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Should you sump fuel tanks when the fuel system is below freezing? the question

should one sump fuel-tanks before flying, if in freezing-conditions?

but this implies another question that boggles me:

Why don't aircraft have "fuel polishing" systems, to remove water from tanked fuel, especially since the cost of having a line block with ice, is so horribly high?

Cruising boats sometimes have such things, & given the peace-of-mind it'd give, why aren't these devices in play, in aviation??

( btw, it essentially is a filter & water-bypass thingy, so the water goes to the bottom of the glass bowl, & the fuel, above that, goes through the filter, if they combine them, or maybe the water-separator & the filter-unit are separate, in sequence, but both are present, before the fuel goes anywhere near the engine... )

I believe a fuel polishing system is run before firing-up an engine.

This removes all the contamination before the engine's fuel-pump even starts.

edit 2:

Marine Diesels, in the stuff above...

Also, this is in light of some events written about years ago...

1 was a small jet that lost power at high altitude due to slush-ice in fuel,

another was a jetliner that smashed after the same problem, ttbomk...

  1. slush-ice in the fuel clogged something in the fuel-system, causing...

  2. a drop in power, so therefore the crew...

  3. increased thrust, which caused ALL the engines to clog/die, so the new recommendation was...

  4. if in that type, at high altitude, in cold, if power reduces unexpectedly,
    then reduce thrust to unclog the lines, & it should clear...

also many apparently have had problems with ice in fuel messing things up for them in piston aircraft...

also I know some are running automotive-conversions,
& if anyone ever has ethanol-blend gas in their aircraft,
& that ethanol has absorbed water, & gets cold, it can separate out,
leaving 2 distinct fluids in the tank:
ethanol-water & gasoline.

( I'm not saying running auto-fuel is legal in an aircraft,
but rather that I know human-nature. )

edit 3:

that link above includes some interesting stuff

also, the whole idea may be bunk,
if the amount of ice-slush in a jetliner's tanks is measured in tonnes, rather than litres,
as no catching-out device is going to handle that kind of volume.

I don't know what volume is involved.

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    $\begingroup$ Depending on what you mean by cruising yachts/boat, the fuels used may differ huuuuugely from what is used in aviation. $\endgroup$
    – Jpe61
    Commented Jan 5, 2021 at 7:55
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    $\begingroup$ A cruise boat is also a heck of a lot heavier than an airplane. $\endgroup$
    – Mast
    Commented Jan 5, 2021 at 19:23
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    $\begingroup$ Interesting question, but it looks like you're writing poetry! $\endgroup$
    – rclocher3
    Commented Jan 6, 2021 at 1:56
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    $\begingroup$ It isn't that poetry is the intent, but rather clarity: brain damage makes thinking hard, ANY means of keeping complicated thoughts clear helps! $\endgroup$
    – Paragone
    Commented Jan 6, 2021 at 20:21

5 Answers 5

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The simple answer is weight/risk/added complexity: the systems are heavier than they are worth for the problems they avoid in aircraft.

While ice blockage has been an issue for aircraft in the past, planes that fly high enough to warrant it have fuel heating systems to avoid this very issue.

The reality is that the chances of water getting into marine fuel are MUCH higher than water getting into AvGas. Water finds its way into airplanes in mainly one way: warm moist air that fills the space in fuel tanks condenses in the cold and the water precipitates out. This often happens over night when planes are not re-fueled after their last flight and left out in the cold. As such its extra important to sump the tanks for the first flight of the day. You can also mitigate this by filling the tanks every night if possible. Pilots are also taught to diligently sump tanks to avoid the issue in the first place.

In my experience, flying in the north east of the US where the air can get humid and the nights can be cool, I've never pulled more than a few bead size drops of water out of the tanks of my archer in the morning. I know of others that have pulled more but not significantly more.

There have been occurrences of a fair bit (hundreds of gallons of water) getting into aircraft tanks but these have generally been caught and handled. On the bright side we have a few nice things in aircraft that make water easy to deal with.

  1. Water is heavier than AvGas so it will pool at the lowest point of the tank which is generally where the outlet/fuel line is connected.
  2. The lowest point of a fuel tank on an aircraft is fairly easy to get to so pulling a test sump of fuel is no problem during pre-flight
  3. It's fairly easy to deal with the fuel once it's in the sump cup on the ramp, chances are you wont anger the EPA all to much.

On the contrary, with marine fuel tanks it can be hard, if not impossible, to access the sump in any reasonable way and once you drain it you run the risk of it getting in the bilge and ending up getting pumped overboard which the EPA will have an issue with.

On a boat you have lots of options for water to get into the tank since there is generally water all around. Maybe it splashed in some gas cans you had on deck, maybe it splashed in during a mid trip re-fuel, maybe it splashed in refueling at a dock, maybe it was raining one of these times or all of these times, maybe the dock kid put the fresh water hose in the fuel inlet in a haste etc. etc. As such fuel/water separators are far more critical on a boat where there is often space and weight to spare and the chance of water getting into the fuel is high.

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    $\begingroup$ My homebuilt has fibreglass fuel tanks (on the wing tips) and I've never had water in the fuel in the 10 years I've had the thing which was always a surprise. I'm not sure why the difference compared to metal tanks. You'd think that metal cold soaked or fibreglass cold soaked would create the same condensation conditions, but apparently not. $\endgroup$
    – John K
    Commented Jan 5, 2021 at 4:29
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    $\begingroup$ @John K: Where do you live? I don't think I've ever had condensation in my fuel tanks, but then I live in an area (US intermountain west) where warm, moist air is a rarity. $\endgroup$
    – jamesqf
    Commented Jan 5, 2021 at 6:24
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    $\begingroup$ @Dave Eastern Canada. With metal tanks in this part of NA you would expect to find drops of water in the fuel from time to time if parked outside. On a preflight of a plane like a 172, I made it a habit to shake the plane at the very start to slosh the fuel in the tanks and wash any water droplets off, then delay checking the sumps until the preflight was done to give the water time to migrate to the low point. You would find water drops that wouldn't appear if you just drained the sumps at the start. $\endgroup$
    – John K
    Commented Jan 5, 2021 at 16:30
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    $\begingroup$ @SolomonSlow I trained out of KDYL with lots of landings at KABE, but the training to not slosh the tanks is what I was always told. It can take a lot of time for the fuel to settle in the tank so I was always told to sump them first before disturbing anything. $\endgroup$
    – Dave
    Commented Jan 5, 2021 at 20:22
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    $\begingroup$ Given the way aviation-safety is supposed to work, I'd be interested in someone putting a quarter-fueled small plane out in the cool, to collect condensation, with a light & encapsulated camera in each tank, sump it in the morn, then shake wings & time the collection of dew in the sump, via the vid: data is better than assumptions, right? Thanks for all these understandings/insights! ( : $\endgroup$
    – Paragone
    Commented Jan 6, 2021 at 20:40
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Sheer volume is the reason.

Aircraft use fuel at an astounding rate.

A Panamax-class container ship is enormous - bigger than the USS Iowa or the Essex class aircraft carriers on display at many major port cities. They burn 9900 litres of fuel per hour.

A 777 burns 7.5 tonnes of fuel per hour, or at 800 g/l for Jet A, that's 9375 litres of fuel per hour.

These burn rates are within 6% of each other.

A 777 would need a polishing plant as big as a Panamax freighter's.

That's a big bag of "not gonna happen". Given that a 777 has complete turnover of its fuel tanks every 24 hours anyway, it simply makes sense to do it on the ground.

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  • $\begingroup$ "a 777 has (a) complete turnover of its fuel tanks every 24 hours" ... meaning the fuel that is in them is completely used and replaced in an average of that time frame? $\endgroup$
    – CGCampbell
    Commented Jan 6, 2021 at 13:01
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    $\begingroup$ CG, I believe Harper simply means, yes, 777s fly continuously (eg back and fore across the Atlantic, or whatever), using all their fuel all the time - they never sit around. $\endgroup$
    – Fattie
    Commented Jan 6, 2021 at 14:50
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Answering from the yacht perspective... the problem here is diesel bug.

Yachts are often powered by low power 1-cylinder diesel engines, and the engine's main use (for many sailors) being to get in and out of harbour, the main power coming from the wind.

As such, a tankful of fuel may last an entire season, and it may not have been fresh in the pumps to start with, in a smaller harbour or marina. Which is long enough to not just absorb moisture, but for diesel bug to grow. This is bacterial growth, and the problem is even greater with the introduction of biodiesel.

Diesel bug turns the fuel to jelly, clogging pipes and injectors, and a failed engine at sea is a safety problem if there is either too little wind, or too much. Fuel polishing can either eliminate diesel bug or keep it out of the engine. (You can also use anti-bacterial additives)

If JP-4 doesn't suffer from "diesel bug", or fuel turnover rate is high enough that fuel isn't lying in the tank for months, this may simply not be an issue for aircraft (though ice is another matter).

I'm taking a wild guess here that jets fill up more than once or twice a year!

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    $\begingroup$ Carnac the Magnificent answers, "propane," to the question, "what fuel will power small outboard motors, and small sailboat auxiliaries in the not-so-distant future?" Maybe not so practical for airplanes though. $\endgroup$
    – Solomon Slow
    Commented Jan 5, 2021 at 20:05
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    $\begingroup$ @SolomonSlow actually quite a few are going electric, with LiFePO4 battery and solar to top it up, like this (local to me) victronenergy.com/blog/2014/01/12/new-blogger-john-rushworth $\endgroup$
    – user_1818839
    Commented Jan 5, 2021 at 20:26
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    $\begingroup$ The technology is advancing. But, for right now, it is limited to prop driven aircraft of limited airspeed, range, and payload capacity. $\endgroup$
    – Dean F.
    Commented Jan 6, 2021 at 3:43
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    $\begingroup$ @DeanF. The reference to going electric was actually in regard to "smal outboard motors, and auxiliaries". But you are of course correct regarding the current limitations wrt electric aircraft propulsion. $\endgroup$
    – user_1818839
    Commented Jan 6, 2021 at 12:59
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Airplanes range from single seat to 600 seaters, and they have varying levels of technical sophistication. Some or all of the following maybe in use if there's a potential fuel contamination (with water) problem:

  • Fuel heaters
  • ice filters
  • Fuel/Oil/Air/Hydraulic fluid heat-exchangers.
  • Checks and crosschecks of fuel by fuel vendor and operator before refueling.
  • physical checks for each tank using purpose built drains plugs.

For airplanes with sweptback + dihedral wings water will collect in the inboard forwardmost part of the wing tanks for tanks following the wing profile shape. Fuselage/body tanks designed to provide a sump-like section that collects water due gravity. (being heavier than fuel)

Airplanes also face icing problems with engine intakes/nacelles/wings and depending on the kind of fuel used, eg Jet, Jet A, Jet A1, JP4, .. . the fuel itself can start freezing depending on the Temperature exposed to, effects of skin friction (due speed through the air), and duration of cold-soak. This happens around -40degC and below - freezing fuel is evidently initially of a soft viscous wax-like consistency.

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    $\begingroup$ that sump-like section of the body-tank is what I meant, but I wonder why that jetliner went-down, if it already had that? So it is the smaller aircraft that don't prevent water/ice in the line from getting past a catch-box, then, it seems... Thx! $\endgroup$
    – Paragone
    Commented Jan 6, 2021 at 20:28
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So, my sailboat's tank hold 20 gals. of diesel, and at the end of each season I top her up with 3-4 gals. For an interesting puzzle you can work out the average age of the fuel in the tank, but the point is that a sailboat tends to carry old diesel in a warm, moist environment for extended periods. Meaning that- even with additives and stabilizers- you have ideal conditions for the growth of microbial life in the fuel. This life (technically known as "goo") will, if left to grow rampant, clog your filters to the point where the engine eventually shuts down from lack of fuel. If it's a windy, wavy day, the goo gets kicked up off the bottom of the tank to kill your engine even faster (on precisely the sort of day you DON'T want to dock under sail).

This goo is what fuel polishers remove, not water or ice (you have a water separator on the engine's filter for water in the fuel, the ice is reserved for my drinks). In fact I do have a fuel polisher myself- I don't keep it on the boat though, as it would only clutter the place up. Its the sort of thing you drag out of the garage only once every 3-5 years or so.

As you can see, the point of a fuel polisher on a yacht is for a completely different set of problems than faced by the typical aircraft.

Have a good one!

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    $\begingroup$ the specific problem being removed ( bacterial guck vs water/ice ) is different, but the Removing Problems Before They Reach The Engine bit is identical in intent, to me. Lots of happy sailing! ( : $\endgroup$
    – Paragone
    Commented Jan 6, 2021 at 20:30

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