Each year a number of aircraft accidents are related to fuel starvation, exhaustion or contamination. There were numerous reasons for these avoidable accidents: ranging from inadequate fuel systems knowledge by the crew, preflight planning issues, takeoff and landing checks and failing to monitor consumption during flight. Or even failing to refuel the correct quantity before the flight.
As preflight of an aircraft is important, preflight of the fuel system is equally so. Determining fuel available, usable quantities and the use of a management log will be the subject here.
Before starting any flight you will need to determine the amount of fuel currently in the tanks. Assuming you have calculated the total amount needed for the flight it is just a matter of filling the tanks with the amount required and you're good to go. Almost. It would be wise to check the available fuel by at least two methods: use an empty tank and fill this with known quantities or you will need to use a calibrated dipstick for this type of aircraft.
Due to the construction of the tank and the location where the fuel inlet port and drain port are located, a certain amount of the fuel is unusable and some part of that is even undrainable. Unusable fuel (which includes undrainable) is the amount that cannot be used in level flight. This amount can vary from model to model and in the aircraft flight manual you will find the specifics for your aircraft.
The experimental aircraft builder/owner must determine for himself what the exact amount of unusable fuel is for each tank in his aircraft and make a note of that in the flight manual. Usable fuel is the only amount which can safely be used in a flight.
Great care must be taken when converting between liters, US and Imperial gallons. Some stations refuel in liters whereas the aircraft fuel gauges show in US gallons and sometimes (to confuse matters even more) on the aircraft wing, near the fuel cap, the amount of that tank is shown in Imperial gallons.
Tank content is displayed on gauges and these should be reasonable correct when the tanks are empty. Compare the indications of the gauges when dipping the tanks so accuracy can be checked. Remember that during some maneuvers, slipping skidding and in turbulence, tank indications can and will vary accordingly. If the aircraft is equipped with a flow indicator its indications should be checked against the level of fuel in the tank after the flight and the expected calculated consumption.
In some countries it is common to dip the tanks to verify the amount of fuel. It is the most accurate way. But as each aircraft (and tank in that aircraft) are somewhat different it is important to use the correct dipstick for your aircraft.
Each dipstick is calibrated to the fuel tanks of that aircraft. It should have the aircraft registration number marked on it too.
When dipping fuel tanks you will need to follow some simple guidelines:
If there is no dipstick for your aircraft, you can start the flight with full tanks and keep a good log during the flight. But as dipsticks can be bought for a reasonable price, buy one and get it calibrated (and labeled too) for your aircraft.
As you normally refill the aircraft after the end of the day to keep the change of moisture as low as possible, it would be wise to dip and drain the tanks in the morning. Water and dirt in the fuel will then have sunk to the lowest part to be drained.
This has also occurred a lot in the past, make sure to dip the tanks before the flight as to prevent any surprise. Note that if the aircraft was parked on an uneven surface fuel could flow from the higher tank to the lower tank and siphon out through the vents. Which would leave you with one tank empty.