172S and R fuel guages.

[Polar742]

Part 23 (and the reg is posted below) only requires that the instrument be calibrated at zero (empty). That means that while it's supposed to tell you how much fuel is in the tank, just because the gauge says 10 gallons doesn't mean you have 10 gallons. The gauge isn't required to be calibrated each gallon...only at zero.

So, no...it's not a myth.

Part 25 planes.........same regulation.

Every part 25 airplane I've flown has had pretty accurate fuel gauges.

It's hard to beat a capacitence (My grammar and spelling functionality have been turned off in my brain, so apologies) system.

 

[minitour]

No, let me help. Note the part in red:

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.
​

The regulation goes on to say:

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);

​

Note that it says "In addition"? This means it's an additional requirement, not the sole one.

Right. There has to be "an indicator" calibrated in appropriate units.

• Layman's terms - There has to be a fuel quantity indicator with marks at gallon points or 1/4, 1/2, 3/4, etc.

That's in part 23 and part 91. Plain enough.

Here's the key though. Note the part I made red.

"(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)"

Note that nowhere does the regulation read that the indicator must be calibrated to read an exact quantity in flight, on the ground, upside down, hanging from a tree, in the hangar, etc. except for at such a time where the quantity of remaining fuel in the tank is equal to the unusable fuel supply as determined under part 23.

It's not a myth.

If you can find the regulation that says the indicator must be calibrated to show 10 gallons or 1/2 tanks or whatever the indicator is set up to display, then you'd have a case. I've looked. A lot...especially lately. There isn't one.

Every part 25 airplane I've flown has had pretty accurate fuel gauges.

It's hard to beat a capacitence (My grammar and spelling functionality have been turned off in my brain, so apologies) system.

Same here, but they only need to be calibrated at zero. That's the reg. Now, whether or not the manufacturer requires more maintenance or calibration per the type certificate of the aircraft and the certification of the instrument is another story.

Going just by the reg, your part 25 airplane only has to be calibrated to say 0 when you're out of usable fuel.

-mini

 

[tgrayson]

Note that nowhere does the regulation read that the indicator must be calibrated to read an exact quantity in flight,

That's an unreasonable expectation, since no gauge measures exactly. They're typically calibrated at a particular point and grow increasingly inaccurate at points far removed from the calibration point. The regulation specifies that the calibration point is zero usable, meaning that the gauge is most accurate as the fuel quantity gets smaller. You're reading some other intent into that paragraph.

The regulation says it must "indicate to the flightcrew members the quantity of usable fuel in each tank during flight," which it would not do if the gauge were not reasonably accurate. The accuracy doesn't have to be defined in the reguations; it may be defined by the courts, or perhaps by maintenance tolerances based on the technology being used to measure the fuel. Again, it's doesn't follow that because the tolerances aren't in the regulation that no tolerances apply.

Moreover, that line of the regulation would serve no purpose if the only thing required was for the gauge to be accurate at zero. And the whole idea of requiring a fuel gauge that's only accurate at zero is absurd. You can recognize that by your engine quitting.

 

[minitour]

That's an unreasonable expectation, since no gauge measures exactly.

One - I didn't say it would have to measure exactliy. I said that the regulation did not require the gauge to read an exact quantity in flight. Just that it is required to indicate a quantity. The only place the regulation requires the indicator to read an accurate quantity is at zero.

However, addressing the "no gauge measures exactly", I'd certainly hope that's not a fact. I would really like to know what my ITT is in each engine...same with my N1 and N2 speeds. I don't want the engines vibrating or burning apart because of an inaccurate instrument.

They're typically calibrated at a particular point

Yes, and per the regulation that point is at the end of the usable fuel or "zero".

and grow increasingly inaccurate at points far removed from the calibration point.

Tough to say that without seeing the mx requirements of the individual instruments and installations. Do you have any you could post or a link to some? I'd enjoy reading about it during my down time (my all-too-much down time lately).

The regulation specifies that the calibration point is zero usable, meaning that the gauge is most accurate as the fuel quantity gets smaller. You're reading some other intent into that paragraph.

No. It means that it is required to be calibrated at zero, just like the paragraph says. When it's at zero, you're out of usable fuel. Period. There's nothing else to read into that paragraph, though I think you might be doing that yourself. It has nothing to do with how accurate the indicator is at other quantities.

The regulation says it must "indicate to the flightcrew members the quantity of usable fuel in each tank during flight," which it would not do if the gauge were not reasonably accurate.

Yes, there must be a gauge and that gauge must be marked in gallons or pounds or fractions "of the tank" (1/2 tank, 3/4 tank), but you'll find no regulation requiring the calibration of the fuel quantity indicator to actually read 1/2 when the tank is 1/2 full.

The only thing that would require that is the mx specs or installation requirements of the specific fuel quantity indicator. 23.1337 doesn't require it. It may happen (reference most transport category planes and even some of the larger part 23 aircraft), but it isn't required.

The accuracy doesn't have to be defined in the reguations; it may be defined by the courts, or perhaps by maintenance tolerances based on the technology being used to measure the fuel. Again, it's doesn't follow that because the tolerances aren't in the regulation that no tolerances apply.

That depends on the installation. Take your typical 172 fuel gauge. What are the mx requirements and tolerances of that gauge? I can only imagine how they're different from a LR-35 or a B-737's gauges. Even different from other part 23 airplanes like a King Air 300 or Citation 525 or 501/551. That doesn't change the fact that the regulation only requires the gauges to be calibrated at zero.

Moreover, that line of the regulation would serve no purpose if the only thing required was for the gauge to be accurate at zero. And the whole idea of requiring a fuel gauge that's only accurate at zero is absurd. You can recognize that by your engine quitting.

I agree 100%, but that's what the regulation says. We don't have to like it, but that doesn't change that the verbiage of the rule only requires calibration at zero.

That is the minimum. Can your fuel gauge tell you when you've got 600lbs in the tank? Sure. Can it tell you when you're down to 1/4 tank? Yes. Can it tell you that you've got 82.6 gallons left? You betcha. Does it have to by part 23 and 25 regulations? No. Only at zero.

-mini

 

[wjmiller3]

We were just talking about this today, all three S model cessna's we have the right fuel guage is bouncing.

 

[JordanD]

We were just talking about this today, all three S model cessna's we have the right fuel guage is bouncing.

What model years are they? The ones in question in my case are 2001s.

 

[wjmiller3]

What model years are they? The ones in question in my case are 2001s.

2000, 2002, and 2005

 

[SpiceWeasel]

Our gauges aren't even necessarily that accurate (CRJ-200, CRJ-900). In a climb it indicates different than in level flight, and in level flight it depends on where the CG exactly is. What is accurate, and what we really use to help us with fuel planning, is the FMS getting the starting fuel quantity. I've even seen the gauges change their reading when sitting empty at the gate to when we've got our pax and bags.

 

[AviatorGator]

the 172 M/N model fuel gauges were a real treat when I flew them

I did however find the R models to be quite accurate and the occasional hiccup was easily fixed by mx when squawked.

The 'Van is always interesting. The gauges were accurate, but the aircraft has to be perfectly coordinated all the time or the fuel in the "high" wing will allow gravity to do it's work and the fuel will flow to the "low" wing.

 

[minitour]

The 'Van is always interesting. The gauges were accurate, but the aircraft has to be perfectly coordinated all the time or the fuel in the "high" wing will allow gravity to do it's work and the fuel will flow to the "low" wing.

That's why you should really consider shutting off both selectors after shut down.

-mini

 

[AviatorGator]

That's why you should really consider shutting off both selectors after shut down.

-mini

I always do. Everytime.

 

[mikecweb]

That's why you should really consider shutting off both selectors after shut down.

-mini

I got a good story bout that...Over beers of course.

 

[AviatorGator]

I got a good story bout that...Over beers of course.

I believe it.

Nice thing in the Caravan is the fuel selector off horn. Another reason I shut the selectors off after the flight was so you could test it when you turn the battery master back on.

 

[minitour]

I got a good story bout that...Over beers of course.

That makes two of us.

-mini

 

[tgrayson]

That doesn't change the fact that the regulation only requires the gauges to be calibrated at zero.

It also requires that they indicate the amount of usable fuel in flight, which you claim is not a requirement. I don't know why you think an English sentence doesn't mean exactly what it says.

BTW, 91.13 doesn't give any criteria for what constitutes careless or reckless. Do you think, therefore, the regulation doesn't exist? What about "congested area"? That's not defined either. Your argument has no merit.

This author also thinks your position is greatly mistaken:

http://www.av8n.com/fly/fuel-gauges.htm

He offers nothing that would persuade you otherwise, but does provide some suggestions about reasonable tolerances.

 

[Holocene]

In our clubs '65 Cherokee, you have to tap on the right-side fuel gauge to get the needle off zero. Yeah, I'm gonna trust that instrument....:cwm27:

 

[AviatorGator]

How about we all go to a fuel gauge like a J3 cub? For those that haven't seen it, It's a wire with a float on the bottom that sticks through the fuel cap. That's idiot-proof isn't it? :banghead:

 

[tgrayson]

Here's a procedure from the FAA Flight Test Guide for Transport Category Rotocraft. The equivalent publication for Part 23 aircraft doesn't detail the procedure, but the requirement for rotocraft is substantially the same.

AC 29.1337. § 29.1337 (Amendment 29-13) POWERPLANT INSTRUMENTS - (Paragraph (b) - FUEL GAUGE CALIBRATION).
a. Explanation. Section 29.1337(b) requires, in part, a means to indicate to the flightcrew the quantity of useable fuel in each tank in flight. Since the flight attitude of a rotorcraft may vary significantly with CG (center of gravity) and airspeed, a standard attitude for calibration of the fuel quantity gauge is needed. In addition, guidelines for gauge accuracy and comments regarding other fuel quantity gauging aspects are offered.
b. Procedures.
(1) Determine the rotorcraft pitch attitudes for most forward and most aft CG at a median gross weight and at an airspeed of 0.9 VNE or 0.9 VH, whichever is less. The mean attitude of the extremes defined above, further adjusted for lateral CG effects, if necessary, define the rotorcraft attitude for fuel gauge calibration.
(2) After establishing the calibration attitude, the requirements of § 29.1337(b) can be accomplished. The aircraft should be placed in the calibration attitude. Add fuel to the filler neck spillover level. Defuel the aircraft in increments corresponding to fuel gauge increment markings or at least 10 increments until gauge zero is obtained. Precautions should be taken during this step to be sure that the fuel transmitter is sensing fuel level and not simply reflecting a physical "STOP" or end point in the system range. The fuel remaining in the tank below the "ZERO" mark must not be less than that amount determined by flight testing under § 29.959. (Otherwise, the zero point must be adjusted upward.) The gauging system accuracy is acceptable when it meets a tolerance of ±2 percent of the total useable fuel plus ±4 percent of the remaining usable fuel at any gauge reading, provided that the gauge indicates zero fuel with unusable fuel in accordance with § 29.959 in the tank. (For a 100-gallon tank this formula would allow a ±6-gallon error at the full level, ±4-gallon error at 50-gallon level, converging to a ±2-gallon error at low fuel with the further provision that the zero mark accurately reflects unusable fuel.)
(3) Certain other aspects of a fuel gauging system need attention in order to minimize fuel exhaustion incidents:
(i) Gauge reading with the aircraft at ground attitude is frequently used by the crew in calculating range, weight and balance, and actual gross weight. Significant gauge errors in either direction during this reading can introduce hazards to the operation of the aircraft. If a calibration at this attitude indicates an unconservative error in excess of 6 percent of the gauge reading, corrective information should be applied adjacent to the fuel quantity gauge or be made available to the crew in other handbook data.
(ii) Flight during hover with maximum rearward wind may introduce significantly different fuel gauge readings. A check should be made to assure that the gauge is either accurate or at least does not read high (unconservative) in this attitude.
(4) Fuel gauging system transmitters which are strictly volumetric measuring devices (float-actuated variable rheostats) introduce a gauge readout error of about 5 percent if calibrated with a fuel temperature of 0° C and subjected to -55° C fuel or +55° C fuel. This error may be minimized by calibrating the gauge with fuel temperature in the middle of the useful range; i.e., 15° C.
(5) Capacitance transmitters have become the standard for most modern fuel systems. These transmitters ordinarily need no temperature compensation since the fuel volume and the fuel dielectric constant vary inversely as temperature changes. The basic capacitance transmitter does not compensate for the different dielectric values to be expected with different type fuels. An add-on capacitance located so as to be submerged in fuel at all times can be devised to automatically compensate for other fuels.

 

[jonny]

If I recall correctly from my CFI days in LEX, the biggest problem was with the "right side fuel sending unit" on the 172r's and SP's. (99's - 2001's) Not sure if it was just the right side, but that seemed to be the one that had the biggest problems. Also, on the 172sp's if you go a long time between overhauls the rear exhaust manifold gasket can corrode (on both sides, the cylinders closest to the firewall) and when you're doing power-off stalls (at altitude, 3000msl for me) the engine essentially chokes itself with too much air and quits. It makes for an interesting lesson, however the engine will run fine on the ground. Your DOM should already know about it, if not ask someone who is in charge of maint. at the place you rent from.

 

[OldTownPilot]

if you fill er up they are full, if the engine pukes its empty, quite accurate