General aviation airplanes such as Cessna's are certificated to Vref airspeeds 30 percent above stall speed in the landing configuration. This speed is known as Vso. With the landing gear down and flaps deployed, Vref is known as 1.3 times Vso. Vref airspeeds are normally based on Maximum Gross Weight (MGW).
Calculate the approach speed needed (Vref) for landing after a long flight. Consider the amount of fuel that has been used. Vref will be reduced approximately .8 knots for each 100 pounds under MGW. Look at your weight and balance chart to determine the maximum gross weight of your aircraft. Then calculate the actual weight at takeoff including passengers, baggage and fuel.
Determine the amount of time in flight and calculate how many gallons of fuel will be burned during the flight. Be sure to take into account the different burn rates for taxi, climb out, cruise, and descent. Then calculate how much weight loss will be used from the fuel burn.
Subtract the burned fuel weight from the takeoff weight, to get the landing weight and then subtract landing weight from MGW. Round to the nearest hundred pounds. Divide this number by 100 then multiply X .8
Take the Vso speed (the stall speed in the landing configuration with full flaps and gear down - shown on the bottom of the white arc on the airspeed indicator) and subtract the number that you just calculated. This is your new Vref approach speed based on the actual landing weight.
Takeoff weight - fuel burn weight = Landing Weight.
MGW - Landing Weight = Z, then round off Z to nearest hundred = Y.
Y divided by 100 times .8 = X.
Vso - X = new Vref approach speed
