Approach categories group aircraft of similar instrument approach airspeeds. These categories are used to determine the lowest altitude (minimums) the pilot is allowed to fly during an instrument approach and the protected area around the destination for circular approaches.
Approach categories are determined based on the aircraft's speed—the slower the speed, the lower the approach category.
Approach categories are grouped from category A through E. Aircraft with the slowest approach speed, 0-90 KIAS, fall under category A. Aircraft with the fastest approach speed, 166 KIAS and greater, operate under category E.
Every other aircraft will be grouped under one of the remaining categories: B, C, or D.
The manufacturer determines the approach category for any particular aircraft. To determine the approach category, the manufacturer will determine the Vref for the aircraft at its maximum certified landing weight. The calculated Vref value will determine the respective category for that particular aircraft.
If an aircraft does not have published Vref speeds, then a speed of 1.3Vso is used.
Once the proper approach category is determined, the pilot will then find the corresponding category minimums shown on the approach plate.
In many instances, different categories will share the same minimums.
As shown in the chart above, for the straight in ILS 4R approach (S-ILS 4R), the minimums are the same for all categories with a decision height of 212ft and a runway visual range (RVR) of 1,800ft.
The straight-in localizer 4R (S-LOC 4R) lists two different minimums. One minimum is shared between categories A and B, and the other being shared between categories C and D.
For the S-LOC 4R, category A and B aircraft have a minimum descent altitude (MDA) of 540ft and an RVR of 2,400ft. Category C and D aircraft have MDAs' of 540ft and an RVR of 5,500ft.
Finally, the same applies to the circling approach with category A and B aircrafts having an MDA of 640ft and visibility of 1 mile. The circling approach for category C and D circling has a separately published minimum for each.
Category C has an MDA of 680ft and visibility of 1.75 miles.
Category D has an MDA of 680ft and a visibility of 2 miles.
Certain factors may cause a pilot to elect to fly an approach faster than typical, such as icing, no flap approaches, and requested by air traffic control.
Suppose a pilot must fly at a higher approach speed and the higher speed puts them in the next higher category. In that case, they must use the next higher category minimums.
For example, a pilot that typically flies at 80 knots during an approach elects to fly the approach at 95 knots. Since these extra 15 knots put the aircraft into category B, they must use category B minimums.
It would be reasonable to think then that an aircraft with a typical approach speed of 95 knots (Category B) flown at 80 knots (Category A) would then be allowed to elect to use category A minimums.
However, this is not true.
This is because the manufacturer determines the approach category based on the aircraft Vref, or 1.3Vso, at its maximum certified landing weight.
Suppose the manufacturer determines that the Vref for a particular aircraft is 125 knots. In that case, that aircraft is placed into Category C. A pilot may elect to fly slower than 125 knots, such as landing at low weights or on short runways. However, they are not allowed to use category B or A minimums even at a slower speed. The lowest category the pilot could, therefore, choose to use is category C.