CATEGORY OF A/C- is calculated according to the A/c speed on approach to land or Vref, which is calculated by using 1.3 of Vso in landing configuration.
For Example a Aircraft has Stalling Speed of 100 kts, therefore 100 ×1.3 = 130 kts.
Therefore Vref will be 130 kts.
The A/c which falls into a category for a straight in landing approach can fall into another category when it is on a Circle to land Approach.
A Circling Approach procedure is used when the approach path is not aligned within 30° of the landing Runway.
Or
A circling approach is required whenever the A/c has to maneuver more than 30° to be able to land on the Runway.
A – Below 90 knots.
B – 90-120 knots.
C – 120-140 knots.
D – 140-166 knots.
E – Above 166 knots