De facto : sorry, but the Cat is not determined with the speed you fly in the portion of the manoeuvre.
Any Category whether A, B , C , or D has a wide variety of speed range to allow flexibility to the operator and the maximum speeds are used to calculate the protection.
Again, a C aircraft can fly up to 180 kts during a circle to land . Please refer to your jeppesen or wikipedia.
I accept the fact that FAA might have different rules. But this is not the case in ICAO, and Eu Ops world.Aircraft category is based on Vref which is the threshold crossing speed, regardless of your downwind speed or circling speed.
Now the example given in your FAA document is not clear:
*"For example, an airplane which fits into Category B, but is circling to land at a speed of 145 kts must use the category D minimum"
Firstly you are not supposed to exceed the maximum speed for your aircraft category, for a circle to land. As previously sated there is a range of speeds within which you must be inside during your approach.
For a CAT B aircraft the maximum speed authorised during a circle to land is 135 kts
For a Cat C aircraft the maximum speed authorised for a circle to land is 180 kts
For a D aircraft the maximum speed authorised for a circle to land is 205 kts
So if your aircraft category is B and you are doing 145 in a circling you are not respecting the speeds for your aircraft category.*
Could be that FAA does not specify a range of speeds for each aircraft category, or a maximum speed in circle to land.. But ICAO does.
Circling to land approaches with PAN OPS criteria , have been designed according to ICAO document 8168 volume 2. Which means they are calculated with Maximum speeds for each category as laid down in the document.
The design is based on the maximum speed for the category. For a C aircraft , the maximum speed in a circle to land is 180 kt. So all margins , areas of protections are designed and based on that maximum speed.
If you are doing lower than 180 kt, you will be inside the protection as your radius will be lower.
I recall a question that many examiners ask during oral test, or type rating : what is the maximum speed allowed in a circle to land for a C aircraft? Answer : 180 kts.
Moreover I have seen many approach charts with pan ops criteria specifying under the circle to land box, the max speed for the category just in front of minimas.
A-->100*
B-->135
C-->180
D-->205