Learning objective

You can explain how water enters avgas, why it sits at the lowest point, and when/how to sample — kept separate from vapour lock.

60 seconds

Water is denser than avgas and immiscible → sump and gascolator. Sample after every refuel and before the first flight of the day, ideally before the aircraft is moved. A blocked tank vent is fuel starvation with a full tank — not water.

Water in fuel is a frequent cause of engine failure shortly after take-off. Water does not burn and interrupts fuel flow.

How does water enter the tank?

Source	Mechanism
Condensation	In half-empty tanks at temperature changes (day/night); hence the recommendation to fill after flight. The highest humidity absorption occurs when fuel tanks are nearly empty — much air in the tank, which condenses water on cooling.
Rainwater	Via defective or loose tank-cap seals
Contaminated fuel truck	Rare, but possible (refueller pre-flight check)
Tank sweating	When refuelling from a ground-cooled tank into warm air

Water properties

Denser than Avgas (~1.0 vs. ~0.72 g/cm³) → condensation water collects at the lowest point of the tank and in the gascolator, because water is heavier than fuel.
Does not mix with Avgas — forms visible droplets.

Pre-flight check — sample obligation

A fuel sample must be taken and checked after every refuelling and before the first flight of the day — best before the aircraft has been moved. Reason: water settles to the lowest point during a quiet period; movement would mix it in the tank and it would no longer concentrate at the drain.

When exactly to sample?

Before the first flight of the day (ideally before first aircraft movement).
After every refuelling — whether 1 litre or full tanks.
After long ground time (overnight outside).
After rain or in suspect conditions.

How?

Drain the gascolator (central filter, lowest point of the fuel system).
Drain each tank bottom (wing sump drains, usually 1-2 per wing).
Pour the fuel into a clear glass container (sampler / fuel tester).
Visual check:
- Correct colour for the grade (100LL = blue, UL91 = clear/yellow).
- No droplets (water visible as a clear bottom layer — distinct interface).
- No particles (dirt/rust).
- Smell like Avgas, not kerosene (Jet A).

If water is found: continue draining until only Avgas comes. For large amounts or suspected wrong grade: do not start, contact maintenance.

Tank ventilation

The tank vent prevents an underpressure in the tank caused by fuel consumption (which would block fuel supply to the carburettor). It must be checked at every pre-flight for blockage (insects, ice, dirt) — a blocked vent can cause engine failure through fuel starvation even with a full tank.

Overheating damage — cylinder head and gasket

If the engine overheats (lean mixture, blocked cooling path, hot climb), there is a risk of damage to the cylinder head and the cylinder head gasket:

Aluminium cylinder head expands strongly when overheated.
The gasket between cylinder and head can fail under pressure and heat.
Result: compression loss, fuel loss, possible power loss or engine fire.

→ Keeping CHT below the AFM limit is the key measure.

Exam knowledge

Water sinks (density), does not mix, visible droplets.
Condensation especially in nearly empty tanks.
Sample: gascolator and wing sumps; before taxi.
This is not vapour lock.
Keep the tank vent clear.

Check yourself

1. Why does water collect at the lowest point of tank and gascolator?

A) Water is lighter than avgas and floats
B) Water is denser than avgas (≈ 1.0 vs ≈ 0.72 g/cm³), immiscible, visible droplets
C) 100LL blue dye binds water
D) Water dissolves then falls out as vapour lock

2. When to take a fuel sample, and why before moving the aircraft?

A) Only after filling the tanks
B) After taxi to the run-up
C) Gascolator only; skip wing sumps
D) After every refuel and before the first flight of the day; at rest the water has already settled

B — water is denser and immiscible.
D — even after 1 L. Moving the aircraft redistributes water.