There are two types of instrument approaches called precision and non-precision.
Precision: A precision approach is able to guide the airplane in both the vertical (altitude) and horizontal (alignment with runway)
Non-Precision: A non-precision approach is only able to guide the airplane in the horizontal plane.
The most common precision approach is the ILS approach. It uses a narrow beam of radio waves that the plane (or pilot) follows to the runway. Planes and pilots can be certified to perform different types of ILS approaches, called “categories”.
Each category has a set of restrictions that outline how close the plane can get to the ground before aborting the takeoff if they don’t see the runway. The following is from Wikipedia:
Category I – A precision instrument approach and landing with a decision height not lower than 60 m (200 ft) above touchdown zone elevation and with either a visibility not less than 800 m or a runway visual range not less than 550 m.
Category II – A precision instrument approach and landing with a decision height lower than 60 m (200 ft) above touchdown zone elevation but not lower than 30 m (100 ft), and a runway visual range not less than 350 m.
Category III A – A precision instrument approach and landing with: a) a decision height lower than 30 m (100 ft) above touchdown zone elevation, or no decision height; and b) a runway visual range not less than 200 m.
Category III B – A precision instrument approach and landing with: a) a decision height lower than 15 m (50 ft) above touchdown zone elevation, or no decision height; and b) a runway visual range less than 200 m but not less than 50 m.
Category III C – A precision instrument approach and landing with no decision height and no runway visual range limitations. A Category III C system is capable of using an aircraft’s autopilot to land and guide the aircraft to the terminal if required.
The following video outlines what a Category III ILS approach looks like. Feel free to fast forward until there are 40 seconds left in the clip.
I’ll be able to do Category I ILS approaches when I get my IFR rating.
