This weekend, I learned what happens when you forget to put the gas cap on after topping off.
Air pressure comes into the tanks from the cabin top vent and pressurizes the tank with the cap. Fuel flows through the selector valve (when it is in the "both" position) and up into the tank without a cap. The tank without a cap then dumps fuel overboard at the approximate rate of 20 GPH.
I was flying along and smelled gas, but attributed it to sloppy fueling. When I noticed the left tank showing half empty, I knew I had left the cap off. I expected the cap was off the left tank, and was surprised to see it was the right. It did make sense after I noodled it out, though.
Luckily, I stopped at an airport with a maintenance shop that had a used cap that fit.
Dave
What I learned this weekend.
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rudymantel
- Posts: 451
- Joined: Sat Nov 23, 2002 4:03 pm
Well, Dave, ..that was an event that has bit more than one guy.
What happens is not so much the gooseneck vent pressurizing the fuel overboard,...but the fact that the fuel caps are in a low-pressure area of the wing and Bernoulli's theorem goes to work sucking fuel out the open cap. When the other full tank realizes the fuel valve is in the both postion (depending upon which fuel selector valve you have) it will equalize fuel and send half it's fuel over to the emptied tank....helped by the gooseneck as you stated....where Bernoulli continues to suck it out the open filler neck. It's good you noticed when you did.
I once was given a check-out in a C-182 for insurance from a flight school rental by their check airman...a retired Continental captain. On the first climbout the tower radioed they observed "something" blowing off our upper wing. We made the circuit and landed and found the right cap held on only by it's chain. Two ATP's and what do you get?
I lost some fellow Hawker pilots to this problem once. They were refueled in the dark at Sonora, Mexico and the wing away from the floodlights of the terminal went unnoticed that the fuel cap was off. On takeoff they lost 1,000 lbs of fuel per minute and lost control of the airplane before they could get around the circuit and back onto the runway with a 3,000 lbs fuel imbalance. (Hawker solved the problem with a mandatory service bulletin that put a rubber "flapper" on a piano hinge just forward of the filler neck inside the tank. The onrush of fuel exiting the tank slams the flapper shut and closes the fuel filler neck from below.)
What happens is not so much the gooseneck vent pressurizing the fuel overboard,...but the fact that the fuel caps are in a low-pressure area of the wing and Bernoulli's theorem goes to work sucking fuel out the open cap. When the other full tank realizes the fuel valve is in the both postion (depending upon which fuel selector valve you have) it will equalize fuel and send half it's fuel over to the emptied tank....helped by the gooseneck as you stated....where Bernoulli continues to suck it out the open filler neck. It's good you noticed when you did.
I once was given a check-out in a C-182 for insurance from a flight school rental by their check airman...a retired Continental captain. On the first climbout the tower radioed they observed "something" blowing off our upper wing. We made the circuit and landed and found the right cap held on only by it's chain. Two ATP's and what do you get?
I lost some fellow Hawker pilots to this problem once. They were refueled in the dark at Sonora, Mexico and the wing away from the floodlights of the terminal went unnoticed that the fuel cap was off. On takeoff they lost 1,000 lbs of fuel per minute and lost control of the airplane before they could get around the circuit and back onto the runway with a 3,000 lbs fuel imbalance. (Hawker solved the problem with a mandatory service bulletin that put a rubber "flapper" on a piano hinge just forward of the filler neck inside the tank. The onrush of fuel exiting the tank slams the flapper shut and closes the fuel filler neck from below.)
'53 B-model N146YS SN:25713
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention.
An originality nut (mostly) for the right reasons.
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention.
An originality nut (mostly) for the right reasons.
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rudymantel
- Posts: 451
- Joined: Sat Nov 23, 2002 4:03 pm
WRONG WAY GAR
I encountered a strange set of circumstances while ferrying an Er-coupe from Minnesota to Louisiana some years ago. The "Coupe" has a "pressurized" fuel transfer system that uses ram air through holes in the fuel cap to push fuel to the center tank up on the firewall. It is supposed to be impossible to put the caps on the wrong way but in the bone chilling cold, windy, dark pre-flight at Ft. Dodge Iowa, I failed to check the orientation of the caps after the fueler had put them back on. (Actually I really didn't know untill the incident what could happen if they were put on backwards) It was only after we were airborne that my faithful co-pilot, MS DEANA, advised me- - "You are losing fuel out of the cap over here"! I leaned over and sure enough, a steady stream of vapor was exiting the hole in the cap!
We returned to the airport and after checking the quantity, and deciding to fill it up again, I installed the cap correctly. We lost about two gallons during that 30 minute flight! The "Fail Safe system used to prevent improper cap orientation is un-even tabs on the cap and un-even slots in the filler neck. It is difficult to do, but by forcing it, the cap can be put on backwards. Another instance of less than acceptable familiarity with the craft I was flying and trusting the fueler to use the correct procedure!
We returned to the airport and after checking the quantity, and deciding to fill it up again, I installed the cap correctly. We lost about two gallons during that 30 minute flight! The "Fail Safe system used to prevent improper cap orientation is un-even tabs on the cap and un-even slots in the filler neck. It is difficult to do, but by forcing it, the cap can be put on backwards. Another instance of less than acceptable familiarity with the craft I was flying and trusting the fueler to use the correct procedure!
OLE GAR SEZ - 4 Boats, 4 Planes, 4 houses. I've got to quit collecting!
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zero.one.victor
- Posts: 2271
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