ILS Cat. III

25.10.2004

Submission:

Please allow me to address you the following general/specific matter:

On chart AD 2-LEST IAC/2 dated 6 SEP 01, the Spanish State Authority has published that ILS rwy 17 CAT II/III operations are authorised without restrictions (later came the limitation of RVR 200m); when I looked at the minima box of the respective chart, I noticed that CAT II OCA(H)s are calculated in standard conditions (missed approach climb 2.5%) and additional lower values for gradient of 4%; both values give the indication that the basic ILS surfaces are penetrated by objects other than the frangible obstacles as defined by PANS-OPS Doc 8168, otherwise the CAT II OCHs would be only defined by the height/loss margins (Table III-21-4) and CAT III operations WOULD NOT BE restricted. But, as I mentioned, this seems not to be the case in Santiago. Same PANS-OPS document mentions in para., 21.4.10.2 "Promulgation of ILS category III approach procedures" that:

"Cat III ops may be permitted subject to the appropriate Cat II OCA(H) (logically, the additional values also) being below the height of the Annex 14 inner horizontal surface. Cat III ops may ALSO be permitted with a Cat II OCA(H) between the height of the inner horizontal surface and 60m provided the Annex 14 Cat II inner approach, inner transitional and balked landing surfaces are extended to protect that OCA(H)."

As I presume, the protection CAT III operation condition stated above was applied by the State Authority and that, normally, means that Cat III is applicable without any limitations; however when operating CAT III at LEST, the aircraft has to fly the standard missed approach climb gradient, or, has to strictly apply the raised gradient of 4% ? This question is only coming from my uncomfortable feeling that such a situations (different CAT II OCA(H) corresponding to better gradients than the nominal) are not clearly covered and explained by Doc. 8168 statements for CAT III ops.

Sorin Onitiu - Senior Design Analyst - Jeppesen

Answer or Commentary:

(I.W.

As I read the Pans-Ops manual, Category III operations are possible if (among other technical points) specific procedure design factors are fulfilled. That is, that an unrestricted category II approach is applicable to the runway concerned and the Annex 14 inner approach, inner transitional and the balked landing surfaces are not penetrated.

Efficient minimums depending on the missed approach climb gradient is a valid way of giving the benefit of lower minimums to aircraft able to perform better than the restrictive nominal (2.5%) gradient employed by Pans-Ops. However the less performance orientated aircraft must still be accommodated for, as is stated in paragraphs 21.4.10, 21.4.8.7.6 and 7.1.6

The three methods of ILS minimum determination utilise either the ILS Basic surfaces, the Obstacle Assessment surfaces or the Collision risk model and the methods increase in precision and complexity. So an obstacle that infringes the ILS missed approach Basic surface (2.5%) but does not infringe the amended (3 -> 5% missed approach) Obstacle Assessment surface could be effectively eliminated making the approach an unrestricted category II approach. By definition the balked landing surface, which has a slope of 3.33 %, must not be infringed. However this surface is quite small, it starts 1800m from the threshold, splays at 10% and continues up to between 45 and 60 meters above the reference. At normal airports it is difficult to infringe this surface. So in answer to your question, yes it is possible but I personally would take exceptionally great care in designing a Category III approach where the missed approach climb gradient is greater than 2.5%.

I would like to take this opportunity to draw the attention of all interested procedure designers to Chapter 1 of the "Control of Obstacles part 6 of the Airport Services Manual (Doc. 9130)". It has some very interesting historical material on the parameters for ILS.)