The pilot of a Grumman AA1-B reported an engine failure due to fuel system mismanagement; resulting in an off-field landing.

Narrative

I executed an off-field forced landing after the engine on a Grumman AA1B ceased operation while turning base to final on Runway XXR at ZZZ. No serious injury or damage resulted from the forced landing. The cause of the incident was fuel exhaustion resulting from pilot error.I am a Private Pilot with an instrument rating and 350 hours as PIC. I have flown 60 hours in the last 12 months and 20 hours in the last 90 days. My instrument rating; flight review and medical are all current. The departure and destination fields are very familiar to me; as is the route and aircraft.I have owned the aircraft for several years. I have 145 hours in the aircraft. The aircraft recently underwent two maintenance processes. The first was an annual inspection completed months earlier. The annual inspection included an overhaul of the magnetos and replacement of the lines used for delivery of fuel and oil to the engine. The second recent maintenance process was the installation of a new attitude indicator; transitioning to a glass instrument with additional functionality. The aircraft had been flown more than 19 hours since completion of the annual inspection and once since installation of the new attitude indicator. The last flight was a brief maintenance flight with the IA who installed the instrument to check its operation and those of related systems (i.e.; the pitot static system).The aircraft has four fuel tanks: two main tanks in the spar of each wing which each hold 12 gallons; 11 usable. The pilot must manually switch between the main tanks as needed. Each wing also has a 5 gallon auxiliary tank which can be pumped to the main tanks. The amount of fuel on board and in each tank can be assessed through several indicators. First; the aircraft has two fuel gauges; one for each wing which show the amount of fuel in each wing. Those gauges are a sort of transparent tube with a red ball that floats up and down based on the fuel in each wing. The gauges only provide reliable readings when the aircraft is level on the ground and even then their readings are approximate (e.g; ¼; ½; ¾) I had never tested their accuracy when the tanks were empty prior to the forced landing. In addition; the aircraft has a fuel totalizer. I have tested the totalizer iteratively and found its measurements to be accurate. Finally; the fuel system also has an electric boost pump. The POH calls for operation of the boost pump during take off and landing. The aircraft burns ~8 GPH in full power cruise and ~6 GPH in low power cruise.On the day of the flight I was well rested and in good health. No stress or external pressures to fly. No alcohol or medicine had been consumed. I was excited to get up and experiment with the newly installed attitude indicator; practice some basic maneuvers and potentially get night current. The conditions were VFR. A thorough pre-flight inspection revealed nothing amiss. My fuel totalizer showed more than 6 gallons of usable fuel available for the flight; more than enough for my intended flight of several minutes from ZZZ1 to ZZZ; plus a 30 minute reserve. The fuel gauges were not checked. I mentally noted that I was low on fuel; but had enough for the flight; plus reserve. I mentally note that I was not sure how much fuel was in each tank.The short flight felt familiar and well-prepared. There were no delays or anything unexpected before departure.I departed ZZZ1; flying uneventfully to ZZZ airspace. I took off and flew on the left tank for the duration of the flight. X minutes into the flight I began descending with clearance to enter the pattern to make right traffic for runway XXR. On descent I reduced power and engaged carburetor heat as usual. I performed my usual 'GUMPS' (Gas; Undercarriage; Mixture; Propeller; Seatbelts) checklist while midfield on the downwind; I did not switch fuel tanks. I engaged first flaps abeam the numbers; then full flaps on base; consistent with my usual practice. My altitude and airspeedwere on target.The first sign that anything was amiss was during the turn from base to final at about 700 feet AGL. My airspeed was between 75 and 80 knots. The engine lurched and then shut down immediately. I advanced the throttle and confirmed the mixture was rich; both to no effect. I immediately focused my attention on maintaining airspeed through a slight nose down attitude and finding the best location for a forced landing. Fortunately; I was just leaving a residential area and entering an empty field. I [requested priority handling] on tower frequency and told them my engine was out. I then focused on getting the airplane down in the field; straight ahead; before I reached a large and crowded street at the end of the field . I touched down more roughly than usual landings; but not so much that I'd describe it as a 'hard landing.' The aircraft rolled through the field for about 100 hundred feet; stopped shorter than usual by a relatively rough surface and full breaks. After I came to a stop I informed controllers that I was ok. Then I turned the master and magnetos off; pulled the mixture to idle cut off; opened the canopy and evacuated.First responders arrived on the scene in just a few minutes. They assessed my condition through a few short questions and then; satisfied I was well; most of them left. One police officer kindly stayed with me until the airport operations crew arrived nearly XX minutes after the landing. The following day; I engaged a A&P/IA located at ZZZ to recover the aircraft from the field and perform an inspection of the systems that may have contributed to the engine failure. Their inspection revealed two potential contributing factors. First; the left (active) fuel tank was low on fuel. The engine ran just less than one minute on the tank when straight and level on the ground. (The right tank was found to have nearly 6 gallons of fuel). The second finding from their inspection concerned the magnetos: the cap to the left magneto was dislodged. The three screws holding the cap were not in place; one was recovered from the engine compartment. Despite the cap being dislodged; the engine ran on both magnetos; left magneto and right magneto; although with notable roughness when 'both' were not engaged. It could not be determined whether the dislodged magneto cap was a contributor to or a result of the forced landing.Other minor cosmetic damage to the fairings and cowling were found. These were determined to be a result of the forced landing.As the PIC I conclude the probable cause of the engine going out and the resulting forced landing was my mismanagement of the fuel system. I departed without being confident of how much fuel was in each tank. Because I was ignorant of the amount of fuel in each tank; I did not take the action of switching tanks as part of my pre-landing GUMPS check. Having failed to switch tanks; I exhausted the active tank. Contributing factors include; low altitude at the time of exhaustion. I have prepared extensively to execute an engine out checklist which includes switching tanks; but with the fuel exhaustion setting in at 700 feet AGL; I felt there was not enough to time to complete the checklist. I instinctively chose to focus on maintaining airspeed and landing in the vacant field (rather than the residential areas or busy streets surrounding the field). As such I did not execute the full checklist and reach the step of switching tanks. Fuel slosh in turn from base to final: a contributing factor was the turn from base to final which appears to have affected the delivery of fuel to the engine. The engine operated on available fuel when straight and level (albeit very briefly).Departing with minimum fuel: my totalizer accurately showed I had adequate fuel to make the flight (+ reserve); but departing with the minimum fuel required leaves insufficient room for error. Dislodged magneto cap: the dislodged magneto cap may have further strained engine operations.It's possible the magneto was not operational; reducing the available spark to the engine at a time of lean mixture. Distraction and excitement: I was excited to get up and test a new instrument. My excitement may have distracted from the rigor required for safe fuel and flight planning. Mitigations [include]; thorough post-flight inspection of aircraft by qualified A&P. (complete)Ground training with certified flight instructor covering fuel planning requirements and fuel system operation. (Complete) Updating personal minimums to include: No departure with less than full tanks unless required for weight and balance purposes. Land with at least 30 minutes fuel without exception. Track usage by tank using totalizer and include notations in logbook or comparable record keeping system. Expectation is to always know with accuracy how much fuel is in each tank. Never depart without 100% confidence status of each tank. (Complete) Share my story with flying communities I am involved with to reinforce my own commitment to these minimums and encourage others to do the same.