Do you know your aircraft’s unusable fuel amount?

General Aviation News often reprints US National Transportation Safety Board (NTSB) accident reports, including one from October 2009 that involved a Cessna 180 in Mitchell, Oregon, where the aircraft was severely damaged plus two people were injured. 

According to the accident report, the pilot had acquired the aircraft several years before the accident. On the day of the accident, the aircraft’s engine lost power due to fuel exhaustion – forcing the pilot to make an off-airport landing.

During the subsequent investigation, the pilot informed the FAA that the fuel capacity on the airplane was 58 gallons thanks to a Supplemental Type Certificate (STC) installation and that the previous owner had told him that all 58 gallons were usable. However, the FAA reviewed the paperwork and found that the actual usable fuel was only 51.4 gallons. In addition, the FAA showed the pilot the fuel selector decal which had also been placarded with 51.4 gallons usable fuel.

The pilot did note that he had already made the same trip eight or nine times before the accident but he would also normally refuel en route. However and on the day of the accident, he did not refuel.

Moreover, the STC installation information noted that to determine the aircraft’s usable fuel, its type certificate data sheet needed to be referenced and then the unusable fuel amount listed on the sheet needed to be subtracted from the new total fuel capacity. The aircraft’s datasheet showed that five gallons of fuel were unusable – meaning the aircraft’s actual usable fuel capacity was 51.4 gallons. Moreover and as part of the STC:

The airplane flight manual and the fuel selector valve placard were supposed to be updated with the revised usable fuel quantity. This action was noted on the FAA Form 337 for the STC installation. The pilot reported that there was no AFM supplement for the STC included in his paperwork, and that he did not ever look at the quantities indicated on the fuel selector valve placard in the cockpit due to its location between the seats.

The cause of the accident was ruled the pilot’s lack of understanding about his aircraft’s fuel system – leading to fuel exhaustion. Hence and its worth asking: Do you understand your aircraft’s fuel system and more importantly, how much of that fuel will be unusable?