Narrative:

This is to report and document an in-flight issue that led to me making an unplanned landing while flying my be-58. After leveling off at 20000 feet I noticed that the left outboard fuel cap was missing. At the time; no fuel was visibly exiting the aircraft; but the fuel cap was clearly missing; and I didn't know whether any fuel had already leaked from the left wing. On-board fuel sensors did not indicate fuel levels on the left were lower than on the right; but I understood that fuel could begin exiting the aircraft if I was not careful; creating a possible fuel imbalance situation; or possible fuel starvation of the left engine; if enough fuel vented the left wing. I called ATC and stated that I needed vectors to an airport for an immediate landing. I switched over the left engine to cross-feed from the right fuel tank. I knew that this would keep both engines running; but would eventually leave a fuel imbalance that would make control of the aircraft increasingly difficult. I would need to land soon. I was given initial vectors and I began a descent on direction of the controller on duty. At about a 2.5 degree nose low pitch attitude; however; fuel began venting from the left wing. I stabilized the aircraft at a level pitch attitude; then tried extending spoilers; then landing gear; and eventually full flaps; to allow for a more rapid descent from altitude; but at a relatively flat pitch attitude (it appeared to be 1-1.5 degrees nose low). This procedure was successful in preventing further venting of fuel from the outboard filler location with the missing fuel cap.I assessed the situation and realized that due to my high altitude; I would easily make it but then still have to descend significantly prior to landing; likely with a series of turns that might risk venting significant amounts of fuel. I assessed that it would be safer for me to instead fly to my home airport that I am well familiar with. I assessed that as I am also very familiar with the terrain and also as there are large areas to the south and southwest of that airport that are relatively uninhabited; this would be the safer airport for me to attempt a potentially difficult landing. I was given a heading that would take me well to the east of the airport. I assessed this would not be the safest situation; as I would not want to be at a lower altitude; needing to cross mountains to get to the airport; with possible controllability issues if I by then had developed a fuel imbalance. I asserted that I was in an emergency condition and asked for vectors directly to the airport. I noted that I would pass safely above another airport at an altitude that would not conflict with aircraft on the ILS. I had previously been vectored above this approach traffic during previous flights; and felt comfortable that my stabilized descent; without significant turns; would keep me safely above such traffic; and allow me to get to my airport environment; without venting more fuel; and leaving me with a controllable aircraft for my final approach. As I got closer I realized my altitude and descent rate would leave me too high for such an approach without more maneuvers that might possibly cause fuel venting. I assessed that I instead could pass overhead and execute a gentle left-hand turn to [the] runway; and could adjust my flight path to ensure that by the time I hit field elevation I would be over the runway; without having to significantly adjust my descent rate or execute sharp turns. Controllers provided me the path I requested. When coming up on tower's frequency; I asked for 'the airspace to the south of the airport;' which they gave me. I was able to adjust my flight path so that I was able to execute a shallow; coordinated left-hand turn back to [the] runway. In the turn I witnessed very brief moments in which fuel vented from the left wing; which I was able to arrest each time by reducing my bank angle and downward pitch attitude. By the time I was linedup with [the] runway for landing; I believe I put in a small amount of power to keep at normal approach speed. I believe I crossed the threshold at between 100-105 knots; and I executed a normal landing. Upon pulling off the runway I returned to my hangar; where I was met my airport personnel; who interviewed me.prior to departure I had checked the fuel caps; as I do as part of pre-flight on all flights. I opened and closed the left outboard fuel cap; in the same manner as I always have prior to flight; nothing appeared to be different than in the past. Also of possible interest; the left fuel cap had a plastic spacer that I understood helped keep the fuel cap snugly held in place. Without the plastic spacer; the fuel cap was somewhat loose. Adjustment of the fuel cap itself is not apparently possible; so use of the plastic spacer is apparently what is and has been used for fuel caps of this sort; on barons; to ensure they are safely secured to the aircraft wing. Note that the left outboard fuel cap had been particularly tight since my maintainers had replaced the O rings on all fuel caps some months ago. I had pointed out the tightness of the fuel cap after replacement of the O rings; and maintenance personnel examined it and assessed that it secured safely; albeit with more force than required when securing the other fuel caps. I have placed my baron in my hangar and ordered a new fuel cap. I have asked them to inspect all fuel caps for safety; including the new one on order; and to assess that safety of the aircraft prior to it flying again.

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Original NASA ASRS Text

Title: A Pilot reported the left outboard fuel cap missing while airborne.

Narrative: This is to report and document an in-flight issue that led to me making an unplanned landing while flying my BE-58. After leveling off at 20000 feet I noticed that the left outboard fuel cap was missing. At the time; no fuel was visibly exiting the aircraft; but the fuel cap was clearly missing; and I didn't know whether any fuel had already leaked from the left wing. On-board fuel sensors did not indicate fuel levels on the left were lower than on the right; but I understood that fuel could begin exiting the aircraft if I was not careful; creating a possible fuel imbalance situation; or possible fuel starvation of the left engine; if enough fuel vented the left wing. I called ATC and stated that I needed vectors to an airport for an immediate landing. I switched over the left engine to cross-feed from the right fuel tank. I knew that this would keep both engines running; but would eventually leave a fuel imbalance that would make control of the aircraft increasingly difficult. I would need to land soon. I was given initial vectors and I began a descent on direction of the controller on duty. At about a 2.5 degree nose low pitch attitude; however; fuel began venting from the left wing. I stabilized the aircraft at a level pitch attitude; then tried extending spoilers; then landing gear; and eventually full flaps; to allow for a more rapid descent from altitude; but at a relatively flat pitch attitude (it appeared to be 1-1.5 degrees nose low). This procedure was successful in preventing further venting of fuel from the outboard filler location with the missing fuel cap.I assessed the situation and realized that due to my high altitude; I would easily make it but then still have to descend significantly prior to landing; likely with a series of turns that might risk venting significant amounts of fuel. I assessed that it would be safer for me to instead fly to my home airport that I am well familiar with. I assessed that as I am also very familiar with the terrain and also as there are large areas to the south and southwest of that airport that are relatively uninhabited; this would be the safer airport for me to attempt a potentially difficult landing. I was given a heading that would take me well to the east of the airport. I assessed this would not be the safest situation; as I would not want to be at a lower altitude; needing to cross mountains to get to the airport; with possible controllability issues if I by then had developed a fuel imbalance. I asserted that I was in an emergency condition and asked for vectors directly to the airport. I noted that I would pass safely above another airport at an altitude that would not conflict with aircraft on the ILS. I had previously been vectored above this approach traffic during previous flights; and felt comfortable that my stabilized descent; without significant turns; would keep me safely above such traffic; and allow me to get to my airport environment; without venting more fuel; and leaving me with a controllable aircraft for my final approach. As I got closer I realized my altitude and descent rate would leave me too high for such an approach without more maneuvers that might possibly cause fuel venting. I assessed that I instead could pass overhead and execute a gentle left-hand turn to [the] runway; and could adjust my flight path to ensure that by the time I hit field elevation I would be over the runway; without having to significantly adjust my descent rate or execute sharp turns. Controllers provided me the path I requested. When coming up on tower's frequency; I asked for 'the airspace to the south of the airport;' which they gave me. I was able to adjust my flight path so that I was able to execute a shallow; coordinated left-hand turn back to [the] runway. In the turn I witnessed very brief moments in which fuel vented from the left wing; which I was able to arrest each time by reducing my bank angle and downward pitch attitude. By the time I was linedup with [the] runway for landing; I believe I put in a small amount of power to keep at normal approach speed. I believe I crossed the threshold at between 100-105 Knots; and I executed a normal landing. Upon pulling off the runway I returned to my hangar; where I was met my airport personnel; who interviewed me.Prior to departure I had checked the fuel caps; as I do as part of pre-flight on all flights. I opened and closed the left outboard fuel cap; in the same manner as I always have prior to flight; nothing appeared to be different than in the past. Also of possible interest; the left fuel cap had a plastic spacer that I understood helped keep the fuel cap snugly held in place. Without the plastic spacer; the fuel cap was somewhat loose. Adjustment of the fuel cap itself is not apparently possible; so use of the plastic spacer is apparently what is and has been used for fuel caps of this sort; on Barons; to ensure they are safely secured to the aircraft wing. Note that the left outboard fuel cap had been particularly tight since my maintainers had replaced the O rings on all fuel caps some months ago. I had pointed out the tightness of the fuel cap after replacement of the O rings; and maintenance personnel examined it and assessed that it secured safely; albeit with more force than required when securing the other fuel caps. I have placed my Baron in my hangar and ordered a new fuel cap. I have asked them to inspect all fuel caps for safety; including the new one on order; and to assess that safety of the aircraft prior to it flying again.

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.