Fuel Stick - Usable vs Total

Which scale should be on a Fuel Tank Dip Stick

  • Usable

    Votes: 26 59.1%

  • Total

    Votes: 15 34.1%

  • Both

    Votes: 3 6.8%
  • Total voters
    44
S

SONICMASD

Filing Flight Plan
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Oct 21, 2012
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SONICMASD
Hi Everyone,

I'm in the process of making a Fuel Tank Dip Stick for a C-172P and am just wondering if I should go with Total Gallons or Usable Gallons on the stick? I think it should be one or the other because I don't want anyone to get confused and read the wrong number since this airplane is used by lots of pilots with varying degrees of experience.

I see that the FuelHawk brand of sticks does usable fuel only but I think there are pros and cons to both options but what do you all think when it comes to not only seeing how much fuel you have but also how both options relate to calculating weight & balance, fuel burn, and flight planning?
 
The unusable fuel is always there (by definition is its really unusable). It should always be included in your W&B and never in your flight planning. What's the downside to calling that '0'?
 
When I made my fuel stick, I poured 5 gallons at a time and started with an empty tank. I use the measurement to know how much to put in the tank at the pumps and I calculate the usable fuel for flying.
 
Pour 1 gallon and mark the stick (if its enough to show up on the stick). Repeat one gallon at a time until full.

On my airplane the first full gallon that read was 3 gallons so that is the first mark on my stick.
 
FuelHawk uses useable fuel because that’s what matters. When you dip the tank, you’re only worried about how much fuel is readily available for combustion by the engine.

The only time unusable fuel comes into play is during your W&B, and how you would even measure unusable fuel on a stick is beyond me.

I would just spend $10 and buy a stick that’s already made and calibrated correctly for your airplane, no need to make one.
 
Every stick I have seen so far has been usable fuel. If I am doing the weight and balance I know to add in the unusable before calculating the weight.
 
Timbeck2 said:
How would one measure unusable fuel on a stick?
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On a Cessna, drain all the fuel from fuel selector drain and measure the level of unusable fuel left in the tank. Calibrate for zero at that level.
 
Clip4 said:
On a Cessna, drain all the fuel from fuel selector drain and measure the level of unusable fuel left in the tank. Calibrate for zero at that level.
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being certain your aircraft is level when doing that.
 
Clip4 said:
On a Cessna, drain all the fuel from fuel selector drain and measure the level of unusable fuel left in the tank. Calibrate for zero at that level.
Click to expand...
Don't do that. That is not unusable fuel. Unusable fuel is defined as the fuel that will not flow out of the tank to the engine in the most critical attitude, which in most airplanes will be either a Vx climb or a steep, full-flap power-off glide. It's the fuel that won't get to the tank's outlet.
If you drain all the fuel while in a level attitude, you'll be way under unusable fuel level, and if you count on that number you could come to grief.

Here's what the FARs say about unusable fuel:

§ 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. Fuel system component failures need not be considered.

The TCDS for most airplanes lists the unusable fuel. And the TCDS will also likely say that the weight and balance must include the unusable fuel so the pilot ignores it altogether when doing is W&B for flight, since it's already in the empty weight IF the last guy who weighed the airplane did it right.

The TCDS for the 172M has this to say, for example:

Fuel Capacity 42 gal. total, 38 gal. usable (two 21 gal. tanks in wings at +48)
See Note 1 for data on unusable fuel.

So we have a look at Note 1 and see this, relevant to the M model:

Serial Nos. 17261578, 17261445, 17265685 through 17274009
The certificated empty weight and corresponding center of gravity location must include unusable
fuel of 24 lbs. at (+46) through 172M (17267584) or 18 lbs. at (+46) 17267585 and on and full
oil of 11.3 lb. at (-14).

So a dipstick should measure only useable fuel. And the placards near the fuel filler should list TOTAL fuel and the placard at the selector should show usable fuel. I have found many airplanes missing their placards, btw, and while some TCDS don't require capacity placards at the filler, most do. The TCDS for the 172M says this about those required placards:

Model 172I through 172M (except 17261445 and 17261578)
"FUEL
80/87 min. grade aviation gasoline
Cap. 21 US. gal."
 
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Clip4 said:
On a Cessna, drain all the fuel from fuel selector drain and measure the level of unusable fuel left in the tank. Calibrate for zero at that level.
Click to expand...

Undrainable vs unusable ... not the same thing.
 
Clip4 said:
On a Cessna, drain all the fuel from fuel selector drain and measure the level of unusable fuel left in the tank. Calibrate for zero at that level.
Click to expand...
Incorrect. That would essentially be undrainable fuel, which is part of the aircraft’s empty weight.

Use the book value for unusable fuel and be done with it.
 
Ryanb said:
Incorrect. That would essentially be undrainable fuel, which is part of the aircraft’s empty weight.

Use the book value for unusable fuel and be done with it.
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No actually you are incorrect. The fuel remaining in the tanks is the unusable value because the fuel system is a gravity system. The undrainable fuel would be the value if you drained the fuel from the wing sump and the fuel selector drain.
 
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You planning on running it down to the last gallon????
I use the fuel stick to give me a basic idea of how much fuel I have.... Not to a 1/2 gallon accurate measurement.
Buy the damn fuel stick already made for the Cessna :confused:
 
pigpenracing said:
You planning on running it down to the last gallon????
I use the fuel stick to give me a basic idea of how much fuel I have.... Not to a 1/2 gallon accurate measurement.
Buy the damn fuel stick already made for the Cessna :confused:
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i agree and disagree. Don't run it down to the last gallon. But don't buy a damn Cessna.

I need to find a picture of my highly calibrated fuel stick for the RV6.
 
Clip4 said:
No actually you are incorrect. The fuel remaining in the tanks is the unusable value because the fuel system is a gravity system.
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No actually you are incorrect, as been pointed out already by several different posters, plus you chose to ignore the explanation as to why you are incorrect. Unusable fuel is a number determined during certification which involves flight testing in various attitudes such as steep climbs, glides, and slips and skids. Draining the fuel on the ground and seeing what's left, again, is not the same thing as "unusuable fuel". Gravity fed or not is irrelevant.
 
jaymark6655 said:
Every stick I have seen so far has been usable fuel. If I am doing the weight and balance I know to add in the unusable before calculating the weight.
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Scratch that, I am incorrect and have been doing this wrong since my first flight. Might have been taught wrong or instructor never noticed I was doing it wrong. From the 172S manual:

Standard Empty Weight is the weight of a standard airplane, including unusable fuel, full operating fluids, and full engine oil. Basic Empty Weight is the standard empty weight plus the weight of any installed optional equipment.

This means I shouldn't add the weight of the unusable fuel during a weight and balance and having only usable fuel on the stick makes even more sense. Learn something new everyday.
 
dmspilot said:
No actually you are incorrect, as been pointed out already by several different posters, plus you chose to ignore the explanation as to why you are incorrect. Unusable fuel is a number determined during certification which involves flight testing in various attitudes such as steep climbs, glides, and slips and skids. Draining the fuel on the ground and seeing what's left, again, is not the same thing as "unusuable fuel". Gravity fed or not is irrelevant.
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I wanted to point that out, but figured I would be typing in vain, because Clifford can’t ever accept the fact that he’s wrong.

Sorry Clip, but you’re mistaken on this one bud.
 
dmspilot said:
No actually you are incorrect, as been pointed out already by several different posters, plus you chose to ignore the explanation as to why you are incorrect. Unusable fuel is a number determined during certification which involves flight testing in various attitudes such as steep climbs, glides, and slips and skids. Draining the fuel on the ground and seeing what's left, again, is not the same thing as "unusuable fuel". Gravity fed or not is irrelevant.
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No, you are ignoring physics. Fuel cannot drain below the level of the fuel pick ups in the tank. Any value left is unusable regardless if it is consumed by the engine or drained from the fuel selector drain. The fuel selector drain is the lowest point in the fuel system of a 172.
 
Clip4 said:
The fuel selector drain is the lowest point in the fuel system of a 172.
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No it isn’t, the fuel line drain plug and strainer would be the lowest. Fuel has to pass through there before it ever reaches the engine for combustion.
 
Clip4 said:
No, you are ignoring physics. Fuel cannot drain below the level of the fuel pick ups in the tank. Any value left is unusable regardless if it is consumed by the engine or drained from the fuel selector drain. The fuel selector drain is the lowest point in the fuel system of a 172.
Click to expand...

No, you are ignoring physics. What you wrote does nothing to dispute my previous post. The level of the fuel pickups will change with the orientation of the aircraft.
 
Clip4 said:
No, you are ignoring physics. Fuel cannot drain below the level of the fuel pick ups in the tank. Any value left is unusable regardless if it is consumed by the engine or drained from the fuel selector drain. The fuel selector drain is the lowest point in the fuel system of a 172.
Click to expand...

Look here. The fuel outlet on a lot of wing fuel tanks is in about the middle of the inboard side of the tank, roughly halfway between the front and back of the tank. In level attitude, most of the fuel (not all of it) will run out to the engine. But in a steep nose-up or nose-down attitude, fuel will slosh forward or back and the outlet ends up dry with anything less than the published unusable fuel. If you drain the tank as you described, and trust that level as being unusable, you could easily run out of fuel on final or in the overshoot at a time when you think you still have your 30 minute VFR reserve. I've seen it happen twice. Dipsticks calibrated the way you proposed did it.

It's that simple. Anyone who has maintained airplanes and seen the systems understands that.

Many aircraft, as the models were updated, installed outlets at the front and back of the tanks to reduce the necessary unuseable fuel level. That 172 TCDS reflects that, with later serials having a lower figure.

Here's an earlier 172 tank with the single outlet. You're looking down at the top and inboard and aft sides of the tank. The outlet is the little nipple a little more than halfway back on the inboard side:
0526007-4_111.JPG


Now, if you are a little below the published unusable fuel, the engine will get fuel in a steep climb, but if you go into a steep glide, it won't, since there's so much volume ahead of that outlet.

A later 172 tank:

images


Here we're looking at the left tank, at its inboard side. Two outlets, one near the front and another near the rear. Tubes run from these down the front and rear doorposts, connecting under the floor, and so the unusable fuel required is considerably less.
 
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Ryanb said:
No it isn’t, the fuel line drain plug and strainer would be the lowest. Fuel has to pass through there before it ever reaches the engine for combustion.
Click to expand...

Next time you see a 172 look under at the fuel selector drain location and the level of inlet line to the fuel strainer and ask yourself how fluid flows from a lower to a higher level once the fuel is below the strainer. This is why newer Cessnas have a fuel selector drain valve and why older Cessnas are some times retro fitted with them.

Also feel free to search low point 172 fuel system.
 
Dan Thomas said:
Look here. The fuel outlet on a lot of wing fuel tanks is in about the middle of the inboard side of the tank, roughly halfway between the front and back of the tank. In level attitude, most of the fuel (not all of it) will run out to the engine. But in a steep nose-up or nose-down attitude, fuel will slosh forward or back and the outlet ends up dry with anything less than the published unusable fuel. If you drain the tank as you described, and trust that level as being unusable, you could easily run out of fuel on final or in the overshoot at a time when you think you still have your 30 minute VFR reserve. I've seen it happen twice. Dipsticks calibrated the way you proposed did it.

It's that simple. Anyone who has maintained airplanes and seen the systems understands that.

Many aircraft, as the models were updated, installed outlets at the front and back of the tanks to reduce the necessary unuseable fuel level. That 172 TCDS reflects that, with later serials having a lower figure.
Click to expand...

Actually on Cessna 172 there is a forward and aft pick up in each tank to compensate for pitch attitude. That is an advantage of a high wing gravity system vs low wing system requiring pumps.
 
Clip4 said:
Actually on Cessna 172 there is a forward and aft pick up in each tank to compensate for pitch attitude. That is an advantage of a high wing gravity system vs low wing system requiring pumps.
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See my post just above. I edited it and added some pictures. There are many thousands of 172s with single tank outlets. And there are many tens of thousands of other airplanes with single tank outlets.

The FARs describe unusable fuel, and the manufacturer's have to determine and publish it. They use much more complex and reliable methods than just draining the system from the selector valve.
 
Dan Thomas said:
See my post just above. I edited it and added some pictures. There are many thousands of 172s with single tank outlets. And there are many tens of thousands of other airplanes with single tank outlets.

The FARs describe unusable fuel, and the manufacturer's have to determine and publish it. They use much more complex and reliable methods than just draining the system from the selector valve.
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That may be, but in a 172 you are safe to use the method I described.
 
Clip4 said:
Next time you see a 172 look under at the fuel selector drain location and the level of inlet line to the fuel strainer and ask yourself how fluid flows from a lower to a higher level once the fuel is below the strainer. This is why newer Cessnas have a fuel selector drain valve and why older Cessnas are some times retro fitted with them.

Also feel free to search low point 172 fuel system.
Click to expand...

I spent plenty of years maintaining and rebuilding 172s and other airplanes. I am familiar with the system. I also taught college-level Aircraft Systems courses. And I was a flight instructor.
 
Dan Thomas said:
I spent plenty of years maintaining and rebuilding 172s and other airplanes. I am familiar with the system. I also taught college-level Aircraft Systems courses. And I was a flight instructor.
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Good for you. Drain the fuel down below the tank pick up in a 172, pull the tank sump and put what is left into calibrated cans and see what you have left.

As a flight instructor you know that some manufactures warn against slips with low fuel because the fuel ports can be uncovered. Useable does not mean useable under all conditions.
 
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Clip4 said:
some manufactures warn against slips with low fuel because the fuel ports can be uncovered. Useable does not mean useable under all conditions.
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It was already stated that flight testing involving slips is part of determining the usable fuel quantity, which is why it can't be determined on the ground, and your rebuttal was that we don't believe in physics. :loco:
 
Statics vs. Dynamics is a big jump for some people. Regardless of what low level of fuel you can measure with the plane parked on the ground, what you can use in maneuvering flight is different. Because one set of conditions is V=0, 1g vertical down [Statics, the study of objects at rest], and one has both a velocity vector and an acceleration vector in addition to the vertical gravity vector [Dynamics, the study of objects in motion], because the airplane cannot fly other than a straight line without accelerating.

Unusuable fuel should be included in the Empty Weight and associated Empty CG calculations of your plane, somthere is no need for you to "account for it" during preflight planning.
 
Clip4 said:
Good for you. Drain the fuel down below the tank pick up in a 172, pull the tank sump and put what is left into calibrated cans and see what you have left.

As a flight instructor you know that some manufactures warn against slips with low fuel because the fuel ports can be uncovered. Useable does not mean useable under all conditions.
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I have done exactly that, when preparing to remove the tanks. Drain all the fuel out through the system, in level attitude, and then drain the rest out of the sumps. I get about a quart, maybe a quart and a half, out of the sump of each tank. The TCDS says there are four gallons unusable, two in each tank, which is six and a half quarts more per tank than what's left using your draining method.

In a high-wing aircrfaft, with the selector on both, slipping won't matter unless you're below unusable level. In a low-wing it is indeed a bigger risk, so you silp away from the tank you have selected.

I'm not spending time trying to convince you. I just want the rest of the lurkers to know what's right. There's too much myth and misinformation in the aviation community already.
 
The problem I have with this is, why people must do things the hard way. When really nice ones for sale ---> cheap.
 
Tom-D said:
The problem I have with this is, why people must do things the hard way. When really nice ones for sale ---> cheap.
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I am thinking along the same lines of splitting hairs between usable and unusable fuel. If a few gallons is the difference between engine running or not, you've already made some poor decisions.
 
I didn't read the entire pizzing match... but interested to know what OP decided …:p
 
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