Aero-Tips!

A good pilot is always learning -- how many times have you heard this old standard throughout your flying career? There is no truer statement in all of flying (well, with the possible exception of "there are no old, bold pilots.")

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Aero-Tips 11.14.06

Fuel Vents... UNOBSTRUCTED. It's a common step on an airplane's Preflight Inspection checklist. But what are you really looking for? Why is it so important to check fuel vents for obstruction?

Tank types

There are three main types of fuel tank commonly installed in general aviation airplanes (note: some airplane models may have more than one type of tank installed):

• Bladder-type tanks-flexible rubber tanks, like big, thick "balloons" in the wings or fuselage.
• Metal or fiberglass tanks-sealed containers for fuel set into the wings or fuselage.
• "Wet wings" -- portions of the wing structure themselves that are sealed to contain fuel and prevent leakage.

Fuel vents

Just like a portable gas tank for a lawn mower or boat, your airplane's fuel tank requires a "push" of air pressure to keep fuel flowing toward the engine. A blocked fuel vent can cause a vapor lock that cuts off fuel flow -- causing a fuel starvation engine failure even when there's a lot of fuel remaining in the tank.

Fuel vents come in many designs and location, but usually consist of a stick, or probe, extending below the wing or fuselage. Orientation of the vent is critical -- if it is not properly slanted into the slipstream it may actually create suction from the tank, instead of pressure into it.

Each individual fuel tank may have its own vent, or several tanks may plumb into a single vent line. Secondary vents may be recessed in the wing, or consist of a simple hole drilled in the aft side of the primary vent. Secondary vents are your back-up in case bugs, dirt or ice block the primary.

Fuel tanks have another reason for proper venting. If a vent becomes obstructed the suction created by a fuel pump (or even a carbureted engine) can be so great that the tank collapses.  Bladder tanks may bunch up so fuel can't get from parts of the tank into the engine; hard tanks can dimple and bend; and wet wings may collapse to the point the airfoil shape is disrupted and aerodynamics severely compromised.

A metal "tip tank" that collapsed in flight under the suction of fuel draw, after its vent line was obstructed by an insect. (photo courtesy American Bonanza Society)

Few Pilots Operating Handbooks tell precisely where fuel tank vents are located, how they need to be oriented, whether they have secondary holes that must also be open, or even how many fuel vents there are on the airplane. Your best bet is to contact a user group ("type club") for your particular airplane type for the proper tank vent inspection requirements.

Aero-tip of the day: Check fuel vents for orientation and lack of obstruction during every preflight check.

FMI: Aero-Tips