This is not what you want to see happening just after takeoff or ANYTIME in flight! And certainly NOT in an IMC situation

But, note how the fuel cap can be seen to be cocked in it's placement in the opening. The leading edge of the cap is clearly tilted upwards and the aft edge is down. This should have been a clue to this pilot while on the ground after fueling.

The least of your CSOB worries is the lost 100LL fuel. And no, this is not my video!

I will admit, this has happened to me once. It happened as a result of one end link of the little chain on the cap getting caught in the sealing lip. The cap, for all cursory visual clues, looked properly closed. It was NOT!

Being a CSOB, I almost always self serve my fuel and even if the FBO fuels me I will always be standing behind the line guy to check the fuel loading and be ready to seal the caps. I can count on one hand the number of times I have allowed FBO line services to fuel my bird without me physically being present at the time of fueling.

Just after lift off from my favorite CSOB northern tundra fuel stop in KCLI, at maybe 400 AGL, my commercial ME pilot buddy in the right seat notices fuel streaming from the right wing aux tank cap and says: "OH NO, Fuel leak!"

I tell him it's OK and say out loud two-times: "Fly the Airplane". I fly a normal pattern at ~800 AGL and land uneventfully. When my buddy and I landed (fortunately severe clear VMC prevailed) I was able to see the chain link and how it had gotten wedged up under the cap and sealing ring of the airframe.

From that moment forward I now always purposefully clear the chain of any kinks and see it drop down and away from the metal sealing surface of the fuel opening in the airframe.

Also, keeping your fuel cap O-rings in great shape is good fuel cap leak prevention. Just look at what happened to this really nice B58 Baron from water contamination of the fuel past a poorly maintained cap and o-ring:

The right engine sputtered to a stop just after takeoff. Here is the fuel cap center shaft from the above splashed Baron, you decide!

See the whole article on this B58 engine failure HERE

See the O-Ring page HERE for some popular fuel cap O-ring PNs. While you're there, give some thought to upgrading to the long life Blue Fluoro-Silicone O-rings listed there.

Baron Owner & IA, Stuart S., posted this pirep on his Fluoro Silicone o-rings: "Mine are like new and they were installed in 1996."

Here's a little tip for when your plane has been hangared outside after a big rain has come through. A little while after the rain storm, look inside the tops of your fuel caps. If I see water pooled there behind the locking tab, to me that's indicative of decent inner O-ring sealing condition on the rotating shaft of the locking tab. If I don't see any water pooled behind the tab, I'm going to get a little concerned and do an extra special sump job. This won't, however, tell you anything about the big O-ring condition. A visual inspection for cracking and checking of the outer O-ring can be a good way to determine fitness for use.

Fuel Cap Tip #2: In addition to your normal pre-flight fuel sumping routine, be sure to add an extra look and focus on sumping when hangared outside after a rain storm or after washing your plane. This is one of the greatest opportunities for water to enter the fuel tanks through poor O-rings or bad fuel cap sealing/mating surfaces.

Fuel Cap Tip #3: After fueling and just prior to getting in the plane for startup, I make just one last purposefully SLOW walk to each fuel cap and make that last check for proper sealing of the caps and I also make a scan under the airplane for any evidence of leaks, chocks AND towbars!

Fuel Cap Tip #4: This is what can happen if you try to snug up the cap sealing force on the closure tab shaft and you run out of threads on the center shaft or corrosion is built up on the nut and shaft

Don't ask me how I know!!! You don't want to be on a trip and have this happen to you as you will be AOG!

My caps are all fine and have been for over 4 years but I still carry a spare in my little "AOG box" that I got from a Beech lister way back when. You might consider carrying one or vow never to mess with your caps while on a trip or just before a big trip.

Here are pics of my O-Ring swap out to Fluoro-Silicone units:

This is why you need to look at your O-Rings carefully AND OFTEN!

Especially if they are the 1940s technology nitrile rubber versions.

See a Shaw 531 Fuel Cap O-Ring Change Article written by BeechTalker, David M. HERE

Thanks for the contribution David

Here are some pics (courtesy of V35 owner, Dale B.) of clamping a Gabb Cap for the center shaft o-ring change:

No O-rings for this "Old School" Fuel Cap for very early Bonanza airframes. Rumor has it these puppies are quite pricey! Click on the pics to go to Spruce for pricing.

As always, if you have no idea what an O-ring is or what a good one is supposed to look like or what a bad one looks like, close this browser page and ask your A&P licensed mechanic to have a look at your fuel caps and let you know if they need to be changed.