve. And when I stick the tanks before filling the gauges are accurate.

 

[Dan Thomas]

I have read many posts about 172 fuel gauges being off and not working but not mine. Is it an earlier 172 problem?

No. It's a combination of poor maintenance, cheap owners, complacency, and the old myth about being required to be accurate only when the tanks are empty.

The senders are the biggest problem. They wear out. If the airplane is parked outside, the wind rocks it a bit, sloshing the fuel in the tanks a bit, and the float arm is moving all the time, for years and years, and stuff gets worn. Other problems include dirty ground connections and corroded crimp terminals at the sender. All the same factors that trash other stuff in the airplane.

New senders are available. And they're better senders. They use a carbon-track rheostat instead of the old wirewound type.

 

[martym]

§ 25.1337 (b)1. They have to read empty in level flight when the only fuel remaining is unusable.

good to know for those flying a transport category plane certificated under Part 25.

 

[MauleSkinner]

good to know for those flying a transport category plane certificated under Part 25.

Be better to know the other 99+% of the requirement, too, but reading is not a critical skill when spreading old wives’ tales.