munckton

ILS Approaches in SA

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i'm not sure if this is the correct forum for this topic, but i'm having a problem with the CAA South Africa approach charts. nowhere is there any mention of a RA, DH, or DA. i want to make a full auto land at RNW19 FACT, and don't know if i should use Barometric or Radio Altitude setting for the Cat11 landing. i think this is my problem as i am just assuming that it should be a RA100. i am flying the PMDG 737-800ngx and every time i land i get a touchdown of -550 to -750 fpm. this unfortunately rejects some of my flights submitted via ACARS. i also have the same issue at FAOR.

 

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On the bottom left hand panel is an obstacle clearance section. 

In common with many state produced charts, this is the raw data and it is up to you to convert this to a DA/H.

The CAT I minima are easy: take the OCH for a Category C aircraft  (154 ft).

This is lower than the system minima for a CAT I approach (200 ft) so we use 200 ft as the DH. Add the threshold elevation of 147 ft on to this and you get a DA of 347 ft (referenced as always against the barometric altimeter).

CAT II is a bit more awkward because here we are required to use the radio altimeter.

The system minimum for a CAT II approach is 100 ft. This is higher than the published OCH, so we use 100 ft as our DH. BUT we have to take in to account the terrain below the aircraft at the point at which we are 100 ft above the threshold. 

This leads us to the Precision Approach Terrain Chart which gives us the survey of the terrain under the approach. We can then find the point at which we are 100 ft above the threshold and measure our actual height above the ground - I haven't had a ruler out but it looks to me as if the ground slopes slightly down at that point such would give a radio DH slightly more than 100 ft.

All the above said I don't think this is your problem with the aircraft: it doesn't care what minima you have set, all it sees is an ILS. Are you engaging both autopilots? Do you get a LAND annunciation? Is the FMA displaying FLARE at the appropriate moment?

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Hi, thanks for the reply. as usual CAA SA has to make things difficult. i don't see why they don't just publish an RA. i do engage both CMD's. i get a FLARE display but sometimes too late or not at all. I have noticed that if the FLARE and RETARD do not go from white to green. not engaging. i thought that it was my DA was incorrectly selected. also i have both courses and RNW heading dialed in.

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It's not really the South African CAA -- this is common with pretty much all state-produced charts including the UK CAA and all the Eurocontrol area CAA charts that I've seen. They don't publish DAs etc because minima (particularly <CAT I) are dependent upon many individual, aircraft and company-specific factors.

Hence CAA-produced charts are generally not designed with pilots in mind: they are designed for reference use by all sorts of users, from performance engineers to airport planners, commercial chart producers and much more. As I say, they provide the raw data for each of these different user groups to work with. Thus if the CAA were to publish a CAT I DA of 347 ft, for instance, this would be misleading to a private pilot IR(R) holder who is subject to different minima, or to a pilot of a light aircraft required to add an altimeter Position Error Correction (PEC) to the DA. It would also not be very useful to a performance engineer working on airline-specific procedures (emergency turns etc) as they would need to know the obstacle clearance heights for each category and so on.

Commercially-produced plates such as Lidos or Jepps on the other hand are designed for use in the cockpit and thus have pre-calculated minima (each airline will have their own bespoke chart package created by Lido/Jepp etc which will take in to account their fleet -- users such as Aerosoft etc that distribute these charts to simmers get a generic package suitable for most airline operations).

Not sure why FLARE etc is not engaging properly as I am not very familiar with the 737; you may need to investigate the PMDG forums!

Simon

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