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Topic Options
#406575 - 09/17/20 05:48 PM Fuel Discrepancy
Marcos Diaz Offline
Safety Pilot

Registered: 03/06/15
Posts: 47
Loc: Venezuela
Hello everyone, I need help.

You probably have seen me talk a lot about fuel, and that for two reasons. First, in Venezuela very few airports have AvGas, so I must be very sharp on my fuel to reach the next airport with AVGas.

Second reason, from the first flights after my restoration was done, I been noticing that numbers don’t add up, they stilll don’t, I have only one more test to do, but I am going crazy, please guide into what I might be doing wrong.

Tanks were removed, cleaned and inspected, they seem like every other 172 tank. No deformation, no bent. Purchased acrylic calibrated dip stick from spruce. Installed new fuel computer JPI FS-450 and set it to 37 gal usable.

1. when I would top of the tanks, reset computer, and fly, after landing dip stick would show 3 gal less than the computer.

2. When fueling full from the truck, fuel in computer + fuel served according to truck, matches 37 gal.

Today I decided ignore the dipstick and went for another test. Full fuel, reset computer, did a flight, landed with 23.8 Gal according to computer. So now lets drain all fuel from tanks and see how it compares.

Drained both tanks, and had 23.9 gal. Great ? No, I am supposed to have 23.9 + 5 non usable gal, right ?

My tanks have only one draining point, and there was no more fuel coming out of it, from top I could see nothing neither.

Where are those non usable 5 gal ? In corners that I don’t see and wont come out ? In the fuel lines ? Could be that my tanks hold less than indicated ? I have pictures of them if you want to see them.

My next test will be drain all fuel again, fill them, and see how much goes in.

What I am doing wrong ? Please help.

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#406579 - 09/17/20 11:14 PM Re: Fuel Discrepancy [Re: Marcos Diaz]
magman Offline
Gold Pilot

Registered: 10/10/12
Posts: 1463
Loc: buffalo NY
I’ve always considered Usable Fuel as fuel that will get to the engine

In normal fashion ( through Carb).

Draining at Tank Drains is draining Unusable Fuel as well.

This may not agree with others.

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#406581 - 09/18/20 04:20 AM Re: Fuel Discrepancy [Re: magman]
Joeman434 Online   content
Diamond Pilot

Registered: 02/19/14
Posts: 6922
Loc: Ocala, Florida
To get a true indication, you would have to drain the system at the gascolater, since the tank drain is lower than the supply line to the selector valve.
_________________________
Do it right the first time.

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#406584 - 09/18/20 07:25 AM Re: Fuel Discrepancy [Re: Joeman434]
wildduk Offline
Second in Command

Registered: 07/07/17
Posts: 180
Loc: georgia
So are you saying 5 unusable total? So 2.5 a side.
How level is the airplane when doing the calibration of fuel computer?

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#406586 - 09/18/20 10:01 AM Re: Fuel Discrepancy [Re: wildduk]
magman Offline
Gold Pilot

Registered: 10/10/12
Posts: 1463
Loc: buffalo NY
There is a note in TCDS.re unusable fuel.

I believe your W & B data outlines the Cessna procedure too.

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#406588 - 09/18/20 10:23 AM Re: Fuel Discrepancy [Re: magman]
Marcos Diaz Offline
Safety Pilot

Registered: 03/06/15
Posts: 47
Loc: Venezuela
I have been reading about it.

It seems that when Cessna says 42 Gal is including fuel lines and gascolater, which I didn’t drain, so there should be some more to add.

Due to the dihedral wings, and the fuel inlet been towards the cabin, the tank will never be totally full, some air will remain in the outside. Only if you horizontal level the wing can be totally full. I really hope Cessna is including that missing fuel in the non usable.

At the Cessna 170 forum, few guys drained the tanks from the drain sump, and none managed to get in more than 20 gal. Most of them 19 gal.

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#406611 - 09/19/20 10:47 AM Re: Fuel Discrepancy [Re: Marcos Diaz]
Showboatsix Online   content
Gold Pilot

Registered: 12/04/10
Posts: 1884
Loc: KUAO, Aurora, Oregon

At the Cessna 170 forum, few guys drained the tanks from the drain sump, and none managed to get in more than 20 gal. Most of them 19 gal.“

All I could get in was 19.2 & 19.1 after the restoration of N6863A.
_________________________

Note: "Speed and 172 are not used in the same sentence!"


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#406618 - 09/19/20 10:20 PM Re: Fuel Discrepancy [Re: Marcos Diaz]
Flubber4.0 Offline
Gold Pilot

Registered: 08/22/14
Posts: 1241
Loc: blue marble
If you‘d add the exact airceaft type and tank version to the discussion. My 172H is on standard tanks of 2x 19.5 = 39 Gallons total and 2x 18 = 36 Gallons usable. When litering the tank I get rather exact the 39 Gallons. The designator <usable> is the amount of fuel in tanks under which test flight examination at the factory showed restrictions to operations of the aircraft, i.e. limited bank angles or limited acceleration values - if you are down that low you are starting to get increasing danger of fuel starvation for certain maneuvers, it is not a static physical measure of the tank.
_________________________
Never give up, never surrender. Activate the Omega-13.

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#406622 - 09/20/20 06:55 AM Re: Fuel Discrepancy [Re: Flubber4.0]
Marcos Diaz Offline
Safety Pilot

Registered: 03/06/15
Posts: 47
Loc: Venezuela
Sorry, sometimes I think everyone knows my airplane. It’s 172A 1960. 42 Gal total, 37 usable as per type certificate.

Maybe my effort to reach precision in 1960 airplane is pointless, but I am an engineer, and my instinct is to understand as much as I can.

My impression after draining usable fuel is that the airplane will turn into a glider in a matter of minutes, even in level flight.

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#406625 - 09/20/20 08:16 AM Re: Fuel Discrepancy [Re: Marcos Diaz]
C420sailor Offline
Gold Pilot

Registered: 11/18/16
Posts: 1568
Loc: Virginia
Those sticks are accurate within a couple gallons, in my experience. They’re much better for before/after readings (‘oh look, I burned 8 gallons this flight’ vice ‘oh look, I have 14 gallons to empty’).

It’s much more accurate than taking a peek and estimating how much fuel you have left, but I would always trust a computer with calibrated sensors over it.

That said, I wouldn’t trust anything when i’m on vapors...
_________________________
Currently transitioning from the F/A-18E to the C-172M...to a damn desk.

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#406626 - 09/20/20 08:43 AM Re: Fuel Discrepancy [Re: Marcos Diaz]
Flubber4.0 Offline
Gold Pilot

Registered: 08/22/14
Posts: 1241
Loc: blue marble
Originally Posted By: Marcos Diaz
My impression after draining usable fuel is that the airplane will turn into a glider in a matter of minutes, even in level flight.

I don't recall the exact definition, but I think you are wrong. 'Usable' refers to a margin for usual flight conditions not to ZF2E - Zero Fuel to engine and does have a safety margin which is not zero.

With a fuel totalizer and a calibrated (takes a while to adjust the parameter though) fuel flow meter you should be able to get a pretty good view on your actual fuel content. On a friends C172RG we tried it al the way to engineers delight and he now is frequently within 0.1 Liter when refueling averaged over time.


Edited by Flubber4.0 (09/20/20 08:44 AM)
_________________________
Never give up, never surrender. Activate the Omega-13.

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#406628 - 09/20/20 10:22 AM Re: Fuel Discrepancy [Re: Flubber4.0]
Showboatsix Online   content
Gold Pilot

Registered: 12/04/10
Posts: 1884
Loc: KUAO, Aurora, Oregon
I hold 42 (listed) in reality could only get 19.2 & 19.1 in totally empty tanks, = 38.3 gallons total at 8 gallons an hour, at 2 hrs = 16 gallons burned 22.3 left, and at 2 hrs I have to P so I always have more fuel than bladder, and 3 hrs is really going to be a race to the B room!
_________________________

Note: "Speed and 172 are not used in the same sentence!"


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