Fuel Gauge Calibration

[8144]

One fuel gauge is pretty accurate, the other won’t go past 3/4, but does show empty when it is....So, is there a procedure to calibrate the gauges? Where do you purchase new gaskets? Thanks in advance for your help.

[6896]

I calibrated the gauges for my tanks when they were removed from the aircraft by filling with water. I weighed the water and back computed the volume for each setting on the mechanical gage. The FAA regulation 23.1337(b)(1) requires that "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)."

The round Buna-N gasket to seal the gauge against the protruding boss from the tank is available from AC Spruce as Rochester P/N 15-4. See attached drawing. Carefully tighten the 1/4-20 screws in opposite corners to avoid cracking the Scott Gauge housing as happened with one of the gauges that I inherited for my C140 restoration project. Once the gage is cracked, it is prone to leakage.

You can make your own round gaskets too using a fly cutter and flat gasket stock. I would use Viton rather than the Buna-N, which you can get from McMaster Carr. Viton provides superior chemical resistance to fuels and oils. Secure the gasket to a board using double stick tape. Then using a drill press, cut the inside hole first, then the outer diameter. You could also sandwich the gasket on the board using a thin sheet of aluminum as a clamping fixture.

There is a square version of the gasket too PN 15-79, but Rochester doesn't usually sell it to private individuals, but in 2012 they did quote me a price of $12.50 each, but for a minimum PO value of 10 pieces or $125 with 2-3 week delivery. I did not take them up on their offer.

Tony

[6896]

Photo of one of my cracked fuel gages, which is a common problem I understand because of over tightening the 1/2-20 screws.

[6643]

The FAA regulation 23.1337(b)(1) requires that "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)."

Tony: Is this what you did? Our planes are not supposed to meet the requirements of part 23. If you calibrate the gauge according to Part 23 standards it will (must) read "empty" when there is roughly 3 gallons left.

§ 23.959 Unusable fuel supply.
(a) The unusable fuel supply for each tank must be established as not less than that quantity at which the first evidence of malfunctioning occurs under the most adverse fuel feed condition occurring under each intended operation and flight maneuver involving that tank.

[6896]

All I did was to measure the volume of water I put into each tank and read what the gauge said for that volume, or vice versa, read the volume of water at a specific gage indication. I did not attempt to compute unusable fuel by running an experiment to determine when the engine would quit with fuel remaining in the tank and fuel lines that could not be scavenged. The only way that I know of to adjust the calibration of the mechanical gage in a C140 is to change the length of or bend the arm attached to the fuel float. All I have are raw data as to what my gages currently read for a given volume of water in the tank at approximately level attitude--that's all.

Does CAR 3 state a fuel gage requirement analogous to FAA Part 23.1337(b)(1)?

[6643]

Does CAR 3 state a fuel gage requirement analogous to FAA Part 23.1337(b)(1)?

Do you mean 4a? That's what the plane was certified under (unless you have a 140A). Here's all it says:

§ 4a.609 Gauge. A satisfactory gauge shall be
so installed on all airplanes as to readily indicate to a
pilot or flight mechanic the quantity of fuel in each
tank while in flight. When two or more tanks are
closely interconnected and vented, and it is
impossible to feed from each one separately, only one
fuel-level gauge need be installed. If a glass gauge is
used, it shall be suitably protected against breakage.

Apparently, Cessna saw fit to differentiate between total fuel and "usable" fuel and added the "No takeoff" range after the fact. If 1/4 of the tank is unusable in takeoff, then a Part 23 gauge would have to show empty with 1/4 tank left. Not very practical. Car 3 and Part 23 systems were redesigned to allow most of that 1/4 tank to be utilized.

Car 3 did get more specific:
§ 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.

3.437 is pretty long winded so I won't post it unless someone wants me to.

[6896]

Thanks John. If I read CAR 4a.609 as it is written, the Cessna 120/140 fuel gage must function and read the fuel quantity in the tank--from empty to full--and that's about it. However, there is no hard requirement about accuracy or how any level in the tank corresponds to engine fuel starvation like under FAA Part 23 and CAR 3. It sounds like the 1/4 tank minimum for takeoff markings that I have seen on some Cessna 140 gages that I have seen is likely the best basis to think about filling up. Interestingly enough, there is no requirement for a placard to this effect under the original Type Certificate A-768, nor could I find such a note in the Operating Manual. In fact, my project came with Scott gages that did not have the 1/4 tank full minimum take off markings.

[6643]

I don't remember the story behind the takeoff restriction. (That seems to happen more frequently these days...) It was after the fact, though, and the planes were produced without the markings, at least the early ones. Hopefully, someone else will chime in.