Fuel gauge accuracy

Jcook1

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James
I am the proud owner of a 1966 Cessna 150f. I'm wondering if there is a requirement for how accurate a fuel gauge has to be. Maybe a strange question but I have my PPL check ride coming up soon and wouldn't want it to be a problem. One of the tanks reads empty until you reach half tank then it comes up and reads accurate from there. I'm guessing a sending unit problem. Since a fuel gauge is required per FAR, I'm wondering if there are any specific accuracy requirements or just a no go? Side note, will be flying the check ride with less than half a tank due to gross weight issues (large examiner & student) lol but still want it right.
Blessings, James
 
This is a question which has been debated heatedly in forums like this. Here's the relevant regulation:
(b) Fuel quantity indicator. 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 [Sec. 23.959(a);]
(2) Each exposed sight gauge used as a fuel quantity indicator must be protected against damage;
(3) Each sight gauge that forms a trap in which water can collect and freeze must have means to allow drainage on the ground;
(4) There must be a means to indicate the amount of usable fuel in each tank when the airplane is on the ground (such as by a stick gauge);
(5) Tanks with interconnected outlets and airspaces may be considered as one tank and need not have separate indicators; and
(6) No fuel quantity indicator is required for an auxiliary tank that is used only to transfer fuel to other tanks if the relative size of the tank, the rate of fuel transfer, and operating instructions are adequate to--
(i) Guard against overflow; and
(ii) Give the flight crewmembers prompt warning if transfer is not proceeding as planned.
There are those who say that subparagraph (1) means the gauge must be accurate only at zero. However, if you read the regulation carefully, you'll see that this subparagraph only defines the specific conditions under which it must read zero ("during level flight when the quantity of fuel remaining in the tank is equal to the unusable fuel supply "), not that it may be inaccurate at other fuel levels. In fact, the paragraph of which that subparagraph is a part specifically says "There must be a means to indicate to the flightcrew members the quantity of usable fuel in each tank during flight." Since your fuel gauge does not do that, it does not meet the regulations governing fuel gauges, that gauge is considered "inoperative" and per 91.213, your airplane is unairworthy until it is repaired.

As for a practical test, based on things I've seen examiners do in the past, I imagine that if you get in that plane with a DPE and turn on the master switch, and that gauge reads "zero" with the tank full, the examiner is going to get a gold-plated opportunity to see by your actions whether you understand that rule. If you don't abort the flight at that point, you are almost certain to fail on Area I, Task B:

Task B: Airworthiness Requirements (ASEL and ASES)​
References: 14 CFR parts 39, 91; FAA-H-8083-25.​
Objective:​
To determine that the applicant exhibits satisfactory
knowledge of the elements related to airworthiness​
requirements by:​
1. Explaining—​
a. required instruments and equipment for day/night VFR.​
b. procedures and limitations for determining​
airworthiness of the airplane with inoperative​
instruments and equipment with and without an MEL.
Now, nobody's going to quibble about a gauge which isn't exactly perfectly correct. But if one of your two fuel gauges reads empty when the tank is full down to half-full, you can't reasonably argue that it "indicate to the flightcrew members the quantity of usable fuel in [that] tank during flight" to any standard of accuracy. Further, the operation you describe would fail to indicate a rapid depletion of fuel from that tank until you'd lost 25% of your usable fuel, and that could have serious safety implications.

Of course, if you arrive for the test with half tanks so the gauge will read accurately during the flight test, you can probably get away with it. But I'd still recommend you get that fixed at your first opportunity, and for cryin' out loud, don't give the examiner even a hint that there's a problem with it. And if it does happen to act up on the ride, bite the bullet and put the plane on the ground as soon as practicable (or cancel the flight portion if it happens before takeoff), even if that means discontinuing the ride and finishing it up later. Demonstrating good judgment goes a long way towards convincing an examiner you deserve that Airman Certificate, and can even have a bit of a "halo" effect if you are marginal on some maneuvers.
 
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So, you will be explaining why you are taking off for the check ride even though the gauge reads empty? And claim that the gauge "functions"?

The accuracy of the gauge is not specified.

Get it fixed. Then there is no issue with the check ride or if something like this happens to your airplane.
 
So, you will be explaining why you are taking off for the check ride even though the gauge reads empty? And claim that the gauge "functions"?

The accuracy of the gauge is not specified.

Get it fixed. Then there is no issue with the check ride or if something like this happens to your airplane.

+1 Get it fixed, the examiner may not want to fly,if he's not sure of the amount of fuel available.
 
Can't remember which CFI of mine came up with this: "Does it work? Yes or no."
 
The FAR specifying it must read empty at zero usable fuel is for a very good reason. Some fuel tanks can have several gallons of fuel in them that are unusable due to size/shape. The tank is not empty. But the gauge is required to read empty, because it is fuel that you can't use. If you have 4 gallons of unusable fuel in an oddly shaped tank, would you want the gauge to read 1/4 tank or Empty when you reach that point? Obviously it needs to read empty.

In a 150 and most trainers, it is not large quantity of unusable fuel. So empty usable is virtually the same as empty literally. It's not a difference the gauge can see. So that helps perpetuate the myth that empty = empty and the FAR is stupid. Get in a plane with many gallons of unusable fuel and you'll appreciate that it isn't stupid, it's just misunderstood.

That line of the FAR is for this reason and this reason only. It is not the only requirement and does not negate the line above it.

With that said, fuel level readings in small GA aircraft are notoriously vague at anything other than full and empty. This is compounded by the fuel sloshing around, pitch and roll attitude, etc. So we all know to visually check the levels.

In flight, they're good to recognize something is way off. You'll never get the precision you're accustomed to in your car. You'll never look at the gauge and say "hmm, it looks like I'm 1/8th of a tank below where I should be." But you will see large discrepancies. You'll see it and think "whoa why is that all of a sudden really low!?"
 
Everyone can quote the regulations all day long but it doesn't mean that the maintenance data to accurately check and calibrate them even exists.

Start researching the maintenance/service manuals.
 
Everyone can quote the regulations all day long but it doesn't mean that the maintenance data to accurately check and calibrate them even exists.

Start researching the maintenance/service manuals.

Very true.

The twin Cessna's have a capacitance system which is fairly accurate. The Hughes (Schwiezer) helicopters used a resistance type which is the same as many single engine airplanes. The test on those was a resistance reading from empty to full, and bending the float wire slightly.

The problem with many SE airplanes are the sending units are shot, and the owners are reluctant to replace them.
 
Very true.

The twin Cessna's have a capacitance system which is fairly accurate. The Hughes (Schwiezer) helicopters used a resistance type which is the same as many single engine airplanes. The test on those was a resistance reading from empty to full, and bending the float wire slightly.

The problem with many SE airplanes are the sending units are shot, and the owners are reluctant to replace them.

Agreed.

In my experience they read empty when the tanks are empty, but also read empty with several usable gallons remaining. Buss voltages between engine running and off (alternator on/off) looks as much as a needle width. There are many many variables. Flying weigh and power settings have an impact on level flight pitch, how accurate is the gauge gonna be? The answer is know your airplane and get the gauges to "work". Fuel dipsticks are a great way to determine fuel quantity on the ground, but only if the conditions are repeatable, ramp level, tire and nose strut inflation, calibrated dipstick etc.


Here is the CAR 3 version of the rule.


 
I am the proud owner of a 1966 Cessna 150f. I'm wondering if there is a requirement for how accurate a fuel gauge has to be. Maybe a strange question but I have my PPL check ride coming up soon and wouldn't want it to be a problem. One of the tanks reads empty until you reach half tank then it comes up and reads accurate from there. I'm guessing a sending unit problem. Since a fuel gauge is required per FAR, I'm wondering if there are any specific accuracy requirements or just a no go? Side note, will be flying the check ride with less than half a tank due to gross weight issues (large examiner & student) lol but still want it right.
Blessings, James

It is a "check ride"... I cannot fathom an examiner getting in it with defective fuel gauges.. IMHO...
 
I am the proud owner of a 1966 Cessna 150f. I'm wondering if there is a requirement for how accurate a fuel gauge has to be. Maybe a strange question but I have my PPL check ride coming up soon and wouldn't want it to be a problem. One of the tanks reads empty until you reach half tank then it comes up and reads accurate from there. I'm guessing a sending unit problem. Since a fuel gauge is required per FAR, I'm wondering if there are any specific accuracy requirements or just a no go? Side note, will be flying the check ride with less than half a tank due to gross weight issues (large examiner & student) lol but still want it right.
Blessings, James

http://mcfarlaneaviation.com/Produc...50F&PartNumber=MC0426517-1 &

Read the warning note on that page before buying.

or this guy:

http://www.fuelsenders.com/site/898219/page/364775
 
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Regardless of the regs, that's one of those things I would want to function as advertised.

Fix it.
 
Regardless of the regs, that's one of those things I would want to function as advertised.

Fix it.
Unfortunately, until you fly the plane and compare the gauge reading with what is visually in the tanks, you don't actually know what it is advertising. Sure it might say 1/4 tank on the gauge. Go stick the tanks and see what's really in there compared what the gauge alleges. :confused:

This actually got me thinking. Maybe I'll make a make a cheat sheet to hang under under the gauges. Stick the tanks at the 3/4, 1/2, and 1/4 tick marks and note what the actual gallons are. Also put the usual notes about capacity, usable, minimums, etc.
 
It is a "check ride"... I cannot fathom an examiner getting in it with defective fuel gauges.. IMHO...

they do. airworthiness is part of the pts. i did my oral exam for the comm cert. and the dpe ask about fuel gages. plus its part of the day vfr requirements (having a workig fuel gauge).
 
Maybe I'll make a make a cheat sheet to hang under under the gauges. Stick the tanks at the 3/4, 1/2, and 1/4 tick marks and note what the actual gallons are. Also put the usual notes about capacity, usable, minimums, etc.

That had to remind me of a homebuilt biplane I ferried once:

15272622639_0185dd33dc_z.jpg


IIRC, the gauge read backwards. Quite useless, really.

As an aside, the gauge in my Sky Arrow appears quite accurate, and the tank is translucent so I can compare the really low readings against actual fuel remaining.
 
It is a "check ride"... I cannot fathom an examiner getting in it with defective fuel gauges.. IMHO...
I can certainly imagine an examiner getting in. I can also imagine the examiner getting out again immediately upon discovering the fuel gauge is defective.
;)
 
..........., fuel level readings in small GA aircraft are notoriously vague at anything other than full and empty. This is compounded by the fuel sloshing around, pitch and roll attitude, etc. So we all know to visually check the levels.
Plus, the tanks are three feet square X three inches deep.
 
Thanks for the info. I am planning to have it fixed, I really wanted to verify the accuracy (or lack of) some info I was given. I was quoted the accuracy at zero portion and had my concerns about it. I'd rather have it fixed and good to go before it has the chance to become a problem.
Thanks again for so many responses. James
 
Legal or not, if the fuel gauge could be made to operate as originally intended for a reasonable cost, a non-bold pilot would prioritize its repair.
We know what they say about bold pilots!
 
Just wanted to follow up on my post. Replaced the sending unit and it's working like it should.... Practical check ride scheduled for next weekend, the 25th. James
 
Just wanted to follow up on my post. Replaced the sending unit and it's working like it should.... Practical check ride scheduled for next weekend, the 25th. James

You jinxed me - my left tank sending unit quit a couple weeks. Airplane is going in for annual at the end of this month, so that's on the list.
 
But if one of your two fuel gauges reads empty when the tank is full down to half-full, . . .

So, you will be explaining why you are taking off for the check ride even though the gauge reads empty? And claim that the gauge "functions"?

The issue has been resolved. But I read the problem to be that the gauge read empty from empty to half full. And accurately from half full to full.

Either way, the gauge needed to be fixed.
 
Another "Diamond-ism" that I've been wondering about.

Above Ron quotes the reg on what is required of fuel gauges:

(b) Fuel quantity indicator. 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.

So, here's my question:

The Diamond I've been flying has 20 gallon tanks. But the stock digital gauges only go up to 15 gallons*. When the tanks are full with 20 gallons, the gauges read 15 and they continue to do so until the level drops to below 15. They are dead nuts accurate once they "come off the peg." This is attested to when I top off. The amount of fuel I put in each tank is always within a gallon of what the gauges say I should add. Always.

But...

If the regs require the gauges to "indicate to the flightcrew members the quantity of usable fuel in each tank during flight" then how are these legal? They certainly don't do this for the first 5 gallons out of each tank.

* I think the gauges max out at 15 gallons, it may be 16. It's been a while since I've topped it off. Lately I just put in ten gallons (bringing each tank up to 12 or 13) and go fly for an hour.
 
The FAR specifying it must read empty at zero usable fuel is for a very good reason. Some fuel tanks can have several gallons of fuel in them that are unusable due to size/shape. The tank is not empty. But the gauge is required to read empty, because it is fuel that you can't use. If you have 4 gallons of unusable fuel in an oddly shaped tank, would you want the gauge to read 1/4 tank or Empty when you reach that point? Obviously it needs to read empty.

In a 150 and most trainers, it is not large quantity of unusable fuel. So empty usable is virtually the same as empty literally. It's not a difference the gauge can see. So that helps perpetuate the myth that empty = empty and the FAR is stupid. Get in a plane with many gallons of unusable fuel and you'll appreciate that it isn't stupid, it's just misunderstood.

Even in a 150 there is significant unusable fuel. The airplane has 26 gallons total, 22.5 of which is usable. 3.5 gallons you can't count on, 1.75 per tank, which is more than 13% of the total.

Pilots often don't understand what unusable means. There's a definition somewhere that says it's the fuel that will not get to the engine when the aircraft is in the attitude most critical for flight. In other words, if you're climbing at Vx or approaching power-off with full flaps, the fuel ends up either in the front of the tank or the rear, and in many airplanes, 150s included, the outlet is in the inboard end of the tank, just above the bottom, about halfway between the front and rear of the tank. Steep pitch attitudes mean that there is fuel that isn't going to flow out of that tank in some normal flight attitudes.

Look at the TCDS for the 150: http://www.airweb.faa.gov/Regulator...9938e5586257ed2006b5117/$FILE/3A19_Rev_49.pdf

Others are similar. If you look at the placards next to the fuel fillers (you DO have them, right? They're required) you'll see the total fuel capacity. Compare that with the figures on the fuel selector/shutoff placard, which show usable fuel. The difference is the unusable fuel.
 
Stock fuel gauges suck. Make a dipstick for yoir airplane and use a watch. Funny thing, several years ago I added digital fuel gauges. Accurate to the gallon on each side. I also have an FS-450. When it comes down to go-no go I use my fuel stick and a watch.
 
My fuel gauge was off. I told my mechanic he didn't want to know, but I fixed it by shunting it with a 330 ohm resistor. EI makes a nice digital gauge (though it uses a different sender than most have). After you calibrate it, it does about as well as you can probably expect. Understand that airplane fuel tanks are large shallow pans so there's always going to be a decent amount of error. I actually played with a little Adinio bluetooth AD on my plane in an attempt to see what I could do, but I suspect it won't end up being better than the EI unit.

I always use the most pessimistic view of: my watch, my fuel flow totalizer, and the gauge.
 
It is simple, fuel gauges can only measure GAUGABLE fuel due to the mechanical limitations of the transmitters and the fuel tank design itself, meaning between X gallons and Y gallons they are somewhat accurate. I installed a brand new digital Aerospace Logic gauge last year with two new level transmitters and calibrated it in two gallon increments. Basically, the last two gallons from completely full or empty aren't accurate or may be immeasurable. It also will not read exactly the same on the ground vs flight due to change in pitch attitude, so the airplane was jacked and leveled during calibration per STC instructions.

In this case, the software makes you calibrate in 2 gallon increments and there is no way to manually tell the instrument that the tanks are TCDS placarded 24.5 usable. The highest the gauge ever reads is 24 gallons despite being able to physically get about 25 gallons in it. Unusable in this tank is only .5 gallon. With just .5 added from a dryish tank (drained and sumped), the fuel barely touched the float on the transmitters, IRRC it took around 2 gallons (from dry) to raise the float at all. With all that in mind I told the owner readings above 2 gallons and below 24 should be pretty darn accurate. He's been very happy with it's performance after understanding the limitations of the float type transmitters.

I liked that gauge but hard to justify it when I already have a fuel flow.
 
Sooner or later someone might notice that I revived a two year-old thread to ask an ancillary question instead of starting a new one! :goofy:
 
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