Fuel gauges stink: fact or fiction?

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drotto

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drotto
I am closing in on the completion of my PPL and this topic has just interested me. Is there truly validity to this idea, or is it something based in some 70 year technology, which has been long replaced, but people cling falsely to the idea. This is not meant to tell people (or myself) that proper flight planning, and fuel management is essential for safety, but rather how can we be safer, and how much can a new pilot trust the gauges in front of them. I will always do exactly as the books says, and check both my gauges and the tanks visually, just as I have been instructed, but is one really more reliable than the other? My thoughts for what they are worth.

1. The FFA requires a fuel gauge in a plane. With how strict they are otherwise, would they really go as far as requiring a pilot to have and use an instrument deemed unreliable?

2. In primary training the number one lesson we are given in battling spacial disorientation is trust your instruments!!! They will save your life, and they are more reliable than you are when encountering IMC conditions. IFR is then completely based on "tech" to make it work and keep it safe. Yet the fuel gauge is the one we are told to be careful of.

3. We can make GPS systems that will pinpoint your location within feet if not inches, glass panels are in all the new planes, and in many cases computers can fly safer than a person. But people can't perfect a simple way to measure fuel in a tank?

4. I forget the exact crash, but I saw a documentary on an incident in the late 70's where a commercial airliner ran out of fuel because the pilots became preoccupied with a faulty landing gear light. The big lesson was pilots and crew need to communicate better, and all the flight crew must have the authority to speak up when safety as an issue. But the specific gauge they said the pilot ignored was the fuel gauge, and looking at it would have stopped the crash.

5. Everyday I get in my car and drive. When the little gas pump shows up, I know I need fuel, and I stop when possible. I have mentally checked the fuel gauge to the refuel amount, and it seems very accurate. I know it is not an exact comparison on the safety basis. But that thing works, and millions of people every day trust that fuel gauge. The one in the plane is not just as good?

If this is a stupid topic, I apologize. I do realize the added steps that are taken, and the additional double checks are in the name of safety, so do them. I also realize that pilots still run out of fuel, which I am not sure I understand why. So where did all this fuel gauge mistrust come from?
 
The fuel gauge is only required to be accurate when the fuel tank is empty. Does that make you feel better..?? :lol:

I don't need a gauge. I know the fuel burn per hour and I know how much fuel I put in the tanks.

The plane I fly for a living has really accurate fuel gauges, and yes I trust them. For back up go back to fuel burn per hour and amount put in.

#4. The crash I think you are referring to happened because no one flew the plane. While trouble shooting the landing gear not safe light the plane descended into the ground because no one was paying attention.
 
The question interests me too. I've heard it said that some pilots put her down for fuel reasons with one tank still full. Forgot to switch. Very expensive glass systems to replace perfectly good steam gauges, but no reliable fuel gauge? I'm too green to have answers. I have only questions.
 
It's about redundancy. Your fuel timing presumes known consumption. There are several kinds of faults that might change that.

Your car gauges suck, too. They just don't include the reserve.
 
They do make very advanced fuel gauges. It's a fuel flow indicator and tells you to within a .1 of a gallon how much you have used. It will not tell you if you have a leak in the tank or give you a good answer if you don't tell it how much fuel you had in the tank to start with. Tanks leaks are very uncommon and fairly easy to find in a preflight so it mostly comes down to making sure the inputs are correct to the sensor.

I have learned to trust the fuel gauges in that if they read lower then I expect I believe them completely. If they read higher then I expect I go with my expectations for fuel usage.
 
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Yea....a fuel totalizer is pretty much dead on.
 
I go by fuel burn per hour and add fuel for that trip with an hour reserve. The totalizer is my best fuel friend. I use the fuel gages to verify no inflight leaks, and when to switch tanks. Keeps me from running out of gas..... I hope.
 
Fuel gauge technology may have changed over the years, but that technology has not been adopted, much less retrofitted, into the light aviation sector. What most of us do for better data is install a fuel flow instrument.
 
I rely principally upon my totalizer and my detailed records of fuel loaded and quantities when tanks switched. The fuel gauges are a rough-cut backup - if they read lower than expected, I would presume I had less fuel than expected and divert for fuel. Has not happened yet, though.

My totalizer (a JPI) has typically been correct to within a tenth of a gallon- I regard a good fuel totalizer as a significant and important piece of safety equipment.
 
My fuel gauges with the Dynon EMS in my RV7 were dead on. I also had fuel flow and totalizer. I could tell you within 1/10 of a gallon how much fuel I had at all times. To bad you can't retrofit the Dynon into a certified airplane. Don
 
drotto said:
5. Everyday I get in my car and drive. When the little gas pump shows up, I know I need fuel, and I stop when possible. I have mentally checked the fuel gauge to the refuel amount, and it seems very accurate. I know it is not an exact comparison on the safety basis. But that thing works, and millions of people every day trust that fuel gauge. The one in the plane is not just as good?
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The car fuel gauges do have some degree of error. Try driving on a gradient or a road which has a slight tilt, your fuel gauge may show a slight increase in fuel and then come back one notch. Moral of the story best is to like you mentioned visually check the tanks of the plane and make sure the fuel gauges are working. An instructor once said that the fuel gauge can in most cases can be trusted only when they show an empty tank, in other cases just make sure it shows some reading. I had a full tank once and the fuel gauge showed 75% full.
 
davef22 said:
The car fuel gauges do have some degree of error. Try driving on a gradient or a road which has a slight tilt, your fuel gauge may show a slight increase in fuel and then come back one notch.
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In addition, look how many of the newer electronic ones are automatic adjusting. They don't give you "actual" fuel in the tank, but recalculated on how you're driving and the "relative" amount of fuel left.

I've had a few cars like that. Drive it like you stole it for a bit, then slow it wayyy down and watch the gauge actually come up a little bit.
 
Fact and Fiction - some maybe, some newer ones maybe not... but most older planes I have flown IE- factory mooney's, Grummans, cennsas and pipers, all seem to be suspect to a point that I would never jump in the plane look at the gage and say "Hey I have 1/2 of a tank" then go fly using endurance based on that.

My opinion - Always dip the tank (even the AA5B can use a calibrated dip stick) and do you own endurance calculation and fly by time, not what the little gauge thing says. Unless it drops to zero during the flight, then I would question it.

I have had a few rental planes still show 1/4 or 1/2 tank when it was empty and a few would change to zero when I tap on them abruptly.

And of course, if you have a totalizator that's a different story, but we were talking gauges..
 
The gauges in my Tampico are nearly dead on. They do tend to OVER estimate how much you have once you go below 1/2. It sucks, but it is CONSISTANT. I combat this by never letting the tanks get below 1/4 full. That is a bit over an hours worth of fuel for me.

As for sticking the tank, no stick needed! The insides of the tank have built in visual markings. Easy to read at night with a flashlight. Harder during the day, but a simple white sheet of paper reflects enough light to be able to read them even on the brightest days.

Score 1 for 1980's-1990's technology!

Jim
 
My Taylorcraft fuel gauge was dead on accurate.....from 1946!

images


Sorry, looks like a "Cub."
 
In spite of the fact that the Piggy has fuel gauges that have always been very accurate and reliable, I still use the graduated stick that I made. Unless I have just filled the plane with fuel, I stick the tanks before every flight, unless it is a quick stop at which I know that I have used very little fuel and the fuel gauges confirm it. I also take fuel samples before almost every flight.

With a modern airplane, if you keep fuel going to the carburetor, the chances of power loss is extremely remote. If OTOH, you are cavalier in your fuel management attitude, your chances of having power loss escalate accordingly.

As far as the test goes, yes the FAA only requires fuel gauges to be accurate in indicating an empty tank. In practice, however, it doesn't take much imagination to understand how low that threshold really is.

Keep plenty of fuel in that aircraft folks.
 
Unit74 said:
Yea....a fuel totalizer is pretty much dead on.
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They are very accurate with one caveat - they have no idea how much fuel you started with outside of what you yourself told it. So if you put the wrong number in to begin with all bets are off and in fact having so much confidence in the accuracy of the instrument can draw you into a trap because it's only as smart as the idiot who programmed it.

Despite the inaccuracy of standard fuel gauges they do provide a valuable eye on the situation. If you look at your gauge and ask yourself "Why is it doing that?" then it's probably a good time to find a place to land and make sure you put that fuel cap back on or that you aren't venting or leaking fuel anywhere else.
 
MBDiagMan said:
As far as the test goes, yes the FAA only requires fuel gauges to be accurate in indicating an empty tank. In practice, however, it doesn't take much imagination to understand how low that threshold really is.
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Nope. The law requires that the gauge reads Empty at the unusable fuel level. It also requires that the airplane have a fuel gauge for each fuel tank that enables the pilot, while seated at the flight controls, to be able to determine the amount of fuel in each fuel tank.

That's not word-for-word, but it's close enough. A dead gauge does not meet the requirement. And the law does not stipulate any parameters for accuracy, but it contains enough verbiage to force one to have a functioning gauge.

Aircraft fuel gauges are usually floats on pivoted arms that move through an arc. That right there introduces error due to the sine effect. Then we have large, shallow tanks that are hard to measure just because the attitude of the airplane can have the fuel sitting somewhere other than level, and the fuel sender might not (usually doesn't) have its float in the middle of the tank. Then those little panel gauges are driven by tiny electromagnets, and the gauge's indicator needle bearings get old and dry and dirty, or worn from vibration.

There are capacitive fuel gauge systems available. I've seen them in Cessna 210s. But I think they still use a single probe, while the airliners use multiple probes across the tank and sum the outputs to get an accurate reading no matter what the fuel is doing in there (unless you're inverted, in which case you aren't much concerned with remaining fuel).

Dan
 
On my Archer the left fuel gauge seems to be pretty accurate. The right gauge seems to read more fuel then I think should be in there until it gets below about half empty then it seem to get closer.

I just plan 10gph burn which is higher than real world, use over an hour for reserve, and keep myself diligent about switching tanks on time. Never assume the gauges are accurate.
 
To the OP:
(and to get back on topic)
I asked similar question and have stated that there is ZERO reason, EVER to run out of gas...with one exception: the fuel leak that develops in-flight.

I'd love to be able to trust my gauge. I also wouldn't give up my routine of visually checking.

When I learned to ride motorcycles, I was taught to visually check. I quite doing that once I learned to trust my odometer and that it matched the gauge. I set trip meters A and B for my ride accordingly (A is set per day and B is set per fill-up). I once continued riding for several miles on reserve, sweating bullets as I was hoping there was a gas station 'just up the road a little'.

Back to airplanes: I don't have a reason they aren't accurate, but I tend to glance at the gauges during XC flights in my C-172S and they seem decently accurate, though the scale is very rough. I still land based on hobbs time, as it provides a little more cushion and accounts for the idiocy in working the throttle and mixture controls. I do not have a totalizer :mad:.
 
The gauges on my Stinson are spot on, my 185 so so, all the turbines I've flown ain't using float gauges and are dead nuts on.
 
Unit74 said:
Yea....a fuel totalizer is pretty much dead on.
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Sometimes in some airplanes. But a week ago while ferrying a plane to FL the totalizer was telling me I had almost two hours of range left when the low fuel warning in one tank came on. Not in the mood to bet on either, I stopped and had less than :45 left in the tanks. Turns out the totalizer had been "ball parked" but never refined with real world data.

Lesson learned. Never assume unless you have personal experience with the particular aircraft.
 
I'm aware of a certain PA-28 that has two fuel gages, each marked 'right tank'. Is that a bad thing?

They are both about the same as most light plane gages. Full is full, empty is empty, anything in between means that your fuel is between empty and full.
 
My Lincoln is currently telling me I'm 48 miles to empty.
If my Mustang did that I wouldn't have run it dry any of 6 times.
 
mikea said:
My Lincoln is currently telling me I'm 48 miles to empty.
If my Mustang did that I wouldn't have run it dry any of 6 times.
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Mustang as in car? Do useful load issues keep you from filling? :dunno: :D
 
Zeldman said:
The fuel gauge is only required to be accurate when the fuel tank is empty.
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That's a popular idea, but it is not true. While 14 CFR 23.1337(b)(1) does require that they read empty "when the quantity of fuel remaining in the tank is equal to the unusable fuel supply determined under Sec. 23.959(a)", that is not the same as saying they must be accurate only at that point. If you read 14 CFR 23.1337(b) in its entirety, you'll see that "There must be a means to indicate to the flightcrew members the quantity of usable fuel in each tank during flight." If it's accurate only at "empty", then there is no "means to indicate to the flightcrew members the quantity of usable fuel in each tank during flight" and it does not meet FAA requirements.
 
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The P-92 I was flying had gauges that weren't even accurate when the tanks were bone dry. You fill the tanks, visually check to make sure they are full, chuckle with amusement at the gauge antics while you fly, and make sure you are on the ground in 4 hours.
 
Dan Thomas said:
Nope. The law requires that the gauge reads Empty at the unusable fuel level. It also requires that the airplane have a fuel gauge for each fuel tank that enables the pilot, while seated at the flight controls, to be able to determine the amount of fuel in each fuel tank.

That's not word-for-word, but it's close enough. A dead gauge does not meet the requirement. And the law does not stipulate any parameters for accuracy, but it contains enough verbiage to force one to have a functioning gauge.

Aircraft fuel gauges are usually floats on pivoted arms that move through an arc. That right there introduces error due to the sine effect. Then we have large, shallow tanks that are hard to measure just because the attitude of the airplane can have the fuel sitting somewhere other than level, and the fuel sender might not (usually doesn't) have its float in the middle of the tank. Then those little panel gauges are driven by tiny electromagnets, and the gauge's indicator needle bearings get old and dry and dirty, or worn from vibration.

There are capacitive fuel gauge systems available. I've seen them in Cessna 210s. But I think they still use a single probe, while the airliners use multiple probes across the tank and sum the outputs to get an accurate reading no matter what the fuel is doing in there (unless you're inverted, in which case you aren't much concerned with remaining fuel).

Dan
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If you don't consider the unusable level to be essentially an empty tank, you might want to think long and hard about your fuel management approach.
 
This comes up time and time again.

Everything has limitations.

Here is an example of the ever so common resistive float sender in typical single engine Cessna.

#1 The stops on the transmitter are FIXED and not adjustable on this Leigh sender.

#2 Typically senders are manufactured in a manner to PREVENT the float & arm from resting on the tank walls

What does this all mean? Assume the system is working correctly.

#1 The gauge will read zero with the UNUSABLE fuel in the tanks AND however many USABLE gallons it takes to lift the float off the empty stop.

#2 It will read FULL with the tanks filled and several gallons drained/burned/leaked until the float moves off the FULL stop.


What do you see in the cockpit? "Gaugable" fuel.




(This float is approx. 3 feet from the inboard tank wall. With dihedral in play its going to take probably more than 2 gallons to raise it. 0.5 gallons is the legal unusable)





There is no way to calibrate these. If the gauge reads zero with float against the empty stop and reads full on the other, that's all there is. The system may or many not read reasonably accurate at 1/4, 1/2, 3/4.

Maintenance data may not even exist on testing and/or calibration the fuel quantity system, leaving you and your mechanic shooting from the hip.

On this particular airplane, there is no data on what the resistance range should be of the sender nor is there any data on what voltage range should operate the gauge. Its is a very simple system but this data would help pinpoint issues.

On this particular airplane Cessna really built a mixed bag with Stewart Warner fuel quantity system on the right tank and a Leigh on the left. They do not operate on the same resistance ranges so you cannot use the "good" one to troubleshoot a bad one.

None of this addresses inaccuracies caused by buss voltage. The gauges won't read the same with alternator "OFF" and the battery supplying approx. 12.7 volts to the gauges and alternator "ON" where they get around 14 volts.





I'd be willing to bet 0.5 gallons doesn't even touch the float. The other sender looks to be at approximately the same height as the one shown above.
 
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That and given the fact you're fueltank is shaped like a dishpan and hence slight changes in pitch/roll will slosh fuel around doesn't help.

Years ago we gave up on float style senders on our fire engine water tanks and got much better results. I understand there's a difference between water and fuel, but there are solid state fuel level designs. Even having 4 or 8 discrete fuel levels would be a better idea.
 
Typical fuel gauges are only useful as a rough comparator to your fuel totalizer reading so you can determine if you are drawing from the correct tank (a Comanche will draw from the main when Aux is selected if the vent for the Aux is clogged) or if you have a gross fuel leak.
 
drotto said:
Is there truly validity to this idea,
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You betcha.



drotto said:
...how much can a new pilot trust the gauges in front of them.
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You can only believe the amount of air in the tank, not the amount of fuel, and only if there's more air than you planned to have at any given check point.

drotto said:
1. The FFA requires a fuel gauge in a plane.
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The FAA not FFA. Attention to detail is required when fuel planning.

drotto said:
With how strict they are otherwise, would they really go as far as requiring a pilot to have and use an instrument deemed unreliable?
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It's a conservative backup for your flight log.

drotto said:
2. In primary training the number one lesson we are given in battling spacial disorientation is trust your instruments!!! They will save your life, and they are more reliable than you are when encountering IMC conditions. IFR is then completely based on "tech" to make it work and keep it safe. Yet the fuel gauge is the one we are told to be careful of.
Click to expand...
Those who don't learn from history are doomed to repeat it.

drotto said:
3. We can make GPS systems that will pinpoint your location within feet if not inches, glass panels are in all the new planes, and in many cases computers can fly safer than a person. But people can't perfect a simple way to measure fuel in a tank?
Click to expand...
Hey, go for it! Let us know what you come up with. :)

drotto said:
4. I forget the exact crash, but I saw a documentary on an incident in the late 70's where a commercial airliner ran out of fuel because the pilots became preoccupied with a faulty landing gear light. The big lesson was pilots and crew need to communicate better, and all the flight crew must have the authority to speak up when safety as an issue. But the specific gauge they said the pilot ignored was the fuel gauge, and looking at it would have stopped the crash.
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United at PDX. The flight engineer tried to tell the captain in too wishy-washy of a way that assumed the captain could infer the danger instead of simply saying, "We're gonna run out gas real soon, YOU IDIOT!"

drotto said:
5. Everyday I get in my car and drive. When the little gas pump shows up, I know I need fuel, and I stop when possible. I have mentally checked the fuel gauge to the refuel amount, and it seems very accurate. I know it is not an exact comparison on the safety basis. But that thing works, and millions of people every day trust that fuel gauge. The one in the plane is not just as good?
Click to expand...
Airplanes store fuel in shallow tanks in the wings, usually. They sit for long periods and float mechanisms corrode.
drotto said:
So where did all this fuel gauge mistrust come from?
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:dunno: Experience?

dtuuri
 
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