The facts.
G-EMIN was recently parked fully fuelled in a guarded outdoor compound
(RAF). Witnesses later reported fuel "pouring from the top of the
fuselage. (I would not like to speculate on the result had a smoker
approached the aircraft). Stains from the fuel ran from the upper vent(s)
down both sides of the fuselage, but any fuel on the ground had
evaporated in the sunshine. The tank level was significantly lower,
probably a gallon or more but the sight gauge was not accurately
calibrated at the time.
The explanation ?
Fuelling with the tail down can result in the vent outlet becoming
covered before fuel fills the forward upper part of the tank. The trapped
air expands when heated sufficiently to overcome the
head necessary to expel fuel out of the vent. So the effect is at its
worst when half the volume above the vent outlet is filled with fuel
(neglecting compressibility). Even fuelling in a level attitude would
still leave a considerable volume of air in the fuelling hose to perform
the same action.
The solution ? The vent should be in the top of the tank at the highest
point in any achievable fuelling attitude. What better place than the
fuel cap itself ? (You could then dispense with a piped vent altogether).
The fuel cap supplied actually has four holes already drilled through
the plastic bit into the space above, but hermetic sealing is maintained
by two further O-rings. When I released it just now it hissed ! though
it has been in the garage for days. I propose to remove the small O-ring
surrounding the release shaft to see if this is sufficient. Till then,
keep it cool.
Graham 2
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