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Fuel Caps

A number of owners reported losing fuel caps in flight.

Early on I discovered a fuel cap missing on my aircraft. While flying with another friend, he also lost a fuel cap inflight.

Subsequent discussion with other Bearhawk owners showed that at least 15 fuel caps had gone missing inflight, and that many pilots were carrying a spare cap with them. In short, it was a known issue that required a fix.

The Cause

The factory supplied fuel caps can be very tight fitting with a variation in diameters. The fuel cap uses a simple O ring grommet to hold the cap in place when tightened in the filler neck. Because they are so tight they can be very difficult to fit and remove, often requiring a screw driver.

The common suggestion (as a work around) was to apply a lubricant to the O rings prior to fitting them each time, and this idea had largely become mainstream. Lubricant reduces the friction required to secure the fuel cap in the tank filler neck. The lower portion of the fuel cap can then turn with the upper portion, thereby preventing tightening in the filler neck.

It appears that if the tanks are full and lubricant is applied to the O rings, the sloshing hydraulic action of the fuel may forcibly eject the fuel cap off.

The low pressure area over the wing can then cause fuel to siphon out into the air-stream. If the fuel selector is in the BOTH position (the normal position), fuel may then flow from the opposite side fuel tank to the one with the missing cap. On landing, the tank with the missing cap may be full, and the opposite tank (still containing a fuel cap) may be lower on fuel.

One Possible Solution

Some fuel caps are only marginally tighter and require minimal modification. By fitting O rings that are one size smaller, the fit may be much improved and no lubricant will be required. The O rings are then able to provide a secure friction fit when tightened.

NZ Bearhawk builder Graham Johnson explored several solutions and came up with a soltion. Mine were machined to a slightly smaller diameter on a lathe. We reduced the diameter by .5mm, then fitted one size smaller O rings. They now fit very well, and can be installed and removed easily by hand, providing a tight, secure fit.

In addition, we found that installing a roll pin (shown below) prevented the lower portion of the fuel cap from turning inside the filler neck, (and preventing tightening).


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