Ok i'm using navigraph charts and also nats-uk charts i see a difference in CAT II DA if you guys can have a look at RWY 33 ILS approach.

 

On navigraph says CAT II RA 98

 

Nats uk says under category C (737) is 420 (92)

 

Why the difference between the 2?

 

Thanks vernon!

Might be difficult to check without knowing which airport ;-).

 

But it could have to do with different field/runway elevations (perhaps using different sources for their data). Or perhaps they're using slightly different reference points.

 

Edit: And just to be clear, you're wondering about the difference from 98 to 92 and not 98 to 420, right?

Oh sorry EGBB guys

98 to 420 is added rwy elev

hmm maybe they have different runway elev?

 

My jepps say EGBB has runway elev of 328, mins of 428 are RA 98 (runway +100, terrain very slightly higher from touchdown point)

Lido's chart states a runway elevation of 328 ft, decision height CAT II class C aircraft RA 99 ft. Just to confuse 1 ft more.

 

NATS UK charts state the obstacle clearance height (OCH) by the way. Probably Jeppesen, Navtech, Lido etc. disclose the decision height (DH). It might be that they have added a few more factors to it and this way it ends up higher than the OCH.

 

 

NATS UK charts state the obstacle clearance height (OCH) by the way. Probably Jeppesen, Navtech, Lido etc. disclose the decision height (DH). It might be that they have added a few more factors to it and this way it ends up higher than the OCH.

 

Yeah, that would make sense. For one OCH can be lower than category minimums, DH not so that fits with 100ft above touchdown level.

The title of this post is misleading -  CAT II descision height difference!

 

The CAT II decision height is:

 

2. Decision height. An operator must ensure that the decision height for a Category II operation is not lower than:

(i) The minimum decision height specified in the AFM, if stated;

(ii) The minimum height to which the precision approach aid can be used without the required visual reference;

(iii) The OCH/OCL for the category of aeroplane;

(iv) The decision height to which the flight crew is authorised to operate; or

(v) 100 ft.

An OCA/(OCH) of 420(98) may not affect the Decision Height in this case.

Why will it not be affected mgh?

 

 

Why will it not be affected mgh?

 

Vernon, I think he means in that way, that the OCA/H is still lower than the decision height required for a CAT II approach. If the OCA/H would be higher you could adapt the DH accordingly.

 

For example your decision height normally would be 100 ft and the OCH for your airfield 150 ft, you should opt for the 150 ft in accordance with the rules mentioned above. Remember it should not be lower than the OCH. As long as the DH is higher there's no need to change it.

Why will it not be affected mgh?

 

Because at EGBB the OCA/(OCH) is 420(98). The Decision Height therefore can't be lower than 100 ft so it can never be less than 98 so doesn't affect it

Why will it affect it if its over 100ft?

http://www.twitch.tv/f30gl/c/4439897

Why will it affect it if its over 100ft?

 

Vernon,

 

you should have a closer look at the terms we are using here. OCH stands for OBSTACLE CLEARANCE HEIGHT which means in common words, the height where you are pretty safe from NOT flying into any obstacles like mountains, sky scrappers or whatever can be in the way of your landing path.

 

Let's assume your company's decision height for CAT II (class C aircraft) usually is 100 ft and due to hills and sinks the OCH for your landing runway is for example 175 ft, which height would you opt for? The lower, usual decision height of 100 ft or the higher decision height of 175 ft which might be less dangerous.

 

You can do the math on your own. :smile:

Right i'm onto it, if lets say DH for me is 50 and the OCH is 100 i'll obviously go with 100? Aslong as the OCH is higher than DH there is no need to change it?

 

If am looking at a chart and says OCA 420 is that for cat 1 approaches guys amd will i have to add runway elevation? Will it be THRelev or ADelev?

 

Just trying to grasp the basics first !!!

Also guys i'm using navigraph charts but i don't use them in the uk as they don't get updated like nats uk charts! My question here is nats uk at egph doesn't show CAT III as navigraph shows CAT III and i know edinburgh does have CAT III ? Nat uk only shows CAT 1 + 2

DH will never be shown to be lower than OCH on charts - chartmakers put DH on charts according to several factors, one of them being it cannot be lower than OCH (even if the ILS CAT allowed). Other factors would be airline requirements for example.

 

National authorities typically do not publish DH, rather OCH, because DH is operator specific.

 

Same applies for DA/OCA.

 

 

 

If am looking at a chart and says OCA 420 is that for cat 1 approaches guys amd will i have to add runway elevation? Will it be THRelev or ADelev?

 

The basic difference you need to grasp is whether you are talking H or A.

 

H (DH, OCH) is measured against runway TDZ level. Usually only valid for CAT II and III ILS approaches, in company charts you will often see a slightly different value suffixed by RA - this means that value is to be used with radio altimeter and is corrected for terrain slope (if there is a downslope from the runway end, then RA value will be a bit higher to account for the slope for example).

 

A (DA, OCA, MDA) is measured against sealevel. You use barometric altimeter in reference to this. It is used on ILS CAT I and non-precision, because there might be a bigger difference between TDZ altitude and terrain altitude where you would be, therefore RA is impractical.

Barometric altimeters are a bit less precise than RAs, but are not susceptible to this error.