Aircraft Vref speed is a function of aircraft stall speed (Vref = 1.3Vs). The 1.3 factor over stall (Vs) is used to provide a maneuverability margin. Vs and correspondingly Vref changes with aircraft landing flaps and weight. Accumulation of ice on aircraft surfaces or system failures such as flaps also increases Vs and therefore Vref.
When the aircraft is fully configured for landing the appropriate approach speed flown should never be less than Vref (assuming a 1.3 maneuverability margin over VS is desired). To stay above Vref at all times a speed increment (∆V) is added to compensate for such things as autothrottle reaction and/or jet engine spool-up times. The ∆V added to Vref is the resulting approach speed (Vap) that should be flown. Therefore Vap is as follows:
Vap = Vref + ∆V
The speed increment ∆V equal ½ the steady state headwind component + the gust increment.
In determining Vap there are 3 possible scenarios that limits ∆V to a maximum and minimum. The 3 scenarios are:
⦁ Landing with aircraft system abnormality (QRH Vref NEW)
⦁ Landing in icing conditions (Stall Protection Ice Speed)
⦁ Landing without scenarios 1 or 2 above (routine landing).
∆V should never exceed 20 kt in all three scenarios. ∆V maybe 0 kt for scenarios one & two and 5 kt for scenario three.
Table shown is an Example routine landing