Narrative:

I first arrived at the hangar at about XA00; intending to go out on a flight specifically working on cross-country navigation techniques. I performed a preflight inspection the aircraft; and saw that I had been the last person to fly the aircraft which I verified by checking the hobbs and tachometer meters. Upon receiving the weather briefing for my planned route of flight; I did not quite like the presence of a SIGMET zulu over nearly the entire flight path; considering it would be VFR over the top for the first half at that time (another thing I much prefer to avoid); beyond that; I also decided that I hadn't gotten enough rest the night prior; so should either not make the flight at all or remedy the situation beforehand. I secured the aircraft and the hangar; and went home to get a little bit of work done and rest if possible. I returned to the airport; rested; at approximately XF30; and performed only a cursory inspection of the aircraft and systems before turning the engine; since I had inspected it in the morning. This was clearly a big mistake; as I noticed a few other abnormalities; such as the hangar lock being unlatched; though I am on the ocd side of things; particularly when it comes to locks. I generally also set the lock's combination to 0000 when locking it; and this was not the condition of the lock upon my return. At the time; I noted it was unusual; but saw nothing obvious with the airplane; and so did not think much else of it. I am not saying this to accuse anyone of anything; I believe that during my first; morning inspection of the aircraft; I was not as thorough as I am usually known to be; and attribute this to fatigue.in flight; I noticed abnormal readings on the fuel tank gauges approximately 30 minutes into the flight; namely; only the left fuel tank's gauge was bouncing around; and the right tank's gauge was stable at around the 75% capacity mark. I attempted to 'shake' the plane around to loosen the fuel gauge floats with no success. As the flight continued; I noticed a slightly increased rolling tendency; requiring constant correction; I then made the decision to perform a precautionary landing at the nearest airport with maintenance and fuel services. I chose ZZZ airport based on its available services; proximity to me at the time; and moderate level of current pilot activity. I landed safely; taxied to the FBO; and examined the aircraft. I found the right wing fuel tank cap was missing. Fortunately; I was able to source a spare at the airport and put a new gasket on it before filling up the tanks to full. I then performed a thorough preflight inspection on the aircraft; and cautiously continued with my flight plan. The rest of the flight was without incident.I have several takeaways from this series of events: first and foremost; a complete and thorough inspection shall be performed before every sortie and after every landing; regardless of whether or not the aircraft has been inspected or flown earlier in the day. Second; any abnormalities with the aircraft or facilities shall again result in a complete thorough inspection of the aircraft. Third; this is not my job; thus; I am under no obligations of any sort to fly while not well rested; and moving forward; shall cancel any planned or scheduled flights if I am not absolutely at 100%; in the future.

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Original NASA ASRS Text

Title: A C172 student pilot reported unusual indications in the right fuel gauge and discovered after landing that the right fuel cap was missing. The student pilot concluded that it had not been secured adequately prior to flight.

Narrative: I first arrived at the hangar at about XA00; intending to go out on a flight specifically working on cross-country navigation techniques. I performed a preflight inspection the aircraft; and saw that I had been the last person to fly the aircraft which I verified by checking the Hobbs and Tachometer meters. Upon receiving the weather briefing for my planned route of flight; I did not quite like the presence of a SIGMET Zulu over nearly the entire flight path; considering it would be VFR over the top for the first half at that time (another thing I much prefer to avoid); beyond that; I also decided that I hadn't gotten enough rest the night prior; so should either not make the flight at all or remedy the situation beforehand. I secured the aircraft and the hangar; and went home to get a little bit of work done and rest if possible. I returned to the airport; rested; at approximately XF30; and performed only a cursory inspection of the aircraft and systems before turning the engine; since I had inspected it in the morning. This was clearly a big mistake; as I noticed a few other abnormalities; such as the hangar lock being unlatched; though I am on the OCD side of things; particularly when it comes to locks. I generally also set the lock's combination to 0000 when locking it; and this was not the condition of the lock upon my return. At the time; I noted it was unusual; but saw nothing obvious with the airplane; and so did not think much else of it. I am not saying this to accuse anyone of anything; I believe that during my first; morning inspection of the aircraft; I was not as thorough as I am usually known to be; and attribute this to fatigue.In flight; I noticed abnormal readings on the fuel tank gauges approximately 30 minutes into the flight; namely; only the left fuel tank's gauge was bouncing around; and the right tank's gauge was stable at around the 75% capacity mark. I attempted to 'shake' the plane around to loosen the fuel gauge floats with no success. As the flight continued; I noticed a slightly increased rolling tendency; requiring constant correction; I then made the decision to perform a precautionary landing at the nearest airport with maintenance and fuel services. I chose ZZZ airport based on its available services; proximity to me at the time; and moderate level of current pilot activity. I landed safely; taxied to the FBO; and examined the aircraft. I found the right wing fuel tank cap was missing. Fortunately; I was able to source a spare at the airport and put a new gasket on it before filling up the tanks to full. I then performed a thorough preflight inspection on the aircraft; and cautiously continued with my flight plan. The rest of the flight was without incident.I have several takeaways from this series of events: First and foremost; a complete and thorough inspection shall be performed before every sortie and after every landing; regardless of whether or not the aircraft has been inspected or flown earlier in the day. Second; any abnormalities with the aircraft or facilities shall again result in a complete thorough inspection of the aircraft. Third; this is not my job; thus; I am under no obligations of any sort to fly while not well rested; and moving forward; shall cancel any planned or scheduled flights if I am not absolutely at 100%; in the future.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.