ILS Categories

We often hear mentioned  Cat 1, Cat 2 or  Cat 3 but  what does it all mean ?

 The Answer is below

Each runway ILS Radio Signal is given a category depending on its quality. The better the quality the higher the category. The best being Cat  3C. The worst Cat 1

Please note a  Cat 1 ILS is not Autoland approved. Only Cat 2 and 3 allow aircraft to Autoland. and then all autopilots must be working.

For a pilot to be able to make an approach  to a runway  in bad  weather, the weather  has  to be at or above certain conditions depending on the CAT number of the ILS. If the weather is below the pilot will have to hold or divert and no approach can be made except for aircraft equipped and certified to Cat 3.

 A table below gives these conditions

Signal Strength  CAT 1 CAT 2 CAT 3 A CAT3 B CAT 3C
Minimum Visibility in Meters 800 M 400 M 200 M 50 M 0
Decision Height in feet 200 FT 100 FT 0 0 0

 For example if the ILS was a Cat 1 ILS and the visibility was 400 metres and the base 100ft the pilot would have to hold or divert . The CAT classification for the ILS is published in approach charts .

Before an aircraft can Autoland,the pilot has to confirm with ATC that the ILS signal is secure. For example the signal is not going to be turned off or that no obstruction such as a vehicle is going to be placed in front of the ILS transmitter. The minumum conditions must be available at the final approach fix before an approach can be legally made. Check approach  charts for the minimums.