Per ICAO Annex 6, Part I, section 4.3.6 "Fuel Requirements," airplanes should calculate their required fuel quantity as follows (summary; see below for actual ICAO text):
- Taxi fuel
- Trip fuel (to reach intended destination)
- Contingency fuel (higher of 5% of "trip fuel" or 5 minutes of holding flight)
- Destination alternate fuel (to fly a missed and reach an alternate)
- Final reserve fuel (45 minutes of holding flight for reciprocating engines, 30 minutes for jets)
- Additional fuel (if needed to guarantee ability to reach an alternate with an engine failure or at lower altitude due to a pressurization loss)
- Discretionary fuel (if the pilot in command wants it)
Considering a flight has many unknown, the same connexion using the same airplane will have different fuel requirement and fuel remaining, simply depending on the date you decide to have a look at the data.
Is there any statistics of how much fuel is left on average in aircrafts after landing of commercial flights?
It can be an average for commercial flights in general, a breakdown of short / medium / long haul flights, expressed in percentage of tank volume or in liters / gallons / weight. Or any usable measure I did not think of that can be used to do basic calculations.