An engine does not care how good your landings are. It cares whether the last thing it drank was avgas or water.
The fuel drain check is the least glamorous item on the walkaround, and it is one of the few pre-flight actions that can stop the engine on climb-out if you skip it. In Kenya, where aircraft sit out under a big diurnal temperature swing and upcountry fuel often arrives in a drum, it earns even more of your attention than the textbook implies.
Here is the uncomfortable truth about water in fuel. It gets in on the ground, silently, while the aircraft is parked and you are asleep, and the only person who will catch it is the one holding the fuel tester before the first flight of the day.
The Short Version
- Water is denser than avgas, so it sinks to the lowest point of every tank and line, which is exactly where the sump and gascolator drains are placed for you to catch it.
- Sample every drain point before the first flight of the day, and again after every refuel, allowing a few minutes for water to settle out first.
- Avgas 100LL is dyed blue and Jet A-1 is clear to pale straw, so colour and smell together are your defence against water, sediment and the far rarer but fatal misfuel.
- The classic trap is a full tester of clear liquid that you read as clean fuel when it is actually all water. Confirm colour, not just the absence of bubbles.
- Kenya's warm days and cold highland nights drive condensation inside partly full tanks, and drummed or hand-pumped fuel at bush strips carries a higher contamination risk than a Wilson bowser.
- If you find water, keep draining until the sample runs clean and correctly coloured. One clear sample is not a pass if the one before it had water in it.
Why water, and why it always ends up at the drain
Start with the physics, because it explains the whole procedure. Water does not dissolve into aviation gasoline in any useful amount, and it is heavier than the fuel, so it separates out and sinks.
That single fact is why your aircraft has drain valves at the lowest point of each fuel tank and at the gascolator, the main fuel strainer sitting at the low point of the fuel system. The designers put the drains where gravity delivers the water, so your job is simply to open those low points and look at what comes out.
The water you are hunting is almost always liquid water sitting as a separate layer, not moisture vaguely "in" the fuel. In a clear tester you will see it as a distinct blob or interface line at the bottom, often slightly domed, with the blue avgas sitting cleanly on top.
Sediment, rust and dirt behave the same way. They settle, they collect at the drains, and they show up in the sample as specks or a haze rather than a clean, bright column of fuel.
How water gets into a Kenyan tank overnight
The most common source is not a dramatic one. It is condensation, and Kenya's climate is unusually good at producing it.
Any air space above the fuel in a tank holds humidity. When a warm afternoon gives way to a cold highland night, that air cools, and the water it was holding condenses onto the cool metal of the tank walls and runs down into the fuel.
A partly full tank has a large air space, so it condenses more water than a full one. Park a half-tanked trainer outside at Wilson or up at Nanyuki through a cold night and you have built a small water factory by dawn.
The second source is the fuel itself. Water can arrive already mixed in from a storage tank or, far more commonly upcountry, from a drum that has been sitting in the sun and rain, hand-pumped into your tanks without a proper filter.
The third source is ingress. A perished fuel cap seal lets rain past during a wet-season downpour, and a puddle of rainwater sitting around the filler neck can be drawn in the moment the cap is cracked open.
None of these announce themselves. They are why the sampling routine is a habit tied to events, the first flight of the day and every refuel, rather than something you do when you happen to remember.
Doing the check so it actually finds water
Use a clear fuel tester, the plastic cup or tube with a probe that opens the drain valve. A metal cup you cannot see through defeats the entire point, which is to look at the fuel.
Draw a generous sample from every drain the aircraft has. On a typical high-wing trainer that means both wing tank sumps and the gascolator, and many types add extra drains you will only find by reading the checklist for that specific aircraft.
Then actually inspect the sample against good light, and check four things every time.
- Water: look for a separate layer, blobs or an interface line sitting at the bottom of the tester.
- Sediment: look for specks, rust or cloudiness that has no business in clean fuel.
- Colour: confirm it is the blue of avgas 100LL, which proves it is fuel and proves it is the right grade.
- Smell: avgas has a distinct sharp petrol smell that is nothing like the kerosene smell of jet fuel.
Now the trap that catches tired and rushing pilots. A fuel tester filled to the brim with clear, still liquid and no visible bubbles looks reassuringly like a clean sample.
It can be a tester full of water. Water is clear, and with no blue fuel above it to form an interface line, there is nothing to see except a full cup, which is exactly why colour is the check that saves you and clarity alone is not.
If you are ever unsure, add a known drop of fuel or tip the sample and watch how it behaves, because water and avgas separate and move differently. When in genuine doubt, drain more and keep looking rather than talking yourself into a launch.
When you find water, and when to keep draining
Finding water is not an emergency and it is not a reason to panic. It is the check doing its job, and the response is simple: keep draining.
Drain the affected point repeatedly until the sample runs clean, bright and the correct blue, with no trailing water at the bottom. If a wing has taken on more than a trace, rocking the wings gently and lowering the tail can move trapped water toward the drains so it can come out.
One clean tester in the middle of a run is not a pass. You want the sequence to end clean, which means the last sample you take is the one that has to be clear of water, not just any sample along the way.
If water keeps coming and will not clear, that is your answer for the day. An aircraft that will not give you a clean fuel sample is telling you it is not ready to fly, and forcing it is how a routine training sortie becomes an engine failure after takeoff with a full logbook of reasons you should have known better.
The rarer killer: fuel that is the wrong fuel
Water is the everyday threat. Misfuelling is the rare one, and it is far more lethal because a piston engine fed jet fuel can start, run on the ground, and then fail once you are airborne and loaded.
This is why the colour and smell check is not a formality. Avgas 100LL is blue and smells of sharp petrol, while Jet A-1 is clear to pale straw and smells of kerosene, and those differences are obvious the moment you actually look and sniff.
At a busy Wilson or upcountry apron where turbine safari aircraft and piston trainers share the same fuellers and hoses, the assumption that "they put in the right stuff" is not a safety system. Your sample is.
The habits that keep water out in the first place
Sampling catches water. Good habits reduce how much there is to catch.
Filling the tanks after the last flight of the day shrinks the air space where overnight condensation forms, which is genuinely effective against the highland-night problem. The trade-off is weight, because full tanks the next morning may not suit a hot-and-high departure or a loaded weight and balance, so it is a judgement, not a blanket rule.
Upcountry, treat drummed fuel with respect. Let a freshly refuelled aircraft stand for a few minutes so water can settle before you sample, insist on proper filtration where it exists, and be more suspicious, not less, of fuel that has been sitting in a drum through a rainy-season week.
Keep fuel caps and their seals in good order, and clear standing rainwater from around the filler before you open it. These are small, boring habits, and they are the difference between finding a trace of water and finding a slug of it.
Frequently asked questions
How often should I sample the fuel?
Before the first flight of the day, and again after every refuel. After refuelling, give the fuel a few minutes to settle so any water has time to sink to the drains before you take the sample.
What colour should the fuel be?
Avgas 100LL is dyed blue. Jet A-1 is clear to pale straw. Confirming the blue colour proves both that the sample is fuel and not water, and that it is the correct grade for a piston engine.
I drained a full clear sample with no bubbles. Is that clean fuel?
Not necessarily. A tester full of clear liquid with no blue tint can be all water, because water is clear and there is no fuel above it to form a visible interface. Check for the blue colour, not just the absence of bubbles.
Why is water in fuel worse in Kenya?
Kenya's warm days and cold highland nights drive strong condensation inside partly full tanks parked outside, and a lot of upcountry fuel is stored and pumped from drums rather than a filtered bowser. Both raise the odds of water in the tanks.
Key Takeaways
- Water is denser than avgas and sinks to the drains, so opening every low point and inspecting the sample is how you catch it before it reaches the engine.
- Sample before the first flight of the day and after every refuel, allowing settling time after fuelling.
- Check water, sediment, colour and smell every time, and never read a full clear tester as clean fuel without confirming the blue of avgas.
- If you find water, keep draining until the sample ends clean and correctly coloured. Fuel that will not clear is a no-go.
- Kenya's diurnal temperature swing and drummed upcountry fuel make this check matter more here than the textbook average, so top up overnight where sensible and treat drum fuel with suspicion.
AngaBrief's pre-flight assessment gives the fuel check a place to be recorded rather than assumed, prompting you to confirm you sampled every drain and what you found before you sign the flight off. It does not inspect the fuel for you and it does not clear the aircraft. That judgement stays with the Pilot in Command and the instructor.