A person that i have much respect for and is much wiser than me was doing a line check with me one time at night. Part of our company's after takeoff SOP checks were, Check fuel tank caps for fuel venting, as this was a Bonanza, and Baron with the bladder tanks.
The question was raised, without shining your torch out the window to the caps, what gauge indication will you see if you are in fact venting fuel? Of course i said straight away well your gauge will be decreasing. I was soon told that the gauge would actually show that your fuel quantity is increasing.
I've always remembered that, because one time it did actually happen to me, launched at night and didnt have the fuel caps sitting perfectly level in the recess, and after takeoff checks i came to the fuel vent check and there it was the gauge increasing. I flew a circuit, got out and there was the cap not sitting correctly.
I totally agree with the comment about going slowly because if i had have gone slowly it would not have happend if i did a proper walkaround.
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