ILS "Y" surface

28.10.2004

Submission:

I questioned about ILS RDH calculation a year ago. Answers were very helpful, so I have been visiting this site frequently. Today I ran into another problem. That is an obstacle (the Air Traffic Control Tower) which penetrates the OAS "Y" surface by about 20 ft. In case of an obstacle penetrating the "Z" surface, one can calculate the equivalent approach height of the missed approach obstacle. However if the obstacle penetrates the "Y" surface, how can I deal with it. I couldn't find a clear answer as yet. Is it OK to publish 2 OCHs (1 for 2.5% missed approach climb rate and another for higher missed approach climb rate) in flight procedure plates? Also can I make an approach procedure using the CRM and ignoring the OAS penetration of ATC tower if the probability of collision (individual) is below 1X10 -7?

J S Park - Seoul Korea

Answer or Commentary:

(I.W.

It seems that there are 3 questions here. Firstly, how to deal with the "Y" surface penetrations. As far as I know these two surfaces provide varying protection for aircraft with greater deviations from the nominal glide path and aircraft that commence an early missed approach (i.e. from as early as the FAF). If the obstacle is past the -900m mark then it can be reduced to an equivalent approach obstacle. If before the -900m position, height loss must be added to determine the OCA/H, unless the obstacle is above the GP'' line that originates from the -900m position (Pans-Ops Part III chapter 21 paragraph 21.4.8.8.2).

Secondly two missed approach gradients. I have seen this published before but ICAO does state that only one missed approach should be issued for an instrument approach procedure (Pans-Ops Part III chapter 7 paragraph 7.1.1). One could always get around this by publishing two IFR procedures.

Thirdly does the CRM take precedence over the OAS? As far as I know the CRM does take precedence over the OAS when used correctly. One shouldn't forget to put in all obstacles as the obstacle density does effect the CRM calculated probability.

Finally I would be surprised that the control tower penetrates the OAS surfaces. As when the airport was built an ICAO Annex 14 check is a standard requirement to ensure that buildings (i.e. the terminal and ATC tower) do not infringe the ICAO annex 14 obstacle protection surfaces. As the Annex 14 surfaces are more stringent than the ILS OAS surfaces the tower would have shown up as a penetration well before it was built. I suggest that you check the position and height information you have received.)