Water That Comes Back After a Clear Drain
At a private strip near Ilorin you have just taken 95 L of avgas from the operator's 5,000 L bowser into a Cessna 172 (201 L usable). You sump the gascolator and both wing drains. The first sample from the left wing drain is cloudy; after about 300 mL it runs clear. You wait 20 minutes while loading bags, then sump again: the left drain now shows a distinct water bubble of roughly 20 mL, and the gascolator sample is clean. The bowser's last delivery was six weeks ago and the strip has had heavy rain for three days. The aircraft's filler caps were on and the aircraft was hangared; the fuel strainer was last drained by the previous pilot two days ago. You have 45 minutes of daylight and 120 NM to your destination with no fuel stops.
1. Why is water reappearing after a clear drain a red flag, and what does it imply?
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It tells you the water is not a small residue at the drain point; it is a body of water inside the tank that is still finding its way to the lowest point.
Physical reasoning: a wing tank has ribs, seams and a drain at one low point. Draining clears the line and the immediate pocket, but water lying elsewhere in the tank - or in a different cell - migrates along the bottom as the aircraft is moved, as fuel sloshes, or simply with time, because water is denser than avgas and settles. Twenty millilitres appearing after 20 minutes means there is more where that came from. The amount you can see in a sample is never the amount in the tank.
It also points at the source: the bowser's last delivery was six weeks ago, the strip has had three days of heavy rain, and a part-full tank over six weeks will also collect condensation. That suggests water ingress through the bowser's own sumps, a vent, or a damaged seal - not a one-off splash. Fuel that runs clear once has removed what was sitting at the drain, not what is still in the system.
2. What will you do, who decides the aircraft is fit to fly, and what verification would satisfy you?
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Actions:
- Do not fly. Water in the fuel is a known cause of engine failure shortly after take-off, when the pickups are unported in the climb attitude and the engine is at full power.
- Put the aircraft back in the hangar and have it defuelled, the tanks sampled and inspected by the maintenance organisation - not just re-sumped until it looks clean. Water has to be removed, not diluted.
- Have the gascolator and fuel strainer drained and filters checked; water that reached the drains may already be in the lines.
- Do not sign the aircraft out or let anyone else fly it. Make a clear written entry and tell the operator and the next pilot.
Who decides: the licensed engineer/AMO decides airworthiness after draining and inspecting the tanks. The pilot's decision is only whether to fly, and the answer is no until there is evidence the fuel system is clean.
Verification that would satisfy me: samples from each tank drain and the gascolator taken after the aircraft has been rocked and left to settle, with no water in repeated samples; a sample from the bowser itself; and a maintenance release.
3. What would you check about the bowser, and what would you tell the next crew?
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On the bowser:
- Drain its sumps and take a sample from the delivery hose and filter; check for water and particulate.
- Check the last delivery date and whether it has been sitting part-full for six weeks with a vent open; check filler caps and vents for rainwater ingress.
- Check the water separator/filter and the record of its last service. If there is no record, treat the whole bowser as suspect and quarantine it.
Tell the next crew: the aircraft has water in the left tank and is grounded pending defuel; the bowser is suspect and should not be used by anyone until checked. Tell the operator too, because this is a fuel-supply problem, not a single-aircraft problem - every aircraft that drew from that bowser is a candidate for the same defect, and the remaining stock may need to be quarantined. Finally, consider that the previous pilot's sump check may have been done carelessly or with a dirty sampler, so check the sampler itself before blaming the fuel alone. Losing the daylight is not a reason to fly; a night stop is cheaper than an engine failure at 300 ft.