When I purchased N131JF from Jesse back in 2000, the fuel caps were not equipped with either grounding braid or tethers. Although the fuel caps on a Long-EZ are outside of the prop arc (and even more so since I now use a Catto prop, which has a smaller diameter but a deeper pitch than the original Performance prop), I was sufficiently concerned by reading accident reports where fuel caps had become loose and gone through the prop in flight to ask Jesse to fit tethers to the fuel caps. The tethers are triangular pieces of metal linked to the underside of the cap by a strong chain.
On Friday 3rd November, I lifted off from Lancaster Texas bound for Santa Fe en route to Escalante Utah (see Trip Report – Escalante). I fuelled the plane to 44 gallons before departure, which is the maximum amount of fuel that I can put in the fuel tanks with the plane “kneeling”.
After 40 minutes, I was at 8,500 feet about 5 miles North East of Breckenridge, when I decided to do a periodic scan of the wings. The left wing looked OK, but my peripheral vision of the right wing seemed odd…A more detailed look showed that the right fuel cap was loose. The tether was doing its job and preventing the fuel cap from flying off, but airflow had wedged the cap against the top of the strake. That was good. What was not good was a steady stream of fuel being blown out of the tank over the wing and into the atmosphere. As the fuel sloshed around in the tank, a spray of liquid and vaporizing fuel would be blown back past the prop.
I immediately cut power, and pointed the nose down towards Breckenridge Airport. Less than 10 minutes later I touched down, pulled off the runway and parked next to the self-serve fuel pump.
An examination of the fuel tanks showed that the right tank (the one with the open fuel cap) was still almost full, but the left tank only contained about 13 gallons of fuel. Because of the difference in the fuel tank fuel levels, fuel was siphoning between the tanks via the common sump tank to equalize the fuel levels (on my plane this can be heard as an intermittent ‘clicking’ sound).
The explanation for the difference in fuel levels was not difficult to work out. With the right fuel cap off the top of the fuel tank, there would be no positive pressure above the fuel via the fuel vent line (which is open to the airflow above the dorsum). The airflow past the open fuel tank was also generating suction. The result was that fuel was flowing from the left tank to the right tank as the fuel was sucked out from the right tank in flight.
I decided to re-fill the plane tanks to 44 gallons, the same level as at Lancaster. This required 17.8 gallons of fuel. Since I had been flying for the equivalent of about 45 minutes burning 8.5 gallons per hour (averaged as I climbed to cruising altitude and descended rapidly to land in Breckenridge), this would have burned 6.4 gallons of fuel. This meant that in 50 minutes of flying I lost 11.4 gallons of fuel from the open fuel tank.
Without my noticing the fuel cap being loose, fuel would have continued to escape from the right hand tank, with fuel moving from the left tank to equalize. This is one downside of a common sump arrangement. Since both tanks are linked via the common sump, I could have lost all of my wing tank fuel load, and I might also have lost the fuel in the common sump area due to the suction effect of air flowing over the strake at normal cruising speed.
It would be difficult to prove or disprove this hypothesis due to the inherent dangers in flight-testing with fuel escaping from the plane in the vicinity of the engine. However, a quick calculation shows that if I had not noticed the issue and diverted, I would have (partly) consumed and (mostly) lost all of my onboard fuel well before reaching the border with New Mexico. That might have ruined my day…
Three conclusions:
1. I failed to check that both fuel caps were secured before I boarded the plane. This item must be on the checklist and you must complete the check.
2. Anybody who owns a Long-EZ with a common sump fuel tank system needs to understand that the linkage between the tanks will result in most (if not all) of the onboard fuel gradually being lost overboard if a fuel cap comes loose or is lost in flight.
3. It is a really good idea to have fuel cap tethers. If you lose a fuel cap completely in flight, even if it does not damage the prop, you cannot really fly the plane anywhere until you fit a replacement. With my current fuel cap tethers, I was able to land, solve the problem and continue.