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I learned from this question that a Cessna 152 holds 5.6 litres of fuel that it can't use.

Why would it be designed to hold fuel that it cannot burn, especially since water contamination of fuel is a concern?

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    $\begingroup$ 5.6 liters doesn't seem like a lot of fuel to be worried about... $\endgroup$ – Mehrdad Mar 19 '15 at 21:42
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    $\begingroup$ It's not that I was worried about it, I was confused by it until I read the explanations below. Now it makes sense. $\endgroup$ – FreeMan Mar 20 '15 at 2:11
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    $\begingroup$ unusable fuel is just like unusable toothpaste. $\endgroup$ – kevin Mar 20 '15 at 2:33
  • $\begingroup$ @kevin: So it gets dumped along with the rest of the engine whenever the trip is over? $\endgroup$ – Mehrdad Mar 20 '15 at 2:44
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    $\begingroup$ @Mehrdad: After a flight the tanks can simply refueled. If the wings / tanks have to undergo maintenance, then it may be necessary to drain out the unusable fuel. $\endgroup$ – kevin Mar 20 '15 at 3:50
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Pilot here (with Cessna 152 hours) as well as an engineer who made it most of the way to an Airframe and Powerplant technician license at one point in life.

There are several reasons that conspire to produce 'unusable fuel':

  1. Fuel tanks on airplanes have complex shapes - they include baffles and other walls (with holes in them) to prevent oscillations from building up during in-flight movement. Sometimes these baffles are even engineered into the tanks late in the design cycle as testing finds oscillation issues. These baffles result in 'geometric trapping' of fuel in nooks and crannies.
  2. Drawing fuel from a fuel tank is more complicated in a plane - cars are pretty much flat with minor tilts while airframes can have all sorts of odd and sustained directions that the fuel wants to go due to acceleration. Imagine an extended tilted spiral while descending - that fuel experiences far more lateral acceleration over a longer time period that a car would have. Getting the fuel system to pull fuel out from these different angles is challenging (note that I've picked a non-aerobatic maneuver for which the Cessna 152 of your question is qualified here).
  3. All of this is a cost/complexity tradeoff - solutions exist for 1 and 2 above - fighter jets use fuel bladders that have pressure bladders arranged to squeeze the fuel out so that they can get almost all the fuel even at high G-forces and unusual angles. Aerobatic planes have similarly more expensive and complicated systems than the old and cheap Cessna 152.

There are other considerations as well, but mostly it comes down to whether the plane in question can justify the cost and effort to get every last drop of fuel.

Things get even more complicated in rockets - the moon rockets (properly the Saturn 5 although it was an upper stage specifically) had special extra engines called ullage motors whose only job was to give the stage a little boost in space to push the fuel towards the rear of the rocket so that the main motors could draw fuel and start.

Addendum edit: @supercat below - the definition of unusable fuel is set by federal aviation regulation part 23, section 959 (FAR 23.959) and basically is the worst case under "intended operation" for the particular plane. Since the Cessna 152 is not aerobatically rated, for example, the unused fuel is defined as the fuel level where SOME intended maneuver would be capable of causing a fuel system error. In practice, especially in flat level flight, some of that unusable fuel would be available for use although you are not supposed to plan to use it in your flight planning.

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    $\begingroup$ Ad 2). Actually car may have pretty serious tilt when cutting corners or braking abruptly. Tires have coefficient of friction 0.8-1.0 on dry road, so the effective gravity vector can tilt to some 40-45°. On the other hand airplanes don't have anything capable of producing lateral force comparable to lift, so their effective gravity vector can't till nearly as much and any such forces are avoided by pilots as they cause lot of drag too. In coordinated flight the slip-skid ball is centered and slip-skid ball is just a ball level! $\endgroup$ – Jan Hudec Mar 19 '15 at 7:56
  • $\begingroup$ On the other hand of course the fuel tanks on aircraft are much wider, so any lateral acceleration has bigger effect in them. $\endgroup$ – Jan Hudec Mar 19 '15 at 7:59
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    $\begingroup$ RE #2: "Aerobatically", a spin is an excellent way to move fuel around in the tanks. In fact, a good spin around the CG will move the fuel to the outside of the wing tanks; it'll move anything to the left or right of the CG further to the left or right. A Cessna 152, having gravity feed fuel lines at the wing root, I believe will actually stop running after about 6-8 or so turns, since you've essentially stopped feeding fuel to the engine. It's been a while since I've conducted spin training in that aircraft. And never to the degree of moving unusable fuel around like that. :-) $\endgroup$ – Shawn Mar 19 '15 at 16:03
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    $\begingroup$ @Jan Hudec - good points, I actually had an answer about 3x as long originally and tried to keep it simple. In coordinated flight the acceleration pull will indeed be oriented 'down' to the plane's frame of reference. However, the airframe engineers cannot rely on this for fuel availability calculations because valid maneuvers like side-slips (approved for losing altitude) do not require this coordinated condition. If you always 'step on the ball' and stay coordinated you may have access to some of that fuel which is rated unusable - though I wouldn't count on it for planning! $\endgroup$ – Ezekiel Kruglick Mar 19 '15 at 16:19
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    $\begingroup$ See addendum above. unusable fuel is the worst case - so if you had that much fuel during the worst maneuver (approved for the aircraft) for the fuel system you need to have at least the rated unusable fuel to prevent a problem. $\endgroup$ – Ezekiel Kruglick Mar 19 '15 at 22:12
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especially since water contamination of fuel is a concern?

That's one of the reasons. Fuel floats on water, so water collects in the bottom of the tank, along with other gunk. By not taking the last dregs of fuel from the bottom of the tank you avoid putting this stuff into the engine

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    $\begingroup$ This may be true in some systems but others do try to take the last dregs of fuel from the tank. The fuel pick-up ports in many Cherokees are right by the sump drains (near the lowest inboard point in the fuel tank) so you'll probably wind up getting water before you get avgas. In that system it's the gascolator's job to get rid of (or more accurately, hold on to) water and "gunk" - in fact the gascolator bowl is about half of the unusable fuel capacity in some models. $\endgroup$ – voretaq7 Mar 19 '15 at 6:42
  • $\begingroup$ @voretaq7 I imagine that's why you drain a bit off each tank in a Cherokee during the flight prep. $\endgroup$ – slicedtoad Mar 19 '15 at 13:28
  • $\begingroup$ @slicedtoad Partly, but you check your fuel for water even in tanks that tend to hold more "unusable fuel" because of their design, and those tend to have more sump points right where the unusable fuel collects (e.g. the 800 sump points on some Cessnas with fuel bladders). The fuel pickups are still usually pretty low in the tank, which just means water that moves around while you're maneuvering might slosh out of one spot where it's harmless and get close enough to the pickup to be drawn into the engine. $\endgroup$ – voretaq7 Mar 19 '15 at 16:08
  • $\begingroup$ I would guess that any aircraft that doesn't use a fuel pump (and lots that do) would keep the fuel inputs at the lowest point of the tank so that gravity can reduce the work of the pumps. @voretaq7 has it right. $\endgroup$ – Shawn Mar 19 '15 at 16:09
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Almost any system that moves liquid around will retain a certain volume. In your case, there are various bends in the pipes and some of those bends go up. Yes, it's gravity-fed but notice the shutoff valve on the floor. Now notice the level of the carburetor. Any fuel below the level of the carb bowl is unusable, as we have no way to push it through the rest of the plumbing without more liquid behind it.

If we turn to things with a pump the unusable quantity can go up, a lot. The volume of liquid between the pump and the final destination would be considered unusable for the same reason - once the pump is dry we can't move it along anymore.

One restaurant manager I used to know installed a metered drink dispensing system. All the booze is in the (locked) storage room, the bartenders push a button and exactly one ounce comes out of the tap. The only complaint he had was that the system cost $500 to prime, per item, and he had to keep thousands of dollars of "unusable" alcohol on the books.

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  • $\begingroup$ This is the best answer to the question as asked. If nothing else, the fuel that clings to the walls by surface tension is not usable, so there will always be some. Even if the designers work hard to minimize it, it will not be zero. $\endgroup$ – Ross Millikan Mar 21 '15 at 3:59
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The term "unusable" is a very special definition. It comes from a specific test described in FAA FAR Part 23. It is not the same as the common sense meaning of the word "unusable".

For single engine light planes like Cessna 152, FAA FAR Part 23 requires the available fuel flow to be 125% of max engine consumption. 150% for pure gravity feed systems. At certification test the fuel flow is measured while the level is dropping in the tanks. As soon as the flow rate goes below 125 or 150 % all remaining fuel is declared unusable. This shall be done with the plane supported in climb nose up atitude.

So, the formal term "unusable" means this last amount of fuel might be unavailble in a nose up attitude at full engine power.

In level cruise you can normally empty the fuel tanks to the last drop.

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I pitch in blind here, but I believe in some instances fuel is circulated for heat exchange.

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  • $\begingroup$ Something like the SR-71? $\endgroup$ – fooot Mar 20 '15 at 17:48
  • $\begingroup$ Not in any GA aircraft I know of. Possible in airliners and military aircraft. $\endgroup$ – Skip Miller Mar 20 '15 at 18:01
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    $\begingroup$ Common enough in larger aircraft to have a fuel/oil heat exchanger, cooling the oil & warming the fuel enough to melt any ice crystals that formed in the fuel as it cooled to something close to OAT up at high altitude. The problem being averted isn't water in the fuel per-se, but clogging a fuel filter with the crystals. All that said, this is simply more plumbing in the fuel line, and the dynamics mentioned in other posts wouldn't change all that much with or without the heat exchanger. $\endgroup$ – Ralph J Mar 20 '15 at 23:26

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