Runway Cat markings

[Mariner9]

I know (or thought I knew) that Cat II, Cat III etc relates to autoland. If so, why are there runway Cat markers at the runway holding points....is there an autotakeoff too?

[Thunderbug]

The CAT II / III holding points still all to do with autolands. When doing an autoland, the autopilots need to follow an uncorrupted ILS beam to the runway. These holding points keep taxiing and and departing aircraft out of the ILS critical area whilst aircraft are doing their approch.

The same sort of idea applies in the air with much bigger gaps between aircraft on the approach to ensure a good quality signal to follow.

On the 757 & 767 that I fly; a sustained deviation from the ILS above 200' or any deviation below is reason to call go-around.

There is not auto-takeoff, but we do use a PVD (para-visual display) which enables us follow the centre of the runway during a low vis take-off. The device gets its signal from the Localiser beam and displays a "go left" or "go right" indication. (display is a streaming barbers pole(!) - sounds strange but fairly intuitive for use). It is mounted in the glare sheild in front of the pilot so your vision can go naturally from out the window to the PVD as the visibility dicates. When the wheels are off the deck it closes up and your vision settles on to the instumrnts to guide your departure.

Clear as RVR 100m! :cool:

[moleslayer]

Hi BIK, bit of a catch-all statement there, certainly can't hand fly Cat 2/3 in my current machine.

Cat2....must be automatic approach, at least 1 autopilot engaged, may be disengaged for landing, minimum 80ft.

Cat3A....must be automatic approach,at least 1 autopilot
engaged , landing must be automatically controlled.

Cat3B....must be automatic approach, 2 autopilots engaged,
automatic landing + rollout automatically controlled.

However I do realise that they CAN be HAND FLOWN given the right equipment e.g. HUD

[Spitoon]

The definitions of the various caterories of approach go something like this (if my memory and a few old notes serve)

Category I - A precision approach and landing with a decision height not lower than 200ft and with either a visibility not less than 800m, or an RVR not less than 550m

Category II - A precision approach and landing with a decision height lower than 200ft but not lower than 100ft, and an RVR not less than 300m

Category IIIA - A precision instrument approach and landing with a decision height lower than 100ft and an RVR not less than 200m

Category IIIB - A precision instrument approach and landing with a decision height, if any, lower than 50ft and an RVR less than 200m

Category IIIC - A precision instrument approach and landing with no decision height and no RVR minimum specified.

[Intruder]

The bottom line is that the area surrounding the ILS beam has to be protected to the extent required by the TERPS (or other standards) so it remains stable enough for either the autopilot or the flight director to guide the airplane to the appropriate minimums. Since Cat III minimums can be as late as touchdown, the margin for error is much reduced, hence a larger area is protected.

[Mariner9]

Thanks for your replies guys, it mostly makes sense now. But one further question...surely your holding position is dictated by ATC who presumably will know that A/C are on autoland approach, while you may not. So why the need for the marker signs?

M9

[ETOPS]

Mariner9

The distance from the runway edge is the key here. Cat 1 holding points can be as little as 30m from the runway whereas the Cat2/3 hold will be further away as detailed earlier. ATC will clear you to the appropriate hold which in normal ops would be the Cat1. If low vis procedures are in force then it would be the Cat2/3 hold but at their discretion it could be the Cat 1 if there was no inbound traffic. This would be used to expidite departures if a queue was forming.

[simfly]

Just to add to above, if you were flying into a cat III equipped airport and decided to do an autoland in clear weather say to test the thing out (if you are allowed to) you would need to advise ATC of this so they can inform aircraft on the ground to hold at the cat II/III holding point, otherwise they would taxi into the LLZ sensitive area.

[Intruder]

Airplanes fly coupled approaches and autoland in clear weather on a routine basis, and pilots seldom, if ever, notify ATC they are doing it.

One bonus is that in clear weather the pilot can look out the window and see if there are any airplanes holding short near the runway threshold. If required, he can disconnect the autopilot and fly manually.