Cat II/III App

[TotalBeginner]

I have a question regarding CAT II/III approaches.

1. Why do airports have a holding point which is further from the runway when aircraft are making CAT II/III approaches. Is it something to do with protecting the LLZ signal, or to protect departing aircraft from those making a go-around from a low altitude?

2. Can CAT III approaches be made at all aiports with an ILS. If not, how would a pilot know which CAT of approach he is supposed to follow?

3. Do CAT III approaches have to be performed using the Autopilot?

[square leg]

Good evening,

1) Yes. Basically you don't want aircraft in the sensitive area when an aircraft is approaching during CAT II/III approaches (protection of arriving A/C) (LLZ&GS signals)

2) No. The approach plate (Jepp) tells you what's allowed and what's not allowed. ATC will also tell you if it's Low Visibility procedures that must be applied or not ("LOW VIS in force").

3) Not neccessarily. Some A/C require that the AP is on (most), some that it is off. Airbus, Boeing (afaik) & Avro's have it on. Then some have a HUD which requires that the pilot flies the approach manually (some Swedisch turboprops).

[TotalBeginner]

I've had a look at ILS plates for EGSS in the AIP, and they only publish minima for CAT I/II. Does this mean that CATIII approaches are not approved?


And what is the term OCA(OCH) I havent' come across this before.

[TopBunk]

Beginner

Stansted has CAT 3 facilities.

OCA(H) = Obstacle Clearance Altitude (Height) - ie altitude refers to QNH, height to QFE. Basically the altitude (height) at which you will clear all obstacles within the approach path / go around area. The area the obstacles which are considered are in varies with the type of approach being flown - bigger area with non-precision approaches as the accuracy of the flight path is less than with an ILS, for example.

[Gary Lager]

Cat II/III approval is usually an issue for the operator - in that they are required to demonstrate adequate crew training and aircraft capability etc. before the CAA with authorise the airline. As a result minima are usually found in the airlines operations manuals, since they depend on a few variables as well as the usual regulatory minima.

Example, the B737 can perform Cat 3A approaches and autolands with either landing flap 30 or 40. If 30 is used, the minimum RVR is 250m, using F40 minimum is 200m (due to improved view over the nose with F40).

There are many other factors which determine the approach capability of an airfield (and aeroplane), such as approach/runway lighting, availibility of backup approach aids etc etc. That's why the minima published on plates are fairly generic, and at best usually only for Cat II.

Our company doesn't use plate-published minima anyway, like many others, we are required to refer to our Aerodrome Operating Minima manual, part of the overall aircraft ops manuals.

OCH - Obstacle clearance height - the minium height to clear all obstacles around an airfield within a defined radius (can't remember what it is, but it increases in size with the approach speed category of the aircraft).

OCA - Obstacle clearance altitude, ie the same thing as OCH but measured from sea level rather than airfield elevation.

(D'oh - TB beat me to that last bit!)

[HEATHROW DIRECTOR]

<<Can CAT III approaches be made at all aiports with an ILS.>>

Just to confirm... the ILS has to be up to Cat III standard and there are various other requirements too. The ILS Sensitive Area has to be clear - at major airports ground staff will ensure this with regular checks. It's a fairly complex operation to introduce Low Visibility Procedures and can take half an hour or more to implement.

[chevvron]

Plus the lighting must conform to Cat 2/3 requirements eg supplementary App lights and touchdown zone 'mat' plus glidepath MUST be 3 deg; anything above or below is not Cat 2/3 (cat 1 can be between 2.5 and 3.5 though)

[TotalBeginner]

Thanks for your time guys! Very informative!! :D